Copyright © 2012 The IETF Trust & W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This document specifies XML digital signature processing rules and syntax. XML Signatures provide integrity, message authentication, and/or signer authentication services for data of any type, whether located within the XML that includes the signature or elsewhere.
XML Signature 2.0 includes a new Reference processing model designed to address additional requirements including performance, simplicity and streamability. This "2.0 mode" model is significantly different than the XML Signature 1.x model in that it explicitly defines selection, canonicalization and verification steps for data processing and disallows generic transforms. XML Signature 2.0 is designed to be backward compatible through the inclusion of a "Compatibility Mode" which enables the XML Signature 1.x model to be used where necessary.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a W3C Candidate Recommendation Working Draft of "XML Signature Syntax and Processing Version 2.0". The W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. The XML Security Working Group expects to request that the Director advance this document to Proposed Recommendation once the Working Group has verified two interoperable implementations of the Candidate Recommendation. The XML Security Working Group does not have an estimate of when this will be achieved. There is no preliminary interop or implementation report.
No features have been marked as "at risk".
Candidate Recommendations of Canonical XML [XML-C14N20] and XML Signature Streaming Profile of XPath 1.0 [XMLDSIG-XPATH] are also being published as a companion documents in conjunction with this publication.
A diff-marked version of this specification that highlights changes against the previous version is available. Major changes in this version in response to Last Call comments include:
KeyInfoReference
instead of
RetrievalMethod
.RetrievalMethod
transform usage consistent with XML
Signature 1.1.KeyInfo
in section 7, "The
KeyInfo
Element".
This Candidate Recommendation includes
the ECDSAwithSHA256
signature
algorithm, which is ECDSA over the P-256 prime curve specified in
Section D.2.3 of FIPS 186-3 [FIPS-186-3] (and using the SHA-256 hash
algorithm), as a mandatory to implement algorithm.
Patent disclosures on this specification. W3C has received several patent disclosures regarding the use of Elliptic Curve cryptography in XML Signature 1.1. These also apply to XML Signature 2.0. In accordance with section 7 of the W3C Patent Policy, the staff has launched a Patent Advisory Group (PAG) to address them. Please refer to the PAG charter for more details.
At the time of this publication, the XML Security WG is also producing "XML Signature Version 1.1". The most recent XML Signature Recommendation is the 10 June 2008 XML Signature (Second Edition) Recommendation.
Additional information related to the IPR status of XML Signature 2.0 related to Elliptic Curve algorithms is available at http://www.w3.org/2011/02/09-xmlsec-status.html.
This document was published by the XML Security Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-xmlsec@w3.org (subscribe, archives). W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 12 April 2012. All feedback is welcome.
Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
KeyInfo
ElementKeyName
ElementKeyValue
ElementRetrievalMethod
ElementX509Data
ElementPGPData
ElementSPKIData
ElementMgmtData
ElementEncryptedKey
and DerivedKey
Elementsdsig11:DEREncodedKeyValue
Elementdsig11:KeyInfoReference
ElementObject
ElementCanonicalizationMethod
in "Compatibility Mode"URI
Attribute in "Compatibility Mode"Transform
AlgorithmsThis section is non-normative.
This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element. More specifically, this specification defines an XML signature element type and an XML signature application; conformance requirements for each are specified by way of schema definitions and prose respectively. This specification also includes other useful types that identify methods for referencing collections of resources, algorithms, and keying and management information.
The XML Signature is a method of associating a key with referenced data (octets); it does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed. Consequently, while this specification is an important component of secure XML applications, it itself is not sufficient to address all application security/trust concerns, particularly with respect to using signed XML (or other data formats) as a basis of human-to-human communication and agreement. Such an application must specify additional key, algorithm, processing and rendering requirements. For further information, please see section 12. Security Considerations.
XML Signature 2.0 includes a new Reference processing model designed to address additional requirements including performance, simplicity and streamability. This "2.0 mode" model is significantly different than the XML Signature 1.x model in that it explicitly defines selection, canonicalization and verification steps for data processing and disallows generic transforms. XML Signature 2.0 is designed to be backward compatible through the inclusion of a "Compatibility Mode" which enables the XML Signature 1.x model to be used where necessary.
The Working Group encourages implementers and developers to read XML Signature Best Practices [XMLDSIG-BESTPRACTICES]. It contains a number of best practices related to the use of XML Signature, including implementation considerations and practical ways of improving security.
This specification defines XML Signature 2.0 which differs from XML
Signature 1.x in some specific areas, in particular the use of
various transform algorithms versus a fixed 2.0 transform that implies the
use of Selection and Verification steps in conjunction with
ds:Reference
processing, the corresponding disuse of
the URI
ds:Reference
attribute, the use of
Canonical XML 2.0 [XML-C14N20] in place of
other canonicalization algorithms, and updates to the required
algorithms and other changes.
This specification defines a "Compatibility Mode" that supports an XML Signature 1.x mode of operation. Compliance and other aspects unique to "Compatibility Mode" are outlined in section B. Compatibility Mode.
The body of the document refers to the syntax and processing model for the new 2.0 mode of operation, referred to as "XML Signature 2.0" in the document. Use of the "Compatibility Mode" is noted explicitly when required.
For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types. Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See section 12.2 Check the Security Model.)
This specification provides normative XML Schemas [XMLSCHEMA-1], [XMLSCHEMA-2]. The full normative grammar is defined by the XSD schemas and the normative text in this specification. The standalone XSD schema files are authoritative in case there is any disagreement between them and the XSD schema portions in this specification.
The key words "must", "must not", "required", "shall", "shall not", "should", "should not", "recommended", "may", and "optional" in this specification are to be interpreted as described in [RFC2119].
"They must only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized key words to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the Namespaces in XML specification [XML-NAMES] is described as "required."
The design philosophy and requirements of this specification are addressed in the original XML-Signature Requirements document [XMLDSIG-REQUIREMENTS], the XML Security 1.1 Requirements document [XMLSEC11-REQS], and the XML Security 2.0 Requirements document [XMLSEC2-REQS].
This specification makes use of XML namespaces, and uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics.
Implementations of this specification must use the following XML namespace URIs:
URI | namespace prefix | XML internal entity |
---|---|---|
http://www.w3.org/2000/09/xmldsig# |
default namespace, ds: , dsig: |
<!ENTITY dsig
"http://www.w3.org/2000/09/xmldsig#"> |
http://www.w3.org/2009/xmldsig11# |
dsig11: |
<!ENTITY dsig11
"http://www.w3.org/2009/xmldsig11#"> |
http://www.w3.org/2010/xmldsig2# |
dsig2: |
<!ENTITY dsig2
"http://www.w3.org/2010/xmldsig2#"> |
While implementations must support XML and XML namespaces, and while use of the above namespace URIs is required, the namespace prefixes and entity declarations given are merely editorial conventions used in this document. Their use by implementations is optional.
These namespace URIs are also used as the prefix for algorithm identifiers that are under control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN], [RFC3406] or Uniform Resource Identifiers [URI] defined by the relevant normative external specification.
For instance:
SignatureProperties
is
identified and defined by the disg:
namespacehttp://www.w3.org/2000/09/xmldsig#SignatureProperties
ECKeyValue
is
identified and defined by the dsig11:
namespacehttp://www.w3.org/2009/xmldsig11#ECKeyValue
http://www.w3.org/TR/1999/REC-xslt-19991116
Selection
is identified
and defined by the dsig2:
namespacehttp://www.w3.org/2010/xmldsig2#Selection
The http://www.w3.org/2000/09/xmldsig#
(dsig:
)
namespace was introduced in the first edition of this specification,
and http://www.w3.org/2009/xmldsig11#
(dsig11:
)
namespace was introduced in 1.1. This version does not coin any new
elements or algorithm identifiers in those namespaces; instead, the
http://www.w3.org/2010/xmldsig2#
(dsig2:
)
namespace is used.
No provision is made for an explicit version number in this syntax. If a future version of this specification requires explicit versioning of the document format, a different namespace will be used.
The contributions of the members of the XML Signature Working Group to the first edition specification are gratefully acknowledged: Mark Bartel, Adobe, was Accelio (Author); John Boyer, IBM (Author); Mariano P. Consens, University of Waterloo; John Cowan, Reuters Health; Donald Eastlake 3rd, Motorola; (Chair, Author/Editor); Barb Fox, Microsoft (Author); Christian Geuer-Pollmann, University Siegen; Tom Gindin, IBM; Phillip Hallam-Baker, VeriSign Inc; Richard Himes, US Courts; Merlin Hughes, Baltimore; Gregor Karlinger, IAIK TU Graz; Brian LaMacchia, Microsoft (Author); Peter Lipp, IAIK TU Graz; Joseph Reagle, NYU, was W3C (Chair, Author/Editor); Ed Simon, XMLsec (Author); David Solo, Citigroup (Author/Editor); Petteri Stenius, Capslock; Raghavan Srinivas, Sun; Kent Tamura, IBM; Winchel Todd Vincent III, GSU; Carl Wallace, Corsec Security, Inc.; Greg Whitehead, Signio Inc.
As are the first edition Last Call comments from the following:
The following members of the XML Security Specification Maintenance Working Group contributed to the second edition: Juan Carlos Cruellas, Universitat Politècnica de Catalunya; Pratik Datta, Oracle Corporation; Phillip Hallam-Baker, VeriSign, Inc.; Frederick Hirsch, Nokia, (Chair, Editor); Konrad Lanz, Applied Information processing and Kommunications (IAIK); Hal Lockhart, BEA Systems, Inc.; Robert Miller, MITRE Corporation; Sean Mullan, Sun Microsystems, Inc.; Bruce Rich, IBM Corporation; Thomas Roessler, W3C/ERCIM, (Staff contact, Editor); Ed Simon, W3C Invited Expert; Greg Whitehead, HP.
Contributions for version 1.1 were received from the members of the XML Security Working Group: Scott Cantor, Juan Carlos Cruellas, Pratik Datta, Gerald Edgar, Ken Graf, Phillip Hallam-Baker, Brad Hill, Frederick Hirsch (Chair, Editor), Brian LaMacchia, Konrad Lanz, Hal Lockhart, Cynthia Martin, Rob Miller, Sean Mullan, Shivaram Mysore, Magnus Nyström, Bruce Rich, Thomas Roessler, Ed Simon, Chris Solc, John Wray, Kelvin Yiu.
This section is non-normative.
This section provides an overview and examples of XML digital signature syntax. The specific processing is given in section 4. Processing Rules. The formal syntax is found in section 5. Core Signature Syntax and section 9. Additional Signature Syntax.
In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.
XML Signatures are applied to arbitrary digital content (data objects) via an
indirection. Data objects are digested, the resulting value is placed
in an element (with other information) and that element is then
digested and cryptographically signed. XML digital signatures are
represented by the Signature
element which has the
following structure (where "?" denotes zero or one occurrence; "+"
denotes one or more occurrences; and "*" denotes zero or more
occurrences):
<Signature ID?> <SignedInfo> <CanonicalizationMethod/> <SignatureMethod/> (<Reference URI? > (<Transforms>)? <DigestMethod> <DigestValue> </Reference>)+ </SignedInfo> <SignatureValue> (<KeyInfo>)? (<Object ID?>)* </Signature>
Signatures are related to data
objects via URIs [URI]. Within an XML document, signatures are
related to local data objects via fragment identifiers. Such local data
can be included within an enveloping signature or can enclose
an enveloped
signature. Detached
signatures are over external network resources or local data
objects that reside within the same XML document as sibling elements;
in this case, the signature is neither enveloping (signature is parent)
nor enveloped (signature is child). Since a Signature
element (and its Id
attribute value/name) may co-exist or
be combined with other elements (and their IDs) within a single XML
document, care should be taken in choosing names such that there are no
subsequent collisions that violate the ID uniqueness validity
constraint [XML10].
This is the same example
an as provided for
the XML Signature 1.x, but for
XML Signature 2.0. The only differences are
in the CanonicalizationMethod
and Reference
portions. The line numbers in this example match up with the line numbers
in the "Compatibility Mode" example.
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> [s05] <Reference> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/2010/xmldsig2#transform"> [s07a] <dsig2:Selection Algorithm="http://www.w3.org/2010/xmldsig2#xml" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#" URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126"> > [s07b] </dsig2:Selection> [s07c] <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/> [s07d] </Transform> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [s10] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s03]
In XML Signature 2.0 the Canonicalization Method URI
should be Canonical XML 2.0 (or a later version) and all the parameters
for Canonical XML 2.0 should be present as subelements of this element
[XML-C14N20].
[s05-s08]
Note XML Signature 2.0 does not use various
transforms, instead each reference object has two parts -
a dsig2:Selection
element to choose the data object to be signed, and
a Canonicalization
element to convert the data object to a canonicalized octet stream. To
fit in these two elements, without breaking backwards compatibility
with the 1.0 schema, these elements have been put inside a special Transform
with URI http://www.w3.org/2010/xmldsig2#transform
.
In XML Signature 2.0 the Transforms
element will contain only this
particular fixed Transform
.
[s05]
In XML Signature 2.0, the URI
attribute is omitted from the Reference
. Instead
it can be found in the dsig2:Selection
.
[s07a-s07b]
The dsig2:Selection
element
identifies the data object to be signed. This specification identifies only
two types, "xml" and "binary", but user specified types are also
possible. For example a new type "database-rows" could be defined to
select rows from a database for signing. Usually a URI and a few
other bits of information are used to identify the data object, but the
URI is not required; for example, the "xml" type can identify a local
document subset by using an XPath.
[s07c]
The CanonicalizationMethod
element provides the mechanism to convert the data object into a
canonicalized octet stream. This specification addresses only
canonicalization for xml data. Other forms of canonicalization can be
defined - e.g. a scheme for signing mime attachments could define a
canonicalization for mime headers and data. The output of the
canonicalization is digested.
The followed detailed example shows XML Signature 2.0 in the context of Web Services Security [WS-SECURITY11], showing how the SOAP body can be referenced using an Id in XML Signature 2.0. This example shows more detail than the previous Simple XML Signature 2.0 Example.
Note: This example (and the next example using XPath) show the use of XML Signature 2.0 in the context of Web Services Security. This is illustrative of how a 2.0 signature could be substituted for an 1.x Signature, but has not been standardized in Web Services Security so should only be considered illustrative.
[ i01 ] <?xml version="1.0" encoding="UTF-8"?> [ i02 ] <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"> [ i03 ] <soap:Header> [ i04 ] <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd"> [ i05 ] <wsse:BinarySecurityToken wsu:Id="MyID" [ i06 ] ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" [ i07 ] EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#Base64Binary"> [ i08 ] MIIEZzCCA9CgAwIBAgIQEmtJZc0.. [ i09 ] </wsse:BinarySecurityToken> [ i10 ] <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> [ i11 ] <ds:SignedInfo> [ i12 ] <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2" [ i13 ] xmlns:c14n2="http://www.w3.org/2010/xml-c14n2"> [ i14 ] <c14n2:IgnoreComments>true</c14n2:IgnoreComments> [ i15 ] <c14n2:TrimTextNodes>false</c14n2:TrimTextNodes> [ i16 ] <c14n2:PrefixRewrite>none</c14n2:PrefixRewrite> [ i17 ] <c14n2:QNameAware/> [ i18 ] </ds:CanonicalizationMethod> [ i19 ] <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> [ i20 ] <ds:Reference> [ i21 ] <ds:Transforms> [ i22 ] <ds:Transform Algorithm="http://www.w3.org/2010/xmldsig2#newTransformModel" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#"> [ i23 ] <dsig2:Selection Algorithm="http://www.w3.org/2010/xmldsig2#xml" URI="#MsgBody" /> [ i24 ] <dsig2:Canonicalization > [ i25 ] <c14n2:IgnoreComments>true</c14n2:IgnoreComments> [ i26 ] <c14n2:TrimTextNodes>true</c14n2:TrimTextNodes> [ i27 ] <c14n2:PrefixRewrite>sequential</c14n2:PrefixRewrite> [ i28 ] <c14n2:QNameAware/> [ i29 ] </dsig2:Canonicalization> [ i30 ] <dsig2:Verifications> [ i31 ] <dsig2:Verification DigestDataLength="308"/> [ i32 ] </dsig2:Verifications> [ i33 ] </ds:Transform> [ i34 ] </ds:Transforms> [ i35 ] <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [ i36 ] <ds:DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</ds:DigestValue> [ i37 ] </ds:Reference> [ i38 ] </ds:SignedInfo> [ i39 ] <ds:SignatureValue>kdutrEsAEw56Sefgs34...</ds:SignatureValue> [ i40 ] <ds:KeyInfo> [ i41 ] <ds:KeyValue> [ i42 ] <wsse:SecurityTokenReference> [ i43 ] <wsse:Reference URI="#MyID"/> [ i44 ] </wsse:SecurityTokenReference> [ i45 ] </ds:KeyValue> [ i46 ] </ds:KeyInfo> [ i47 ] </ds:Signature> [ i48 ] </wsse:Security> [ i49 ] </soap:Header> [ i50 ] <soap:Body wsu:Id="MsgBody"> [ i51 ] <ex:operation xmlns:ex="http://www.example.com/"> [ i52 ] <ex:param1>42</ex:param1> [ i53 ] <ex:param2>43</ex:param2> [ i54 ] </ex:operation> [ i55 ] </soap:Body> [ i56 ] </soap:Envelope>
[ i05-i09 ]
The wsse:BinarySecurityToken
is a Web Services Security
mechanism to convey key information needed for signature processing,
in this case an X.509v3 certificate.
[ i12-i18 ]
This example shows explicit choices for
parameters of the ds:CanonicalizationMethod
rather
than relying on implicit defaults. These canonicalization choices
are for the canonicalization of ds:SignedInfo
using
Canonical XML 2.0 [XML-C14N20].
[ i14 ]
The c14n2:IgnoreComments
parameter
is set to true
, the default, meaning that comments will
be ignored.
[ i15 ]
The c14n2:TrimTextNodes
parameter
is set to false
, so white space will be preserved.
[ i16 ]
The c14n2:PrefixRewrite
parameter
is set to none
, the default, meaning that
no prefixes will be rewritten.
[ i17 ]
The c14n2:QNameAware
parameter
is set to the empty set, the default, meaning that no QNames require
special processing.
[ i23 ]
The dsig2:Selection
URI
parameter
is set to #MsgBody
meaning that the element with the
corresponding Id (in this case wsu:Id
) will be
selected.
[ i24-i29 ]
The dsig2:Canonicalization
element again has parameters set explicitly
for ds:Reference
canonicalization.
[ i30-i33 ]
This example uses the new ability in XML
Signature 2.0 for a verifier to receive constraint information that
can be used to verify correctness of the information received, to
mitigate against attacks. The dsig2:Verifications
element contains this verification information. In this case the
length of the ds:Reference
data that was digested is conveyed.
[ i42-i44 ]
Web Services Security uses
its SecurityTokenReference
mechanism to reference key
information conveyed in tokens, such as an X.509 certificate. In
this example this mechanism is used to reference the binary security
token at using the
MyID
Id.
[ i50 ]
The soapBody
has
a wsu:Id
attribute which is used by
the ds:Reference
URI
attribute to
reference the element.
The followed detailed example shows use of XML Signature 2.0 in a Web
Services Security example similar to the
previous example using an
Id reference, but here uses an XPath expression to help
mitigate the possibility of wrapping attacks.
In this case the soap:Body
is signed, but
the ex:param2
is omitted from the signature.
This could correspond to a case where the
the first parameter is known to be
invariant end-end while the second parameter might be expected to
change as the SOAP message traverses SOAP intermediaries, so is omitted
from the signature.
[ p01 ] <?xml version="1.0" encoding="UTF-8"?> [ p02 ] <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ex="http://www.example.com/"> [ p03 ] <soap:Header> [ p04 ] <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" [ p05 ] xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"> [ p06 ] <wsse:BinarySecurityToken wsu:Id="MyID" [ p07 ] ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" [ p08 ] EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#Base64Binary"> [ p09 ] MIIEZzCCA9CgAwIBAgIQEmtJZc0.. [ p10 ] </wsse:BinarySecurityToken> [ p11 ] <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> [ p12 ] <ds:SignedInfo> [ p13 ] <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2" [ p14 ] xmlns:c14n2="http://www.w3.org/2010/xml-c14n2"> [ p15 ] <c14n2:IgnoreComments>true</c14n2:IgnoreComments> [ p16 ] <c14n2:TrimTextNodes>false</c14n2:TrimTextNodes> [ p17 ] <c14n2:PrefixRewrite>none</c14n2:PrefixRewrite> [ p18 ] <c14n2:QNameAware/> [ p19 ] </ds:CanonicalizationMethod> [ p20 ] <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> [ p21 ] <ds:Reference> [ p22 ] <ds:Transforms> [ p23 ] <ds:Transform Algorithm="http://www.w3.org/2010/xmldsig2#newTransformModel" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#"> [ p24 ] <dsig2:Selection Algorithm="http://www.w3.org/2010/xmldsig2#xml" URI=""> [ p25 ] <dsig2:IncludedXPath>/soap:Envelope/soap:Body[1]</dsig2:IncludedXPath> [ p26 ] <dsig2:ExcludedXPath> [ p27 ] /soap:Envelope/soap:Body[1]/ex:operation[1]/ex:param2[1] [ p28 ] </dsig2:ExcludedXPath> [ p29 ] </dsig2:Selection> [ p30 ] <dsig2:Canonicalization > [ p31 ] <c14n2:IgnoreComments>true</c14n2:IgnoreComments> [ p32 ] <c14n2:TrimTextNodes>true</c14n2:TrimTextNodes> [ p33 ] <c14n2:PrefixRewrite>sequential</c14n2:PrefixRewrite> [ p34 ] <c14n2:QNameAware/> [ p35 ] </dsig2:Canonicalization> [ p36 ] <dsig2:Verifications> [ p37 ] <dsig2:Verification DigestDataLength="169"/> [ p38 ] </dsig2:Verifications> [ p39 ] </ds:Transform> [ p40 ] </ds:Transforms> [ p41 ] <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [ p42 ] <ds:DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</ds:DigestValue> [ p43 ] </ds:Reference> [ p44 ] </ds:SignedInfo> [ p45 ] <ds:SignatureValue>kdutrEsAEw56Sefgs34...</ds:SignatureValue> [ p46 ] <ds:KeyInfo> [ p47 ] <ds:KeyValue> [ p48 ] <wsse:SecurityTokenReference> [ p49 ] <wsse:Reference URI="#MyID"/> [ p50 ] </wsse:SecurityTokenReference> [ p51 ] </ds:KeyValue> [ p52 ] </ds:KeyInfo> [ p53 ] </ds:Signature> [ p54 ] </wsse:Security> [ p55 ] </soap:Header> [ p56 ] <soap:Body> [ p57 ] <ex:operation> [ p58 ] <ex:param1>42</ex:param1> [ p59 ] <ex:param2>43</ex:param2> [ p60 ] </ex:operation> [ p61 ] </soap:Body> [ p62 ] </soap:Envelope>
[ p24 ]
In this case the URI
attribute
of the
Reference
element is ""
as XPath is used
rather than an Id based reference.
[ p25 ]
The dsig2:IncludedXPath
element
includes an XPath expression to reference the soap:Body
element. Note
that this expression is written to reference the
specific soap:Body
to mitigate wrapping attacks. The XPath expression is an XML Security
2.0 profile of
XPath 1.0 [XMLDSIG-XPATH].
[ p26 ]
The dsig2:ExcludedXPath
element
specifies that the ex:operation[1]/ex:param2[1]
child of
the soap:Body
not be included in the signature. The
XPath expression specifies the exact
instance to avoid wrapping attacks.
An implementation that conforms to this specification must be conformant to XML Signature 2.0 mode, and may be conformant to XML Signature 1.1 Compatibility Mode.
The following conformance requirements must be met by all implementations, including those in compatibility mode.
This section identifies algorithm conformance requirements applicable to both 2.0 and compatibility mode.
There is currently no consensus on mandatory to implement algorithms; the current draft text represents one possible outcome. Positions of some Working Group members against the currently expressed set of mandatory to implement algorithms include:
The opposing position is that, going forward, this specification needs to have credible algorithm agility for both hash and public-key algorithms: Should one set of algorithms prove weak, this would enable a quick switch-over. Therefore, there should be two mandatory to implement public-key algorithms from different families. At this time, elliptic curve based algorithms are the only credible contenders. They have the additional benefit of providing a reasonable balance between key sizes and security level. As profiles built on top of XML Signature that currently rely on DSA-SHA1 or RSA-SHA1 as the only supported signature algorithm will need to be updated in the future, the Signature core specification should outline a clear way forward in terms of choice of algorithms. This choice should be Elliptic Curve DSA.
Algorithms are identified by URIs that appear as an attribute to the
element that identifies the algorithms' role (DigestMethod
,
Transform
, SignatureMethod
, or CanonicalizationMethod
).
All algorithms used herein take parameters but in many cases the
parameters are implicit. For example, a SignatureMethod
is implicitly given two parameters: the keying info and the output of CanonicalizationMethod
.
Explicit additional parameters to an algorithm appear as content
elements within the algorithm role element. Such parameter elements
have a descriptive element name, which is frequently algorithm
specific, and must be in the XML Signature namespace or an algorithm
specific namespace.
This specification defines a set of algorithms, their URIs, and requirements for implementation. Requirements are specified over implementation, not over requirements for signature use. Furthermore, the mechanism is extensible; alternative algorithms may be used by signature applications.
*note: Note that
the same URI is used to identify base64 both in "encoding"
context (e.g. within the Object
element) as well as in
"transform" context (when identifying a base64
transform).
An implementation that conforms to this specification must support XML Signature 2.0 operation and conform to the following features when not operating in compatibility mode:
URI
attribute in a Reference
elementReference
element must have a single Transforms
element and
that element must contain exactly one Transform
element with an
Algorithm
of "http://www.w3.org/2010/xmldsig2#transform"
Reference
must be an octet stream
with the digest algorithm applied to the resulting data octetsRetrievalMethod
should not be used;
dsig11:KeyInfoReference
should be used instead.This section identifies algorithms used with the XML digital
signature specification. Entries contain the identifier to be used
in Signature
elements, a reference to the formal specification, and definitions,
where applicable, for the representation of keys and the results of
cryptographic operations.
Note that the algorithms required for 2.0 conformance are fewer than for compatibility mode, and that some algorithms required or optional are disallowed in 2.0.
An implementation that conforms to this specification may be conformant to Compatibility Mode. To conform to compatibility mode conformance with the following is required as well as conformance to common conformance requirements described in section 3.1 Common Conformance Requirements.
The following algorithm support is required for compatibility mode (in addition to those required for all modes).
**note: The Enveloped Signature transform removes the
Signature
element from the calculation of the signature when the
signature is within the content that it is being signed. This may be
implemented via the XPath specification specified in 6.6.4: Enveloped Signature Transform; it
must have the same effect as that specified by the
XPath Transform.
When using transforms, we RECOMMEND selecting the least expressive choice that still accomplishes the needs of the use case at hand: Use of XPath filter 2.0 is recommended over use of XPath filter. Use of XPath filter is recommended over use of XSLT.
Note: Implementation requirements for the XPath transform may be downgraded to optional in a future version of this specification.
The sections below describe the operations to be performed as part of signature generation and validation.
The required steps include the generation of Reference
elements and the SignatureValue
over SignedInfo
.
SignedInfo
element with SignatureMethod
,
CanonicalizationMethod
and Reference
(s).SignatureValue
over SignedInfo
based on algorithms specified in SignedInfo
.
For XML Signature 2.0 signatures (i.e. not XML Signature 1.x or
"Compatibility Mode" signatures), canonicalization in this step must
use a canonicalization
algorithm designated as compatible with XML Signature 2.0. This
canonicalization
algorithm should be the same as that used for Reference canonicalization.Signature
element that includes SignedInfo
,
Object
(s) (if desired, encoding may be different than
that used for signing), KeyInfo
(if required), and SignatureValue
.
Note, if the Signature
includes same-document
references, [XML10] or [XMLSCHEMA-1] ,[XMLSCHEMA-2] validation
of the document might introduce changes that break the signature.
Consequently, applications should be careful to consistently process
the document or refrain from using external contributions (e.g.,
defaults and entities).
For each Reference:
dsig2:Selection
.Canonicalization
to convert the data object
into an octet stream. This is not required for binary data.Reference
element, including the dsig2:Selection
element, Canonicalization
element, the digest algorithm and the DigestValue
.
(Note, it is the canonical form of these references that are signed in
section 4.1 Signature Generation and
validated in
section 4.4 Reference Check.)XML data objects must be canonicalized using Canonical XML 2.0 [XML-C14N20] or an alternative algorithm that is compliant with its interface.
The required steps of core validation include
KeyInfo
.
(Note in some environments, the signing key is implicitly known, and KeyInfo
is not used at all).Reference
to to see if the data
object matches with the expected data object.SignedInfo
.Reference
in SignedInfo
.Note, there may be valid signatures that some signature applications are unable to validate. Reasons for this include failure to implement optional parts of this specification, inability or unwillingness to execute specified algorithms, or inability or unwillingness to dereference specified URIs (some URI schemes may cause undesirable side effects), etc.
Comparison of each value in reference and signature validation is over the numeric (e.g., integer) or decoded octet sequence of the value. Different implementations may produce different encoded digest and signature values when processing the same resources because of variances in their encoding, such as accidental white space. But if one uses numeric or octet comparison (choose one) on both the stated and computed values these problems are eliminated.
The absence of arbitrary transforms makes reference checking simpler
in XML Signature 2.0. Implementations process the dsig2:Selection
in each Reference
to return a list of data objects that
are included in the signature. For example each reference in a
signature may point to a different part of the same document. The
signature implementation should return all these parts (possibly as DOM
elements) to the calling application, which can then compare them against
its policy to make sure what was expected to be signed is actually
signed.
Reference Validation is very similar to that in XML Signature 1.x,
except that
SignedInfo
need not be canonicalized, there are no arbitrary
transforms to execute, and there is an optional dsig2:Verifications
step.
Reference
in SignedInfo
:
dsig2:Selection
.
dsig2:Verification
element with
Type="http://www.w3.org/2010/xmldsig2#IDAttributes"
, then its content may
assist in obtaining the intended data object by identifying an ID attribute that
the verifier may not otherwise recognize.dsig2:Verification
element with
Type="http://www.w3.org/2010/xmldsig2#PositionAssertion"
, then the verifier
may confirm that the data object obtained is the same as that which would be obtained
by resolving the XPath expression in the PositionAssertion
attribute.Canonicalization
to compute an octet stream.
dsig2:Verification
element
with Type="http://www.w3.org/2010/xmldsig2#DigestDataLength"
, then
verify that the length of the octet stream computed above is the same as the length
specified in the DigestDataLength
attribute.DigestMethod
specified in its Reference
specification. The canonicalization
and digesting can be combined in one step for efficiency.DigestValue
in the SignedInfo
Reference
; if there is any mismatch,
validation fails.Signature Validation in XML Signature 2.0 is very similar to XML
Signature 1.x, except that KeyInfo
cannot contain any
transforms, and
the canonicalization of SignatureMethod
is not required.
These are the steps.
KeyInfo
or from an external source.CanonicalizationMethod
(which must be
Canonical XML 2.0 or an alternative algorithm that is compliant with its interface)
and use the result (and previously obtained KeyInfo
)
to confirm the SignatureValue
over the SignedInfo
element.The general structure of an XML Signature is described in section 2. Signature Overview and Examples. This section provides detailed syntax of the core signature features. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via an XML Schema [XMLSCHEMA-1][XMLSCHEMA-2] with the following XML preamble, declaration, and internal entity.
Schema Definition: <?xml version="1.0" encoding="utf-8"?> <!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd" [ <!ATTLIST schema xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#"> <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'> <!ENTITY % p ''> <!ENTITY % s ''> ]> <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" targetNamespace="http://www.w3.org/2000/09/xmldsig#" version="0.1" elementFormDefault="qualified">
Additional markup defined in version 1.1 of this specification uses
the dsig11:
namespace. The syntax is defined in an XML
schema with the following preamble:
<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd" [ <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'> <!ENTITY dsig11 'http://www.w3.org/2009/xmldsig11#'> <!ENTITY % p ''> <!ENTITY % s ''> ]> <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dsig11="http://www.w3.org/2009/xmldsig11#" targetNamespace="http://www.w3.org/2009/xmldsig11#" version="0.1" elementFormDefault="qualified">
Finally, markup defined by version 2.0 of this specification uses
the dsig2:
namespace. The syntax is defined in an XML
schema with the following preamble:
<?xml version="1.0" encoding="utf-8"?> <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dsig11="http://www.w3.org/2009/xmldsig11#" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#" targetNamespace="http://www.w3.org/2010/xmldsig2#" version="0.1" elementFormDefault="qualified">
Notwithstanding the presence of a mixed content model (via mixed="true" declarations) in the definitions of various elements that follow, use of mixed content in conjunction with any elements defined by this specification is not recommended.
When these elements are used in conjunction with XML Signature 2.0 signatures, mixed content must not be used.
ds:CryptoBinary
Simple TypeThis specification defines the ds:CryptoBinary
simple
type for representing arbitrary-length integers (e.g. "bignums") in XML
as octet strings. The integer value is first converted to a "big
endian" bitstring. The bitstring is then padded with leading zero bits
so that the total number of bits == 0 mod 8 (so that there are an
integral number of octets). If the bitstring contains entire leading
octets that are zero, these are removed (so the high-order octet is
always non-zero). This octet string is then base64 [RFC2045]
encoded. (The conversion from integer to octet string is equivalent to
IEEE 1363's I2OSP
[IEEE1363] with minimal length).
This type is used by "bignum" values such as RSAKeyValue
and DSAKeyValue
. If a value can be of type base64Binary
or ds:CryptoBinary
they are defined as base64Binary
.
For example, if the signature algorithm is RSA or DSA then SignatureValue
represents a bignum and would be ds:CryptoBinary
.
However, if HMAC-SHA1 is the signature algorithm then SignatureValue
could have leading zero octets that must be preserved. Thus SignatureValue
is generically defined as of type base64Binary
.
Schema Definition: <simpleType name="CryptoBinary"> <restriction base="base64Binary"> </restriction> </simpleType>
Signature
elementThe Signature
element is the root element of an XML
Signature. Implementation must generate laxly
schema valid [XMLSCHEMA-1][XMLSCHEMA-2] Signature
elements as specified by the following schema:
Schema Definition: <element name="Signature" type="ds:SignatureType"/> <complexType name="SignatureType"> <sequence> <element ref="ds:SignedInfo"/> <element ref="ds:SignatureValue"/> <element ref="ds:KeyInfo" minOccurs="0"/> <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
SignatureValue
ElementThe SignatureValue
element contains the actual value
of the digital signature; it is always encoded using base64
[RFC2045].
Schema Definition: <element name="SignatureValue" type="ds:SignatureValueType"/> <complexType name="SignatureValueType"> <simpleContent> <extension base="base64Binary"> <attribute name="Id" type="ID" use="optional"/> </extension> </simpleContent> </complexType>
SignedInfo
ElementThe structure of SignedInfo
includes a
canonicalization algorithm, a signature algorithm, and one or more
references. Given the importance of reference processing, this is
described separately in section 6. Referencing Content.
The SignedInfo
element may contain an
optional ID attribute allowing it to be referenced by other
signatures and objects.
SignedInfo
does not include explicit signature or
digest properties (such as calculation time, cryptographic device
serial number, etc.). If an application needs to associate properties
with the signature or digest, it may include such information in a SignatureProperties
element within an Object
element.
Schema Definition: <element name="SignedInfo" type="ds:SignedInfoType"/> <complexType name="SignedInfoType"> <sequence> <element ref="ds:CanonicalizationMethod"/> <element ref="ds:SignatureMethod"/> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
CanonicalizationMethod
ElementCanonicalizationMethod
is a required element that
specifies the canonicalization algorithm applied to the SignedInfo
element prior to performing signature calculations. This element uses
the general structure for algorithms described in
section 3.2.1 XML Signature 2.0 Algorithm Identifiers and Implementation Requirements. Implementations
must support the required canonicalization algorithms.
Schema Definition: <element name="CanonicalizationMethod" type="ds:CanonicalizationMethodType"/> <complexType name="CanonicalizationMethodType" mixed="true"> <sequence> <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
In XML Signature 2.0, the SignedInfo
element is presented as a
single subtree with no exclusions to the Canonicalization 2.0 [XML-C14N20]
algorithm. Parameters to that algorithm are represented as subelements of
the Canonicalization
element.
XML Signature 2.0 signatures use
the CanonicalizationMethod
element
to express the canonicalization of each Reference
.
SignatureMethod
ElementSignatureMethod
is a required element that specifies
the algorithm used for signature generation and validation. This
algorithm identifies all cryptographic functions involved in the
signature operation (e.g. hashing, public key algorithms, MACs,
padding, etc.). This element uses the general structure here for
algorithms described in section 3.2.1 XML Signature 2.0 Algorithm Identifiers and Implementation Requirements. While there is a
single identifier, that identifier may specify a format containing
multiple distinct signature values.
Schema Definition: <element name="SignatureMethod" type="ds:SignatureMethodType"/> <complexType name="SignatureMethodType" mixed="true"> <sequence> <element name="HMACOutputLength" minOccurs="0" type="ds:HMACOutputLengthType"/> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) external namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
The ds:HMACOutputLength
parameter is used for HMAC
[HMAC] algorithms. The
parameter specifies a truncation length in bits. If this parameter is
trusted without further
verification, then this can lead to a security bypass
[CVE-2009-0217]. Signatures must be deemed invalid if the truncation
length is below
the larger of (a) half the underlying hash algorithm's output length,
and (b) 80 bits.
Note that some implementations are known to not
accept truncation lengths that are lower than the underlying hash
algorithm's output length.
DigestMethod
ElementDigestMethod
is a required element that identifies the
digest algorithm to be applied to the signed object. This element uses
the general structure here for algorithms specified in section 3.2.1 XML Signature 2.0 Algorithm Identifiers and Implementation Requirements.
For "Compatibility Mode" signatures, if the result of the URI
dereference and application of Transforms
is an XPath
node-set (or sufficiently functional replacement implemented by the
application) then it must be converted as described in section B.4.1 The "Compatibility Mode" Reference Processing Model. If the
result of URI
dereference and application of Transforms
is an octet stream,
then no conversion occurs (comments might be present if Canonical XML
with Comments was specified in the Transforms
). The digest
algorithm is applied to the data octets of the resulting octet stream.
For XML Signature 2.0 signatures, the result of processing
the Reference
is an octet stream, and the digest algorithm is applied to the resulting data
octets.
Schema Definition: <element name="DigestMethod" type="ds:DigestMethodType"/> <complexType name="DigestMethodType" mixed="true"> <sequence> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
DigestValue
ElementDigestValue
is an element that contains the encoded value of the
digest. The digest is always encoded using base64 [RFC2045].
Schema Definition: <element name="DigestValue" type="ds:DigestValueType"/> <simpleType name="DigestValueType"> <restriction base="base64Binary"/> </simpleType>
The XML Signature 2.0 specification is designed to support a new, simplified processing model while remaining backwardly-compatible with the older 1.x processing model through optional support of a "Compatibility Mode" defined in a separate section of this document, section B. Compatibility Mode.
A generic signature processor can determine the mode of a signature
by examining the Reference
element's attributes and the child
element(s) of the Transforms
element (if any). If the
URI
attributes is present, "Compatibility Mode" can be assumed.
If the URI
attribute is not present, and the
Transforms
element contains exactly one Transform
element with an Algorithm
of "http://www.w3.org/2010/xmldsig2#transform"
,
then XML Signature 2.0 processing can be assumed. Otherwise,
"Compatibility Mode" is applied.
All the references of a signature should have the same mode; i.e. all XML Signature 2.0, or all "Compatibility Mode".
Reference
ElementReference
is an element that may occur one or more
times. It specifies a digest algorithm and digest value, and optionally
an identifier of the object being signed, the type of the object,
and/or a list of transforms to be applied prior to digesting. The
identification (URI) and transforms describe how the digested content
(i.e., the input to the digest method) was created. The Type
attribute facilitates the processing of referenced data. For example,
while this specification makes no requirements over external data, an
application may wish to signal that the referent is a Manifest
.
An optional ID attribute permits a Reference
to be
referenced from elsewhere.
Schema Definition: <element name="Reference" type="ds:ReferenceType"/> <complexType name="ReferenceType"> <sequence> <element ref="ds:Transforms" minOccurs="0"/> <element ref="ds:DigestMethod"/> <element ref="ds:DigestValue"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="URI" type="anyURI" use="optional"/> <attribute name="Type" type="anyURI" use="optional"/> </complexType>
URI
AttributeThe URI attribute must be omitted for XML Signature 2.0 signatures.
Transforms
ElementEach Reference
must contain the Transforms
element,
and this must contain one and only one Transform
element with an
Algorithm
of
"http://www.w3.org/2010/xmldsig2#transform"
. This signals
the 2.0 syntax and processing (Compatibility mode transforms are
described in section B.5 "Compatibility Mode" Transforms and Processing Model).
Schema Definition: <element name="Transforms" type="ds:TransformsType"/> <complexType name="TransformsType"> <sequence> <element ref="ds:Transform" maxOccurs="unbounded"/> </sequence> </complexType> <element name="Transform" type="ds:TransformType"/> <complexType name="TransformType" mixed="true"> <choice minOccurs="0" maxOccurs="unbounded"> <any namespace="##other" processContents="lax"/> <!-- (1,1) elements from (0,unbounded) namespaces --> <element name="XPath" type="string"/> </choice> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
The semantics of the Transform
element in XML
Signature 2.0 is
that its input is determined solely from within the Transform
itself rather than via the surrounding Reference
. The output
is guaranteed to be an octet stream.
The detailed definition of the XML Signature 2.0 Transform
algorithm definitions can be found in
section 10.5 The Transform Algorithm.
A difference from XML Signature 1.x (and the corresponding
"Compatibility Mode") is that the use of
extensible Transform
algorithms is
replaced with an extensible syntax for reference
and selection processing. This construct is modeled as a fixed Transform,
for compatibility with the original schema, and to ensure predictable failure
modes for older implementations.
Legacy implementations should react to this as an undefined Transform
and report failure in the fashion that is normal for them in such a case.
dsig2:Selection
ElementThe dsig2:Selection
element describes the data being signed
for a "2.0 Mode" signature Reference
. The content and processing
model for this element depends on the value of the required Algorithm
attribute, which identifies the selection algorithm/syntax in use. The required
URI
attribute and any child elements are passed to that algorithm
as parameters to selection processing.
Schema Definition: <xs:element name="Selection" type="dsig2:SelectionType"/> <xs:complexType name="dsig2:SelectionType"> <xs:sequence> <xs:any namespace="##any" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> <xs:attribute name="URI" type="xs:anyURI" use="required"/> <xs:attribute name="Algorithm" type="xs:anyURI" use="required"/> </xs:complexType>
The Algorithm
attribute is an extensibility point
enabling application-specific content selection approaches. Each
Algorithm
must define the parameters expected, how
they are expressed within the dsig2:Selection
element,
how to process the selection, what user-defined object the selection
produces, and what canonicalization algorithm(s) to allow for unambiguous
conversation of the data into an octet stream.
The result of processing the dsig2:Selection
element
must be one of the following:
In the first case, the current Signature
node is implicitly
added as an exclusion, and then a "2.0 Mode" canonicalization algorithm
(one compatible with these inputs) must be applied to produce an octet stream
for the digest algorithm. The contents of the sibling CanonicalizationMethod
element, if present, will specify the algorithm to use, and supply any non-default
parameters to that algorithm. If no sibling CanonicalizationMethod
element is present, then the XML Canonicalization 2.0 Algorithm [XML-C14N20]
must be applied with no non-default parameters.
For an octet stream, no further processing is applied, and the octets are supplied directly to the digest algorithm.
For a user-defined object (the result of a user-defined selection process), processing is subject to the definition of that process.
Signatures in "2.0 Mode" do not deal with XML content to be signed in terms of an XPath nodeset. Instead, the following interface is used:
dsig2:Verifications
Elementdsig2:Verifications
is an optional element containing information
that aids in signature verification. It contains one or more dsig2:Verification
elements identifying the type(s) of verification information available.
Use of the dsig2:Verifications
element by validators is optional,
even if the element is present. For example, validators may ignore a
dsig2:Verification
element of Type
"http://www.w3.org/2010/xmldsig2#PositionAssertion"
,
and rely on ID-based referencing (with the risk of being vulnerable
to signature wrapping attacks unless other steps are taken) for simplicity.
Schema Definition: <xs:element name="Verifications" type="dsig2:VerificationsType"/> <xs:complexType name="VerificationsType"> <xs:sequence> <xs:element ref="dsig2:Verification" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> <xs:element name="Verification" type="dsig2:VerificationType"/> <xs:complexType name="VerificationType"> <xs:choice minOccurs="0" maxOccurs="unbounded"> <xs:any namespace="##other" processContents="lax"/> <xs:element ref="dsig2:QualifiedAttr"/> <xs:element ref="dsig2:UnqualifiedAttr"/> <xs:/choice> <xs:attribute name="Type" type="xs:anyURI" use="required"/> <xs:attribute name="DigestDataLength" type="xs:nonNegativeInteger"/> <xs:attribute name="PositionAssertion" type="xs:string"/> <xs:anyAttribute namespace="##other" processContents="lax"/> <xs:/complexType>
KeyInfo
ElementKeyInfo
is an optional element that enables the
recipient(s) to obtain the key needed to validate the
signature. KeyInfo
may contain keys, names, certificates and other public key management
information, such as in-band key distribution or key agreement data.
This specification defines a few simple types but applications may
extend those types or all together replace them with their own key
identification and exchange semantics using the XML namespace facility
[XML-NAMES]. However, questions of trust of such key information
(e.g., its authenticity or strength) are out of scope of this
specification and left to the application.
Details of the structure and usage of element children
of KeyInfo
other than
simple types described in this specification are out of scope. For
example, the definition of PKI certificate contents, certificate ordering,
certificate revocation and CRL management are out of scope.
If KeyInfo
is omitted, the recipient is expected to be
able to identify the key based on application context. Multiple
declarations within KeyInfo
refer to the same key. While
applications may define and use any mechanism they choose through
inclusion of elements from a different namespace, compliant versions
must implement KeyValue
(section 7.2 The KeyValue Element) and
should implement KeyInfoReference
(section 7.10 The dsig11:KeyInfoReference Element).
KeyInfoReference
is preferred over use of
RetrievalMethod
as it avoids use of
Transform
child elements that
introduce security risk and implementation challenges. Support for
other children of KeyInfo
is optional.
The schema specification of many of KeyInfo
's children
(e.g., PGPData
, SPKIData
, X509Data
)
permit their content to be extended/complemented with elements from
another namespace. This may be done only if it is safe to ignore these
extension elements while claiming support for the types defined in this
specification. Otherwise, external elements, including alternative
structures to those defined by this specification, must be a child of KeyInfo
.
For example, should a complete XML-PGP standard be defined, its root
element must be a child of KeyInfo
. (Of course, new
structures from external namespaces can incorporate elements from the dsig:
namespace via features of the type definition language. For instance,
they can create a schema that permits, includes, imports, or derives
new types based on dsig:
elements.)
The following list summarizes the KeyInfo
types that
are allocated an identifier in the dsig:
namespace; these
can be used within the RetrievalMethod
Type
attribute to describe a remote KeyInfo
structure.
The following list summarizes the additional KeyInfo
types that are allocated an identifier in the dsig11:
namespace.
In addition to the types above for which we define an XML structure, we specify one additional type to indicate a binary (ASN.1 DER) X.509 Certificate.
Schema Definition: <element name="KeyInfo" type="ds:KeyInfoType"/> <complexType name="KeyInfoType" mixed="true"> <choice maxOccurs="unbounded"> <element ref="ds:KeyName"/> <element ref="ds:KeyValue"/> <element ref="ds:RetrievalMethod"/> <element ref="ds:X509Data"/> <element ref="ds:PGPData"/> <element ref="ds:SPKIData"/> <element ref="ds:MgmtData"/> <!-- <element ref="dsig11:DEREncodedKeyValue"/> --> <!-- DEREncodedKeyValue (XMLDsig 1.1) will use the any element --> <!-- <element ref="dsig11:KeyInfoReference"/> --> <!-- KeyInfoReference (XMLDsig 1.1) will use the any element --> <!-- <element ref="xenc:EncryptedKey"/> --> <!-- EncryptedKey (XMLEnc) will use the any element --> <!-- <element ref="xenc:Agreement"/> --> <!-- Agreement (XMLEnc) will use the any element --> <any processContents="lax" namespace="##other"/> <!-- (1,1) elements from (0,unbounded) namespaces --> </choice> <attribute name="Id" type="ID" use="optional"/> </complexType>
KeyName
ElementThe KeyName
element contains a string value (in which
white space is significant) which may be used by the signer to
communicate a key identifier to the recipient. Typically, KeyName
contains an identifier related to the key pair used to sign the
message, but it may contain other protocol-related information that
indirectly identifies a key pair. (Common uses of KeyName
include simple string names for keys, a key index, a distinguished name
(DN), an email address, etc.)
Schema Definition: <element name="KeyName" type="string"/>
KeyValue
ElementThe KeyValue
element contains a single public key that
may be useful in validating the signature. Structured formats for
defining DSA (required), RSA (required) and ECDSA (required) public
keys are defined in section 10.3 Signature Algorithms. The KeyValue
element may
include externally defined public keys values represented as PCDATA or
element types from an external namespace.
Schema Definition: <element name="KeyValue" type="ds:KeyValueType"/> <complexType name="KeyValueType" mixed="true"> <choice> <element ref="ds:DSAKeyValue"/> <element ref="ds:RSAKeyValue"/> <!-- <element ref="dsig11:ECKeyValue"/> --> <!-- ECC keys (XMLDsig 1.1) will use the any element --> <any namespace="##other" processContents="lax"/> </choice> </complexType>
DSAKeyValue
ElementType="http://www.w3.org/2000/09/xmldsig#DSAKeyValue"
(this can be used within a RetrievalMethod
or
Reference
element to identify the referent's type)DSA keys and the DSA signature algorithm are specified in [FIPS-186-3]. DSA public key values can have the following fields:
P
Q
G
Y
J
seed
pgenCounter
Parameter J is available for inclusion solely for efficiency as it
can be calculated from P and Q. Parameters seed and pgenCounter are used
in the DSA prime number generation algorithm specified in
[FIPS-186-3]. As such, they are optional but must either both be
present or both be absent. This prime generation algorithm is designed
to provide assurance that a weak prime is not being used and it yields
a P and Q value. Parameters P, Q, and G can be public and common to a
group of users. They might be known from application context. As such,
they are optional but P and Q must either both appear or both be
absent. If all of P
, Q
, seed
,
and pgenCounter
are present, implementations are not
required to check if they are consistent and are free to use either P
and Q
or seed
and pgenCounter
.
All parameters are encoded as base64
[RFC2045] values.
Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are
represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema Definition:
<element name="DSAKeyValue" type="ds:DSAKeyValueType"/>
<complexType name="DSAKeyValueType">
<sequence>
<sequence minOccurs="0">
<element name="P" type="ds:CryptoBinary"/>
<element name="Q" type="ds:CryptoBinary"/>
</sequence>
<element name="G" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Y" type="ds:CryptoBinary"/>
<element name="J" type="ds:CryptoBinary" minOccurs="0"/>
<sequence minOccurs="0">
<element name="Seed" type="ds:CryptoBinary"/>
<element name="PgenCounter" type="ds:CryptoBinary"/>
</sequence>
</sequence>
</complexType>
RSAKeyValue
ElementType="http://www.w3.org/2000/09/xmldsig#RSAKeyValue"
(this can be used within a RetrievalMethod
or
Reference
element to identify the referent's type)RSA key values have two fields: Modulus and Exponent.
<RSAKeyValue> <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV 5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U= </Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue>
Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are
represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema Definition:
<element name="RSAKeyValue" type="ds:RSAKeyValueType"/>
<complexType name="RSAKeyValueType">
<sequence>
<element name="Modulus" type="ds:CryptoBinary"/>
<element name="Exponent" type="ds:CryptoBinary"/>
</sequence>
</complexType>
dsig11:ECKeyValue
ElementType="
http://www.w3.org/2009/xmldsig11#ECKeyValue"
(this can be used within a RetrievalMethod
or Reference
element to identify the referent's type)The dsig11:ECKeyValue
element is defined in the
http://www.w3.org/2009/xmldsig11# namespace.
EC public key values consists of two sub components: Domain
parameters and dsig11:PublicKey
.
<ECKeyValue xmlns="http://www.w3.org/2009/xmldsig11#"> <NamedCurve URI="urn:oid:1.2.840.10045.3.1.7" /> <PublicKey> vWccUP6Jp3pcaMCGIcAh3YOev4gaa2ukOANC7Ufg Cf8KDO7AtTOsGJK7/TA8IC3vZoCy9I5oPjRhyTBulBnj7Y </PublicKey> </ECKeyValue>
Note - A line break has been added to the dsig11:PublicKey
content to preserve printed page width.
Domain parameters can be encoded explicitly using the
dsig11:ECParameters
element or by reference using the dsig11:NamedCurve
element. A named
curve is specified through the URI
attribute. For named
curves that are identified by OIDs, such as those defined in
[RFC3279] and [RFC4055], the OID should be encoded
according to [URN-OID]. Conformant applications must support the
dsig11:NamedCurve
element and the 256-bit prime field curve as identified by
the OID 1.2.840.10045.3.1.7
.
The dsig11:PublicKey
element contains the base64 encoding of
a binary representation of the x and y coordinates of the point. Its value
is computed as follows:
Schema Definition:
<!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->
<element name="ECKeyValue" type="dsig11:ECKeyValueType"/>
<complexType name="ECKeyValueType">
<sequence>
<choice>
<element name="ECParameters" type="dsig11:ECParametersType"/>
<element name="NamedCurve" type="dsig11:NamedCurveType"/>
</choice>
<element name="PublicKey" type="dsig11:ECPointType"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
<complexType name="NamedCurveType">
<attribute name="URI" type="anyURI" use="required"/>
</complexType>
<simpleType name="ECPointType">
<restriction base="ds:CryptoBinary"/>
</simpleType>
The dsig11:ECParameters
element consists of the following
subelements. Note these definitions are based on the those described in [RFC3279].
dsig11:FieldID
element identifies the finite field over which the
elliptic curve is defined. Additional details on the structures for
defining prime and characteristic two fields is provided below.dsig11:Curve
element specifies the coefficients a
and b of the elliptic
curve E. Each coefficient is first converted from a field
element to an
octet string as specified in section 6.2 of [ECC-ALGS], then
the resultant octet string is encoded in
base64.dsig11:Base
element specifies the base point P on the elliptic
curve. The base point is represented as a value of type dsig11:ECPointType
.dsig11:Order
element specifies the order n of the base point and is
encoded as a positiveInteger
.dsig11:Cofactor
element is an optional element that specifies the
integer h = #E(Fq)/n. The cofactor is not required to support ECDSA,
except in parameter validation. The cofactor may be included to support
parameter validation for ECDSA keys. Parameter validation is not
required by this specification. The cofactor is required in ECDH public
key parameters.dsig11:ValidationData
element is an optional element that
specifies the hash algorithm used to generate the elliptic curve E
and the base point G verifiably at random. It also specifies the
seed that was used to generate the curve and the base point.
Schema Definition:
<!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->
<complexType name="ECParametersType">
<sequence>
<element name="FieldID" type="dsig11:FieldIDType"/>
<element name="Curve" type="dsig11:CurveType"/>
<element name="Base" type="dsig11:ECPointType"/>
<element name="Order" type="ds:CryptoBinary"/>
<element name="CoFactor" type="integer" minOccurs="0"/>
<element name="ValidationData" type="dsig11:ECValidationDataType" minOccurs="0"/>
</sequence>
</complexType>
<complexType name="FieldIDType">
<choice>
<element ref="dsig11:Prime"/>
<element ref="dsig11:TnB"/>
<element ref="dsig11:PnB"/>
<element ref="dsig11:GnB"/>
<any namespace="##other" processContents="lax"/>
</choice>
</complexType>
<complexType name="CurveType">
<sequence>
<element name="A" type="ds:CryptoBinary"/>
<element name="B" type="ds:CryptoBinary"/>
</sequence>
</complexType>
<complexType name="ECValidationDataType">
<sequence>
<element name="seed" type="ds:CryptoBinary"/>
</sequence>
<attribute name="hashAlgorithm" type="anyURI" use="required"/>
</complexType>
dsig11:Prime
fields are described by a single subelement dsig11:P
,
which represents the field size in bits. It is encoded as a positiveInteger
.
Schema Definition:
<!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->
<element name="Prime" type="dsig11:PrimeFieldParamsType"/>
<complexType name="PrimeFieldParamsType">
<sequence>
<element name="P" type="ds:CryptoBinary"/>
</sequence>
</complexType>
Structures are defined for three types of characteristic two fields: gaussian normal basis, pentanomial basis and trinomial basis.
Schema Definition:
<!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->
<element name="GnB" type="dsig11:CharTwoFieldParamsType"/>
<complexType name="CharTwoFieldParamsType">
<sequence>
<element name="M" type="positiveInteger"/>
</sequence>
</complexType>
<element name="TnB" type="dsig11:TnBFieldParamsType"/>
<complexType name="TnBFieldParamsType">
<complexContent>
<extension base="dsig11:CharTwoFieldParamsType">
<sequence>
<element name="K" type="positiveInteger"/>
</sequence>
</extension>
</complexContent>
</complexType>
<element name="PnB" type="dsig11:PnBFieldParamsType"/>
<complexType name="PnBFieldParamsType">
<complexContent>
<extension base="dsig11:CharTwoFieldParamsType">
<sequence>
<element name="K1" type="positiveInteger"/>
<element name="K2" type="positiveInteger"/>
<element name="K3" type="positiveInteger"/>
</sequence>
</extension>
</complexContent>
</complexType>
Implementations that need to support the [RFC4050] format for ECDSA keys can avoid known interoperability problems with that specification by adhering to the following profile:
ECDSAKeyValue
element against
the [RFC4050] schema. XML Schema validators may not support integer
types with decimal data exceeding 18 decimal digits.
[XMLSCHEMA-1][XMLSCHEMA-2].NamedCurve
element.urn:oid:1.2.840.10045.3.1.7
.The following is an example of a ECDSAKeyValue
element
that meets the profile described in this section.
<ECDSAKeyValue xmlns="http://www.w3.org/2001/04/xmldsig-more#"> <DomainParameters> <NamedCurve URN="urn:oid:1.2.840.10045.3.1.7" /> </DomainParameters> <PublicKey> <X Value="5851106065380174439324917904648283332 0204931884267326155134056258624064349885"> <Y Value="1024033521368277752409102672177795083 59028642524881540878079119895764161434936"> </PublicKey> </ECDSAKeyValue>
Note - A line break has been added to the X
and Y
Value
attribute values to preserve
printed page width.
RetrievalMethod
ElementA RetrievalMethod
element within KeyInfo
is used to convey a reference to KeyInfo
information that
is stored at another location. For example, several signatures in a
document might use a key verified by an X.509v3 certificate chain
appearing once in the document or remotely outside the document; each
signature's KeyInfo
can reference this chain using a
single RetrievalMethod
element instead of including the
entire chain with a sequence of X509Certificate
elements.
RetrievalMethod
uses the same syntax and dereferencing
behavior as section B.4 The URI Attribute in "Compatibility Mode"
and section B.4.1 The "Compatibility Mode" Reference Processing Model
except that there are
no DigestMethod
or DigestValue
child
elements and presence of the URI
attribute is mandatory.
Type
is an optional identifier for the type of data
retrieved after all transforms have been applied. The result of
dereferencing a RetrievalMethod
Reference
for all KeyInfo
types defined by
this specification (section 7. The KeyInfo Element)
with a corresponding XML structure
is an XML element or document with that element as the root. The
rawX509Certificate
KeyInfo
(for which there is no
XML structure) returns a binary X509 certificate.
Note that when referencing one of the
defined KeyInfo
types within the same document, or some
remote documents, at
least one Transform
is required to turn an ID-based
reference to a KeyInfo
element into a child element
located inside it. This is due to the lack of
an XML ID attribute on the defined KeyInfo
types.
Transforms in RetrievalMethod
are more attack prone,
since they need to be evaluated in the first step of the
signature validation, where the trust in the key has not yet been
established, and the SignedInfo
has not yet been
verified. As noted in the [XMLDSIG-BESTPRACTICES] an attacker can easily
causes a Denial of service, by adding a specially crafted transform in
the RetrievalMethod
without even bothering to have the
key validate or the signature match.
Note:
The KeyInfoReference
element is preferred over use of
RetrievalMethod
as it avoids use
of Transform
child elements that
introduce security risk and implementation challenges.
Schema Definition <element name="RetrievalMethod" type="ds:RetrievalMethodType"/> <complexType name="RetrievalMethodType"> <sequence> <element ref="ds:Transforms" minOccurs="0"/> </sequence> <attribute name="URI" type="anyURI"/> <attribute name="Type" type="anyURI" use="optional"/> </complexType>
Note: The schema for the URI
attribute of RetrievalMethod erroneously omitted the attribute: use="required"
.
However, this error only results in a more lax schema which permits all
valid RetrievalMethod
elements. Because the existing
schema is embedded in many applications, which may include the schema
in their signatures, the schema has not been corrected to be more
restrictive.
X509Data
ElementType="http://www.w3.org/2000/09/xmldsig#X509Data
"RetrievalMethod
or
Reference
element to identify the referent's type)An X509Data
element within KeyInfo
contains one or more identifiers of keys or X509 certificates (or
certificates' identifiers or a revocation list). The content of
X509Data
is at least one element, from the following
set of element types; any of these may appear together or more than
once iff (if and only if) each instance describes or is related to
the same certificate:
X509IssuerSerial
element, which contains an X.509
issuer distinguished name/serial number pair. The distinguished name
should be represented as a string that complies with section 3 of
RFC4514 [LDAP-DN], to be generated according to the
Distinguished Name Encoding Rules
section below,X509SubjectName
element, which contains an X.509
subject distinguished name that should be represented as a string that
complies with section 3 of RFC4514 [LDAP-DN], to be generated according to the
Distinguished Name Encoding Rules
section below,X509SKI
element, which contains the base64 encoded
plain (i.e. non-DER-encoded) value of a X509 V.3 SubjectKeyIdentifier
extension,X509Certificate
element, which contains a
base64-encoded [X509V3] certificate, andX509CRL
element, which contains a base64-encoded
certificate revocation list (CRL) [X509V3].dsig11:X509Digest
element contains a base64-encoded
digest of a certificate. The digest algorithm URI is identified with a
required Algorithm
attribute. The input to the digest must
be the raw octets that would be base64-encoded were the same certificate
to appear in the X509Certificate element.dsig11:OCSPResponse
element contains a base64-encoded OCSP response in
DER encoding. [OCSP].Any X509IssuerSerial
, X509SKI
, X509SubjectName
,
and dsig11:X509Digest
elements that appear must refer to the
certificate or certificates containing the validation key. All such elements
that refer to a particular individual certificate must be grouped inside a
single X509Data
element and if the certificate to which they refer
appears, it must also be in that X509Data
element.
Any X509IssuerSerial
, X509SKI
, X509SubjectName
,
and dsig11:X509Digest
elements that relate to the same key but
different certificates must be grouped within a single KeyInfo
but may occur in multiple X509Data
elements.
Note that if X509Data
child elements are used to identify a
trusted certificate (rather than solely as an untrusted hint supplemented by
validation by policy), the complete set of such elements that are intended to
identify a certificate should be integrity protected, typically by signing an
entire X509Data
or KeyInfo
element.
All certificates appearing in an X509Data
element must relate
to the validation key by either containing it or being part of a certification
chain that terminates in a certificate containing the validation key.
No ordering is implied by the above constraints. The comments in the following instance demonstrate these constraints:
<KeyInfo> <X509Data> <!-- two pointers to certificate-A --> <X509IssuerSerial> <X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM, L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName> <X509SerialNumber>12345678</X509SerialNumber> </X509IssuerSerial> <X509SKI>31d97bd7</X509SKI> </X509Data> <X509Data><!-- single pointer to certificate-B --> <X509SubjectName>Subject of Certificate B</X509SubjectName> </X509Data> <X509Data> <!-- certificate chain --> <!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4--> <X509Certificate>MIICXTCCA..</X509Certificate> <!-- Intermediate cert subject CN=arbolCA,OU=FVT,O=IBM,C=US issuer CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US --> <X509Certificate>MIICPzCCA...</X509Certificate> <!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US --> <X509Certificate>MIICSTCCA...</X509Certificate> </X509Data> </KeyInfo>
Note, there is no direct provision for a PKCS#7 encoded "bag" of
certificates or CRLs. However, a set of certificates and CRLs can occur
within an X509Data
element and multiple X509Data
elements can occur in a KeyInfo
. Whenever multiple
certificates occur in an X509Data
element, at least one
such certificate must contain the public key which verifies the
signature.
While in principle many certificate encodings are possible, it is
recommended that certificates appearing in an
X509Certificate
element be limited to an encoding of BER
or its DER subset, allowing that within the certificate other content
may be present. The use of other encodings may lead to interoperability
issues. In any case, XML Signature implementations should not alter or
re-encode certificates, as doing so could invalidate their signatures.
Deployments that expect to make use of the X509IssuerSerial
element should
be aware that many Certificate Authorities issue certificates with large,
random serial numbers. XML Schema validators may not support integer types
with decimal data exceeding 18 decimal digits [XML-schema]. Therefore such
deployments should avoid schema-validating the X509IssuerSerial
element, or
make use of a local copy of the schema that adjusts the data type of the
X509SerialNumber
child element from "integer"
to "string"
.
To encode a distinguished name (X509IssuerSerial
,X509SubjectName
,
and KeyName
if appropriate), the encoding rules in
section 2 of RFC 4514 [LDAP-DN] should be applied, except that the
character escaping rules in section 2.4 of RFC 4514 [LDAP-DN] may be
augmented as follows:
Since an XML document logically consists of characters, not octets, the resulting Unicode string is finally encoded according to the character encoding used for producing the physical representation of the XML document.
Schema Definition
<element name="X509Data" type="ds:X509DataType"/>
<complexType name="X509DataType">
<sequence maxOccurs="unbounded">
<choice>
<element name="X509IssuerSerial" type="ds:X509IssuerSerialType"/>
<element name="X509SKI" type="base64Binary"/>
<element name="X509SubjectName" type="string"/>
<element name="X509Certificate" type="base64Binary"/>
<element name="X509CRL" type="base64Binary"/>
<!-- <element ref="dsig11:OCSPResponse"/> -->
<!-- <element ref="dsig11:X509Digest"/> -->
<!-- OCSPResponse and X509Digest elements (XMLDsig 1.1) will use the any element -->
<any namespace="##other" processContents="lax"/>
</choice>
</sequence>
</complexType>
<complexType name="X509IssuerSerialType">
<sequence>
<element name="X509IssuerName" type="string"/>
<element name="X509SerialNumber" type="integer"/>
</sequence>
</complexType>
<!-- Note, this schema permits X509Data
to be empty; this is
precluded by the text in section 7. The KeyInfo Element which states
that at least one element from the dsig namespace should be present
in the PGP, SPKI, and X509 structures. This is easily expressed for
the other key types, but not for X509Data because of its rich
structure. -->
<!-- targetNameSpace="http://www.w3.org/2009/xmldsig11#" --> <element name="OCSPResponse" type="base64Binary" /> <element name="X509Digest" type="dsig11:X509DigestType"/> <complexType name="X509DigestType"> <simpleContent> <extension base="base64Binary"> <attribute name="Algorithm" type="anyURI" use="required"/> </extension> </simpleContent> </complexType>
PGPData
ElementType="http://www.w3.org/2000/09/xmldsig#PGPData
"
(this can be used within a RetrievalMethod
or Reference
element to identify the referent's type)The PGPData
element within KeyInfo
is
used to convey information related to PGP public key pairs and
signatures on such keys. The PGPKeyID
's value is a
base64Binary sequence containing a standard PGP public key identifier
as defined in [PGP] section 11.2]. The PGPKeyPacket
contains a base64-encoded Key Material Packet as defined in [PGP]
section 5.5]. These children element types can be complemented/extended
by siblings from an external namespace within PGPData
, or
PGPData
can be replaced all together with an alternative
PGP XML structure as a child of KeyInfo
. PGPData
must contain one PGPKeyID
and/or one PGPKeyPacket
and 0 or more elements from an external namespace.
Schema Definition: <element name="PGPData" type="ds:PGPDataType"/> <complexType name="PGPDataType"> <choice> <sequence> <element name="PGPKeyID" type="base64Binary"/> <element name="PGPKeyPacket" type="base64Binary" minOccurs="0"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> <sequence> <element name="PGPKeyPacket" type="base64Binary"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> </choice> </complexType>
SPKIData
ElementType="http://www.w3.org/2000/09/xmldsig#SPKIData
"
(this can be used within a RetrievalMethod
or Reference
element to identify the referent's type)The SPKIData
element within KeyInfo
is
used to convey information related to SPKI public key pairs,
certificates and other SPKI data. SPKISexp
is the base64
encoding of a SPKI canonical S-expression. SPKIData
must
have at least one SPKISexp
; SPKISexp
can be
complemented/extended by siblings from an external namespace within SPKIData
,
or SPKIData
can be entirely replaced with an alternative
SPKI XML structure as a child of KeyInfo
.
Schema Definition: <element name="SPKIData" type="ds:SPKIDataType"/> <complexType name="SPKIDataType"> <sequence maxOccurs="unbounded"> <element name="SPKISexp" type="base64Binary"/> <any namespace="##other" processContents="lax" minOccurs="0"/> </sequence> </complexType>
MgmtData
ElementType="http://www.w3.org/2000/09/xmldsig#MgmtData
"
(this can be used within a RetrievalMethod
or Reference
element to identify the referent's type)MgmtData
element within KeyInfo
is a
string value used to convey in-band key distribution or agreement data.
However, use of this element is not recommended and should not be
used. The
section 7.8 XML Encryption EncryptedKey
and DerivedKey Elements describes
new KeyInfo
types for conveying key information.
Schema Definition: <element name="MgmtData" type="string"/>
EncryptedKey
and DerivedKey
Elements<xenc:EncryptedKey>
and <xenc:DerivedKey>
elements defined in
[XMLENC-CORE1] as children of ds:KeyInfo
can be used
to convey in-band encrypted or derived key material. In particular, the
xenc:DerivedKey
> element may be present when the key used in
calculating a Message Authentication Code is derived from a shared
secret.
dsig11:DEREncodedKeyValue
ElementType="http://www.w3.org/2009/xmldsig11#DEREncodedKeyValue"
(this can be used within a RetrievalMethod
or Reference
element to identify the referent's type) The public key algorithm and value are DER-encoded in accordance with the value that would be used in the Subject Public Key Info field of an X.509 certificate, per section 4.1.2.7 of [RFC5280]. The DER-encoded value is then base64-encoded.
For the key value types supported in this specification, refer to the following for normative references on the format of Subject Public Key Info and the relevant OID values that identify the key/algorithm type:
Specifications that define additional key types should provide such a normative reference for their own key types where possible.
Schema Definition: <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" --> <element name="DEREncodedKeyValue" type="dsig11:DEREncodedKeyValueType"/> <complexType name="DEREncodedKeyValueType"> <simpleContent> <extension base="base64Binary"> <attribute name="Id" type="ID" use="optional"/> </extension> </simpleContent> </complexType>
Historical note: The dsig11:DEREncodedKeyValue
element was added
to XML Signature 1.1 in order to support certain interoperability
scenarios where at least one of signer and/or verifier are not able to
serialize keys in the XML formats described in
section 7.2 The KeyValue Element
above. The KeyValue
element is to be used for
"bare" XML key
representations (not XML wrappings around other binary encodings like
ASN.1 DER); for this reason the dsig11:DEREncodedKeyValue
element is not a child of KeyValue
. The dsig11:DEREncodedKeyValue
element is also not a child of the
X509Data
element, as the keys represented
by dsig11:DEREncodedKeyValue
may
not have X.509 certificates associated with them (a requirement for
X509Data
).
dsig11:KeyInfoReference
ElementA dsig11:KeyInfoReference
element within KeyInfo
is
used to convey a reference to a KeyInfo
element at another location
in the same or different document. For example, several signatures in a document
might use a key verified by an X.509v3 certificate chain appearing once in the
document or remotely outside the document; each signature's KeyInfo
can reference this chain using a single dsig11:KeyInfoReference
element instead of including the entire chain with a sequence of
X509Certificate
elements repeated in multiple places.
dsig11:KeyInfoReference
uses the same syntax and dereferencing
behavior as Reference
's URI
(
section B.4 The URI Attribute in "Compatibility Mode") and the Reference
Processing Model
(section B.4.1 The "Compatibility Mode" Reference Processing Model)
except that there are no child elements and the
presence of the URI
attribute is mandatory.
The result of dereferencing a dsig11:KeyInfoReference
must be
a KeyInfo
element, or an XML document with a KeyInfo
element as the root.
KeyInfoReference
element is a desirable
alternative to the use of
RetrievalMethod
when the data being referred to is
a KeyInfo
element and the
use of RetrievalMethod
would require one or
more Transform
child elements,
which introduce security risk and implementation challenges.
Schema Definition <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" --> <element name="KeyInfoReference" type="dsig11:KeyInfoReferenceType"/> <complexType name="KeyInfoReferenceType"> <attribute name="URI" type="anyURI" use="required"/> <attribute name="Id" type="ID" use="optional"/> </complexType>
Object
ElementType="http://www.w3.org/2000/09/xmldsig#Object"
(this can be used within a Reference
element
to identify the referent's type)Object
is an optional element that may occur one or
more times. When present, this element may contain any data. The Object
element may include optional MIME type, ID, and encoding attributes.
The Object
's Encoding
attributed may be
used to provide a URI that identifies the method by which the object is
encoded (e.g., a binary file).
The MimeType
attribute is an optional attribute which
describes the data within the Object
(independent of its
encoding). This is a string with values defined by [RFC2045]. For
example, if the Object
contains base64 encoded PNG, the Encoding
may be specified as 'http://www.w3.org/2000/09/xmldsig#base64' and the MimeType
as 'image/png'. This attribute is purely advisory; no validation of the
MimeType
information is required by this specification.
Applications that require normative type and encoding information for
signature validation should rely on Algorithm
in
the dsig2:Selection
element ("2.0 Mode")
or specify Transforms
with well defined resulting types and/or encodings ("Compatibility Mode").
The Object
's Id
is commonly referenced
from a Reference
in SignedInfo
, or Manifest
.
This element is typically used for enveloping signatures where the object being
signed is to be included in the signature element. The digest is
calculated over the entire Object
element including start
and end tags.
Note, if the application wishes to exclude the <Object>
tags from the digest calculation the Reference
must
identify the actual data object using standard Referencing mechanisms. e.g.
<Object>
tag,
then use an XPath Transform ("Compatibility Mode")
or dsig2:IncludedXPath
("2.0 Mode")
to refer to these nodes. Note in "2.0 Mode" it is not possible to refer to non-element nodes.dsig2:Selection
with a
Algorithm="http://www.w3.org/2010/xmldsig2#binaryfromBase64"
("2.0 Mode")Transform
("Compatibility Mode") or dsig2:Selection
("2.0 Mode").Exclusion of the object tags may be desired for cases where one wants the signature to remain valid if the data object is moved from inside a signature to outside the signature (or vice versa), or where the content of the Object is an encoding of an original binary document and it is desired to extract and decode so as to sign the original bitwise representation.
Schema Definition: <element name="Object" type="ds:ObjectType"/> <complexType name="ObjectType" mixed="true"> <sequence minOccurs="0" maxOccurs="unbounded"> <any namespace="##any" processContents="lax"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="MimeType" type="string" use="optional"/> <attribute name="Encoding" type="anyURI" use="optional"/> </complexType>
This section describes the optional to implement Manifest
and SignatureProperties
elements and describes the
handling of XML processing instructions and comments. With respect to
the elements Manifest
and SignatureProperties
this section specifies syntax and little behavior -- it is left to the
application. These elements can appear anywhere the parent's content
model permits; the Signature
content model only permits
them within Object
.
Manifest
ElementType="http://www.w3.org/2000/09/xmldsig#Manifest"
(this can be used within a Reference
element
to identify the referent's type)The Manifest
element provides a list of Reference
s.
The difference from the list in SignedInfo
is that it is
application-defined which, if any, of the digests are actually checked
against the objects referenced and what to do if the object is
inaccessible or the digest compare fails. If a Manifest
is pointed to from SignedInfo
, the digest over the Manifest
itself will be checked by the core signature validation behavior. The
digests within such a Manifest
are checked at the
application's discretion. If a Manifest
is referenced
from another Manifest
, even the overall digest of this
two level deep Manifest
might not be checked.
Schema Definition: <element name="Manifest" type="ds:ManifestType"/> <complexType name="ManifestType"> <sequence> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
SignatureProperties
ElementType="http://www.w3.org/2000/09/xmldsig#SignatureProperties"
(this can be used within a Reference
element
to identify the referent's type)Additional information items concerning the generation of the
signature(s) can be placed in a SignatureProperty
element
(i.e., date/time stamp or the serial number of cryptographic hardware
used in signature generation).
Schema Definition: <element name="SignatureProperties" type="ds:SignaturePropertiesType"/> <complexType name="SignaturePropertiesType"> <sequence> <element ref="ds:SignatureProperty" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType> <element name="SignatureProperty" type="ds:SignaturePropertyType"/> <complexType name="SignaturePropertyType" mixed="true"> <choice maxOccurs="unbounded"> <any namespace="##other" processContents="lax"/> <!-- (1,1) elements from (1,unbounded) namespaces --> </choice> <attribute name="Target" type="anyURI" use="required"/> <attribute name="Id" type="ID" use="optional"/> </complexType>
No XML processing instructions (PIs) are used by this specification.
Note that PIs placed inside SignedInfo
by an
application will be signed unless the CanonicalizationMethod
algorithm discards them. (This is true for any signed XML content.) All
of the canonicalization algorithms identified within this
specification retain PIs. When a PI is part of content that is signed
(e.g., within SignedInfo
or referenced XML documents)
any change to the PI will obviously result in a signature failure.
XML comments are not used by this specification.
Note that unless the CanonicalizationMethod
removes
comments within SignedInfo
or any other referenced XML
(which [XML-C14N] does), they will be signed. Consequently, if they
are retained, a change to the comment will cause a signature failure.
Similarly, the XML signature over any XML data will be sensitive to
comment changes unless a comment-ignoring canonicalization/transform
method, such as the Canonical XML [XML-C14N], is specified.
This specification defines several possible digest algorithms for the DigestMethod element, including required algorithm SHA-256. Use of SHA-256 is strongly recommended over SHA-1 because recent advances in cryptanalysis (see e.g. [SHA-1-Analysis]) have cast doubt on the long-term collision resistance of SHA-1. Therefore, SHA-1 support is required in this specification only for backwards-compatibility reasons.
Digest algorithms that are known not to be collision resistant should not be used in DigestMethod elements. For example, the MD5 message digest algorithm should not be used as specific collisions have been demonstrated for that algorithm.
The SHA-1 algorithm [FIPS-186-3] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:
<DigestMethod Algorithm="
http://www.w3.org/2000/09/xmldsig#sha1"/>
A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. For example, the DigestValue element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
The SHA-256 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-256 digest is a 256-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 32-octet octet stream.
The SHA-384 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-384 digest is a 384-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 48-octet octet stream.
The SHA-512 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-512 digest is a 512-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 64-octet octet stream.
MAC algorithms take two implicit parameters, their keying material
determined from KeyInfo
and the octet stream output by
CanonicalizationMethod
. MACs and signature algorithms are
syntactically identical but a MAC implies a shared secret key.
The HMAC
algorithm (RFC2104 [HMAC]) takes the output (truncation) length in
bits as a parameter; this specification REQUIRES that the truncation
length be a multiple of 8 (i.e. fall on a byte boundary) because Base64
encoding operates on full bytes. If the truncation parameter is
not specified then all the bits of the hash are output. Any signature
with a truncation length that is less than half the output length of
the underlying hash algorithm must be deemed invalid. An example of an
HMAC SignatureMethod
element:
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"> <HMACOutputLength>128</HMACOutputLength> </SignatureMethod>
The output of the HMAC algorithm is ultimately the output (possibly
truncated) of the chosen digest algorithm. This value shall be base64
encoded in the same straightforward fashion as the output of the digest
algorithms. Example: the SignatureValue
element for the
HMAC-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [HMAC] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
Schema Definition: <simpleType name="HMACOutputLengthType"> <restriction base="integer"/> </simpleType>
Signature algorithms take two implicit parameters, their keying
material determined from KeyInfo
and the octet stream
output by CanonicalizationMethod
. Signature and MAC
algorithms are syntactically identical but a signature implies public
key cryptography.
The DSA family of algorithms is defined in FIPS 186-3 [FIPS-186-3]. FIPS 186-3 defines DSA in terms of two security parameters L and N where L = |p|, N = |q|, p is the prime modulus, q is a prime divisor of (p-1). FIPS 186-3 defines four valid pairs of (L, N); they are: (1024, 160), (2048, 224), (2048, 256) and (3072, 256). The pair (1024, 160) corresponds to the algorithm DSAwithSHA1, which is identified in this specification by the URI http://www.w3.org/2000/09/xmldsig#dsa-sha1. The pairs (2048, 256) and (3072, 256) correspond to the algorithm DSAwithSHA256, which is identified in this specification by the URI http://www.w3.org/2009/xmldsig11#dsa-sha256. This specification does not use the (2048, 224) instance of DSA (which corresponds to DSAwithSHA224).
DSA takes no explicit parameters; an example of a DSA SignatureMethod
element is:
<SignatureMethod Algorithm="http://www.w3.org/2009/xmldsig11#dsa-sha256"/>
The output of the DSA algorithm consists of a pair of integers
usually referred by the pair (r, s). The signature value consists of
the base64 encoding of the concatenation of two octet-streams that
respectively result from the octet-encoding of the values r and s in
that order. Integer to octet-stream conversion must be done according
to the I2OSP operation defined in the RFC 3447 [PKCS1]
specification with a l
parameter equal to 20. For
example, the SignatureValue
element for a DSA signature (r
,
s
) with values specified in hexadecimal:
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
<SignatureValue>
i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>
Per FIPS 186-3 [FIPS-186-3], the DSA security parameter L is defined to be 1024, 2048 or 3072 bits and the corresponding DSA q value is defined to be 160, 224/256 and 256 bits respectively. Special Publication SP 800-57 Part 1 [SP800-57], NIST recommends using at least at 2048-bit public keys for securing information beyond 2010 (and 3072-bit keys for securing information beyond 2030).
Since XML Signature 1.0 requires implementations to support DSA-based digital signatures, this XML Signature 1.1 revision REQUIRES signature verifiers to implement DSA only for keys of 1024 bits in order to guarantee interoperability with XML Signature 1.0 generators. XML Signature 1.1 implementations may but are not required to support DSA-based signature generation, and given the short key size and the SP800-57 guidelines, DSA with 1024-bit prime moduli should not be used for signatures that will be verified beyond 2010.
The expression "RSA algorithm" as used in this specification refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 3447 [PKCS1]. The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
The SignatureValue
content for an RSA signature is the
base64 [RFC2045] encoding of the octet string computed as per RFC 3447 [PKCS1],
section 8.2.1: Signature generation for the RSASSA-PKCS1-v1_5 signature
scheme]. Computation of the signature will require concatenation of the
hash value and a constant string determined by RFC 3447. Signature
computation and verification does not require implementation of an
ASN.1 parser.
The resulting base64 [RFC2045] string is the value of the child text node of the SignatureValue element, e.g.
<SignatureValue> IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw= </SignatureValue>
The ECDSA algorithm [FIPS-186-3] takes no explicit parameters. An
example of a ECDSA SignatureMethod
element is:
<SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"/>
The output of the ECDSA algorithm consists of a pair of integers
usually referred by the pair (r, s). The signature value consists of
the base64 encoding of the concatenation of two octet-streams that
respectively result from the octet-encoding of the values r and s in
that order. Integer to octet-stream conversion must be done according
to the I2OSP operation defined in the RFC 3447 [PKCS1]
specification with the l
parameter equal to the size of
the base point order of the curve in bytes (e.g. 32 for the P-256 curve
and 66 for the P-521 curve).
This specification REQUIRES implementations to support the ECDSAwithSHA256 signature algorithm, which is ECDSA over the P-256 prime curve specified in Section D.2.3 of FIPS 186-3 [FIPS-186-3] (and using the SHA-256 hash algorithm). It is further recommended that implementations also support ECDSA over the P-384 and P-521 prime curves; these curves are defined in Sections D.2.4 and D.2.5 of FIPS 186-3, respectively.
The input to any canonicalization algorithm compatible with XML Signature 2.0 signatures is a set of document subtrees and exclusions in the form of subtrees or XML attributes. The actual representation of these inputs depends on the processing model and may be in terms of DOM nodes or representations suitable for streaming-based processing. The output is an octet stream.
Note: The input passed to "2.0 Mode" canonicalization algorithms
must always exclude the current Signature
element node (i.e., the Signature
must be passed as one of the exclusion elements. This is equivalent to an implicit
Enveloped Signature Transform in "Compatibility Mode", and has no effect for
non-enveloped signatures.
This specification REQUIRES implementation of Canonical XML 2.0 [XML-C14N20].
Applications may support other canonicalization algorithms with the same input model
(subtrees with exclusions). A Reference
to non-XML data may not use
canonicalization at all, or may use a custom canonicalization algorithm with this
input model or a completely different one.
An example of a Canonical XML 2.0 element is:
<CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"
xmlns:c14n2="http://www.w3.org/2010/xml-c14n2">
<c14n2:PrefixRewrite>sequential</c14n2:PrefixRewrite>
<c14n2:TrimTextNodes>true</c14n2:TrimTextNodes>
<c14n2:QNameAware>
<c14n2:QualifiedAttr Name="type" NS="http://www.w3.org/2001/XMLSchema-instance"/>
</c14n2:QNameAware>
</CanonicalizationMethod>
There is no Canonical XML 2.0 Transform
. Instead the same CanonicalizationMethod
element is reused within the dsig2:Selection
element for specifying canonicalization of
referenced data,
The normative specification of Canonical XML 2.0 is [XML-C14N20].
Transform
AlgorithmIn XML Signature 2.0, the Transforms
element contains
exactly one Transform
element with an Algorithm
of
"http://www.w3.org/2010/xmldsig2#transform"
. This transform
encapsulates the process of selecting the content to sign, canonicalizing it,
and attaching optional material that may aid the verifier.
This fixed Transform
element consists of a single required
dsig2:Selection
element, followed by an optional
CanonicalizationMethod
element, and an optional
dsig2:Verifications
element.
dsig2:Selection
AlgorithmsThis dsig2:Selection
algorithm allows the selection of XML documents
or fragments.
The required URI
attribute can be an external or same-document reference.
External references are parsed into an XML document or event stream for the subsequent
selection process to operate upon.
URI=""
) or a fragment (e.g URI="#foo"
). The former refers
to the entire document, while the latter refers to a subtree rooted at the element with
the "ID" contained in the fragment.URI="http://example.com/bar.xml"
)
or refer to fragments of external documents (e.g. URI="http://example.com/bar.xml#chapter1"
).The differences between the processing, and allowed syntax, of this URI
attribute and that of a "Compatibility Mode" Reference
URI
are:
xpointer
syntax is not permitted.The dsig2:IncludedXPath
must not be present, if the URI
contains a fragment identifier.
The dsig2:ExcludedXPath
maybe present even if there is a fragment identifier. I.e
the dsig2:Selection
must have one of the following
URI
attribute with or without a fragment identifier. URI
attribute with or without a fragment identifier, and one dsig2:ExcludedXPath
parameter element. URI
attribute and one dsig2:IncludedXPath
parameter element. URI
attribute, one dsig2:IncludedXPath
parameter element and one dsig2:ExcludedXPath
parameter element. Note: When an IncludedXPath
or ExcludedXPath
selects an element node, it
implies that the whole subtree rooted at that element is included or excluded.
Processing of the selection and parameters is as follows:
dsig2:IncludedXPath
element is present, evaluate this XPath at the root of document
to select element node(s),then add them to the "inclusion" list.dsig2:ExcludedXPath
is present,
evaluate it at the root of the document to select element and or attribute nodes(s),
then add them to the "exclusion list".
Signature
element under computation/evaluation to the "exclusion list".Initialize the XPath evaluation context for the dsig2:IncludedXPath
element
and the dsig2:ExcludedXPath
as follows:
here()
defined in Xpath Filter Transform
must not be used in this context)The result of the selection process is a set of one or more element nodes, and a set of zero or more exclusions consisting of element and/or attribute nodes.
Note: In a "streaming mode" of evaluation, the XPath evaluation, the canonicalization and digesting need to happen in a pipeline. This is described in Section "2.1 Streaming for XPath Signatures" in [XMLDSIG-XPATH].
dsig2:IncludedXPath
ElementThe dsig2:IncludedXPath
element is used in conjunction with XML-based
dsig2:Selection
algorithms to specify the subtree(s) to include in a
selection. The element contains an XPath 1.0 expression that is evaluated in the context
of the root of the XML document.
For example, "/Book/Chapter"
refers to the subtrees rooted
by all Chapter
child elements of all Book
child elements
of the document root.
The XPath 1.0 expression must evaluate only to element nodes, and must conform to the XML Signature Streaming Profile of XPath 1.0 [XMLDSIG-XPATH]. Implementations are not required to use a streaming XPath processor, but the expressions used must conform to the streaming profile to ensure compatibility with implementations that do use a streaming processor.
Schema Definition: <xs:element name="IncludedXPath" type="xs:string"/>
dsig2:ExcludedXPath
ElementThe dsig2:ExcludedXPath
element is used in conjunction with XML-based
dsig2:Selection
algorithms to specify subtree(s) and/or attributes
to exclude from a selection. The element contains an XPath 1.0 expression that is
evaluated in the context of the root of the XML document.
For example, "/Book/Chapter"
refers to the subtrees rooted
by all Chapter
child elements of all Book
child elements
of the document root.
The XPath 1.0 expression must evaluate to element and/or attribute nodes, and must conform to the XML Signature Streaming Profile of XPath 1.0 [XMLDSIG-XPATH]. Implementations are not required to use a streaming XPath processor, but the expressions used must conform to the streaming profile to ensure compatibility with implementations that do use a streaming processor.
Schema Definition: <xs:element name="ExcludedXPath" type="xs:string"/>
dsig2:ByteRange
ElementThe dsig2:ByteRange
element is used in conjunction with binary
dsig2:Selection
algorithms to specify byte range subsets of the
originally selected octet stream to include.
The element value must conform to the Byte Ranges syntax described in section 14.35.1 of [HTTP11].
For example, element content of 0-20,220-270,320-
indicates
that the first 21 bytes, then bytes 220 through 270, and finally bytes 320
through the rest of the stream are included.
Schema Definition: <xs:element name="ByteRange" type="xs:string"/>
This dsig2:Selection
algorithm allows the selection of external binary data.
The required URI
attribute must be an external reference and the result
of dereferencing it is treated as an octet stream.
The dsig2:Selection
element may contain at most one
dsig2:ByteRange
parameter element to modify the selection result.
If present, the range(s) indicated modify the resulting octet stream obtained from
the URI
. The implementation may incorporate the byte range into the
dereferencing process as an optimization.
The final result of the selection process is an octet stream.
This dsig2:Selection
algorithm allows the selection of
base64-encoded binary data from a Text node within an XML document.
The required URI
attribute can be an external or same-document reference.
External references are parsed into an XML document or event stream for the subsequent
selection process to operate upon.
URI=""
) or a fragment (e.g URI="#foo"
). The former refers
to the entire document, while the latter refers to a subtree rooted at the element with
the "ID" contained in the fragment.URI="http://example.com/bar.xml"
)
or refer to fragments of external documents (e.g. URI="http://example.com/bar.xml#chapter1"
).The differences between the processing, and allowed syntax, of this URI
attribute and that of a "Compatibility Mode" Reference
URI
are:
xpointer
syntax is not permitted.The dsig2:Selection
element may contain at most one
dsig2:IncludedXPath
and at most one dsig2:ByteRange
parameter
element to modify the selection result. However dsig2:IncludedXPath
must not be present,
if the URI
contains a fragment identifier.
Processing of the selection and parameters is as follows:
IncludedXPath
element is present, evaluate this XPath at the root of document
to select one element node. It is an error if the XPath returns more than one element node.dsig2:ByteRange
parameter is present, use these range(s) to modify
the octet stream obtained in the previous step.The final result of the selection process is an octet stream.
dsig2:Verification
TypesThe DigestDataLength dsig2:Verification
type contains an integer
that specifies the number of bytes that were digested for the containing Reference
. This can be
used for multiple purposes:
The non-negative integer value is carried within a DigestDataLength
attribute inside the
dsig2:Verification
element.
The PositionAssertion dsig2:Verification
type is used
to increase the resistance of ID-based referencing to signature wrapping attacks.
It contains an XPath expression that must match the referenced content's position
in the document. Thus, instead of "selecting" the referenced element via an XPath,
its position is verified by one (which enables flexibility in the actual use of XPath by
the signer or verifier). The actual selection process remains ID-based, which is simpler for many
implementers.
The XPath expression is carried within a PositionAssertion
attribute inside the
dsig2:Verification
element.
While using the PositionAssertion feature allows more flexibility in
accomodating XPath-unaware signers and verifiers, applications should
favor the use of XPath-based selection via the dsig2:IncludedXPath
element over the use of this feature in most cases. Because verification
of the PositionAssertion is formally optional, verifiers may become subject
to positional wrapping attacks if they choose to ignore the assertion.
This feature is appropriate mainly in applications in which knowledge of the
verifier's support for the feature can be assured.
The IDAttributes dsig2:Verification
type is used
in conjunction with ID-based references, to specify the ID attribute node name
that the signer used. Ordinarily, ID attribute knowledge is imparted through a
variety of normative and informal means, including DTDs, XML Schemas, use of xml:id,
and application-specific content knowledge. A signer is not required to use this
mechanism to identify ID attributes, but may do so to transfer its own ID knowledge
to the verifier through the signature itself. Verifiers may incorporate this knowledge,
or use more traditional means of recognizing ID attributes.
The dsig2:Verification
element specifies exactly one ID attribute node.
This must be the name of the node involved in the containing Reference
.
The dsig2:Verification
element must contain one of the
following two child elements:
dsig2:QualifiedAttr
Name
and
NS
attributes.dsig2:UnqualifiedAttr
Name
attribute,
and required ParentName
and optional ParentNS
attributes to identify
the owning element.Schema Definition: <xs:element name="QualifiedAttr" type="dsig2:QualifiedAttrType"/> <xs:complexType type="QualifiedAttrType"> <xs:attribute name="Name" type="xs:NCName" use="required"/> <xs:attribute name="NS" type="xs:anyURI" use="required"/> <xs:/complexType> <xs:element name="UnqualifiedAttr" type="dsig2:UnqualifiedAttrType"/> <xs:complexType type="UnqualifiedAttrType"> <xs:attribute name="Name" type="xs:NCName" use="required"/> <xs:attribute name="ParentName" type="xs:NCName" use="required"/> <xs:attribute name="ParentNS" type="xs:anyURI"/> <xs:/complexType>
Without a DTD, there is technically no way to define IDness in an XML document.
In practice, this typing was extended to documents validated by an XML Schema,
and then to the creation of xml:id
. Unfortunately, DTDs have mostly
fallen out of use in many contexts, and schemas are expensive, rarely used in many
runtime scenarios, and can't be relied on to be completely known by the verifier
in the presence of extensible XML scenarios.
xml:id
has not yet seen wide adoption, mainly because a lot of the
standards that needed it (SAML, WS-Security) were completed prior to its invention.
The result is that applications that rely on ID-based references for signing have
typically made insecure assumptions about the IDness of attributes based on their
name (ID
, id
, Id
, etc.), or have to provide
APIs for applications to call before verification (which is also a problem in the
face of extensibility). DOM Level 3, which is now fairly widely implemented, also
provides the ability to identify attributes as an ID at runtime, although often
without guaranteeing the uniqueness property.
The IDAttributes verification type provides a deterministic way of defining
an ID attribute used during signing, that is independent of DTD, XML Schema, DOM 3
or other application-specific mechanisms.
Digital signatures only work if the verification calculations are performed on exactly the same bits as the signing calculations. If the surface representation of the signed data can change between signing and verification, then some way to standardize the changeable aspect must be used before signing and verification. For example, even for simple ASCII text there are at least three widely used line ending sequences. If it is possible for signed text to be modified from one line ending convention to another between the time of signing and signature verification, then the line endings need to be canonicalized to a standard form before signing and verification or the signatures will break.
XML is subject to surface representation changes and to processing which discards some surface information. For this reason, XML digital signatures have a provision for indicating canonicalization methods in the signature so that a verifier can use the same canonicalization as the signer.
Throughout this specification we distinguish between the
canonicalization of a Signature
element and other signed
XML data objects. It is possible for an isolated XML document to be
treated as if it were binary data so that no changes can occur. In that
case, the digest of the document will not change and it need not be
canonicalized if it is signed and verified as such. However, XML that
is read and processed using standard XML parsing and processing
techniques is frequently changed such that some of its surface
representation information is lost or modified. In particular, this
will occur in many cases for the Signature
and enclosed SignedInfo
elements since they, and possibly an encompassing XML document, will be
processed as XML.
Similarly, these considerations apply to Manifest
, Object
,
and SignatureProperties
elements if those elements have
been digested, their DigestValue
is to be checked, and
they are being processed as XML.
The kinds of changes in XML that may need to be canonicalized can be divided into four categories. There are those related to the basic [XML10], as described in 7.1 below. There are those related to [DOM-LEVEL-1], [SAX], or similar processing as described in 7.2 below. Third, there is the possibility of coded character set conversion, such as between UTF-8 and UTF-16, both of which all [XML10] compliant processors are required to support, which is described in the paragraph immediately below. And, fourth, there are changes that related to namespace declaration and XML namespace attribute context as described in 7.3 below.
Any canonicalization algorithm should yield output in a specific
fixed coded character set. All canonicalization algorithms
identified in this document use UTF-8 (without a byte order mark (BOM))
and do not provide character normalization. We RECOMMEND that signature
applications create XML content (Signature
elements and
their descendants/content) in Normalization Form C [NFC] and check
that any XML being consumed is in that form as well; (if not,
signatures may consequently fail to validate). Additionally, none of
these algorithms provide data type normalization. Applications that
normalize data types in varying formats (e.g., (true, false) or (1,0))
may not be able to validate each other's signatures.
XML 1.0 [XML10]] defines an interface where a conformant application reading XML is given certain information from that XML and not other information. In particular,
Note that items (2), (4), and (5.3) depend on the presence of a
schema, DTD or similar declarations. The Signature
element type is laxly
schema valid [XMLSCHEMA-1][XMLSCHEMA-2], consequently
external XML or even XML within the same document as the signature may
be (only) well-formed or from another namespace (where permitted by the
signature schema); the noted items may not be present. Thus, a
signature with such content will only be verifiable by other signature
applications if the following syntax constraints are observed when
generating any signed material including the SignedInfo
element:
In addition to the canonicalization and syntax constraints discussed above, many XML applications use the Document Object Model [DOM-LEVEL-1] or the Simple API for XML [SAX]. DOM maps XML into a tree structure of nodes and typically assumes it will be used on an entire document with subsequent processing being done on this tree. SAX converts XML into a series of events such as a start tag, content, etc. In either case, many surface characteristics such as the ordering of attributes and insignificant white space within start/end tags is lost. In addition, namespace declarations are mapped over the nodes to which they apply, losing the namespace prefixes in the source text and, in most cases, losing where namespace declarations appeared in the original instance.
If an XML Signature is to be produced or verified on a system using the DOM or SAX processing, a canonical method is needed to serialize the relevant part of a DOM tree or sequence of SAX events. XML canonicalization specifications, such as [XML-C14N], are based only on information which is preserved by DOM and SAX. For an XML Signature to be verifiable by an implementation using DOM or SAX, not only must XML 1.0 syntax constraints given in the section 11.1 XML 1.0 Syntax Constraints, and Canonicalization be followed but an appropriate XML canonicalization must be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octet stream that was signed.
The XML Signature specification provides a very flexible digital signature mechanism. Implementers must give consideration to their application threat models and to the following factors. For additional security considerations in implementation and deployment of this specification, see [XMLDSIG-BESTPRACTICES].
A requirement of this specification is to permit signatures to
"apply to a part or totality of a XML document." (See
[XMLDSIG-REQUIREMENTS], section 3.1.3].) The Transforms
mechanism meets this requirement by permitting one to sign data derived
from processing the content of the identified resource. For instance,
applications that wish to sign a form, but permit users to enter
limited field data without invalidating a previous signature on the
form might use [XPATH] to exclude those portions the user needs to
change. Transforms
may be arbitrarily specified and may
include encoding transforms, canonicalization instructions or even XSLT
transformations. Three cautions are raised with respect to this feature
in the following sections.
Note, core validation behavior does not confirm that the signed data was obtained by applying each step of the indicated transforms. (Though it does check that the digest of the resulting content matches that specified in the signature.) For example, some applications may be satisfied with verifying an XML signature over a cached copy of already transformed data. Other applications might require that content be freshly dereferenced and transformed.
First, obviously, signatures over a transformed document do not secure any information discarded by transforms: only what is signed is secure.
Note that the use of Canonical XML [XML-C14N] ensures that
all internal entities and XML namespaces are expanded within the
content being signed. All entities are replaced with their definitions
and the canonical form explicitly represents the namespace that an
element would otherwise inherit. Applications that do not canonicalize
XML content (especially the SignedInfo
element) should
not use internal entities and should represent the namespace explicitly
within the content being signed since they can not rely upon
canonicalization to do this for them. Also, users concerned with the
integrity of the element type definitions associated with the XML
instance being signed may wish to sign those definitions as well (i.e.,
the schema, DTD, or natural language description associated with the
namespace/identifier).
Second, an envelope containing signed information is not secured by the signature. For instance, when an encrypted envelope contains a signature, the signature does not protect the authenticity or integrity of unsigned envelope headers nor its ciphertext form, it only secures the plaintext actually signed.
Additionally, the signature secures any information introduced by the transform: only what is "seen" (that which is represented to the user via visual, auditory or other media) should be signed. If signing is intended to convey the judgment or consent of a user (an automated mechanism or person), then it is normally necessary to secure as exactly as practical the information that was presented to that user. Note that this can be accomplished by literally signing what was presented, such as the screen images shown a user. However, this may result in data which is difficult for subsequent software to manipulate. Instead, one can sign the data along with whatever filters, style sheets, client profile or other information that affects its presentation.
Just as a user should only sign what he or she "sees," persons and
automated mechanism that trust the validity of a transformed document
on the basis of a valid signature should operate over the data that was
transformed (including canonicalization) and signed, not the original
pre-transformed data. This recommendation applies to transforms
specified within the signature as well as those included as part of the
document itself. For instance, if an XML document includes an embedded
style sheet [XSLT] it is the transformed document that should be
represented to the user and signed. To meet this recommendation where a
document references an external style sheet, the content of that
external resource should also be signed as via a signature Reference
otherwise the content of that external content might change which
alters the resulting document without invalidating the signature.
Some applications might operate over the original or intermediary data but should be extremely careful about potential weaknesses introduced between the original and transformed data. This is a trust decision about the character and meaning of the transforms that an application needs to make with caution. Consider a canonicalization algorithm that normalizes character case (lower to upper) or character composition ('e and accent' to 'accented-e'). An adversary could introduce changes that are normalized and consequently inconsequential to signature validity but material to a DOM processor. For instance, by changing the case of a character one might influence the result of an XPath selection. A serious risk is introduced if that change is normalized for signature validation but the processor operates over the original data and returns a different result than intended.
As a result:
This specification uses public key signatures and keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified algorithms which may have other models.
With public key signatures, any number of parties can hold the public key and verify signatures while only the parties with the private key can create signatures. The number of holders of the private key should be minimized and preferably be one. Confidence by verifiers in the public key they are using and its binding to the entity or capabilities represented by the corresponding private key is an important issue, usually addressed by certificate or online authority systems.
Keyed hash authentication codes, based on secret keys, are typically much more efficient in terms of the computational effort required but have the characteristic that all verifiers need to have possession of the same key as the signer. Thus any verifier can forge signatures.
This specification permits user provided signature algorithms and keying information designators. Such user provided algorithms may have different security models. For example, methods involving biometrics usually depend on a physical characteristic of the authorized user that can not be changed the way public or secret keys can be and may have other security model differences.
The strength of a particular signature depends on all links in the security chain. This includes the signature and digest algorithms used, the strength of the key generation [RANDOM] and the size of the key, the security of key and certificate authentication and distribution mechanisms, certificate chain validation policy, protection of cryptographic processing from hostile observation and tampering, etc.
Care must be exercised by applications in executing the various algorithms that may be specified in an XML signature and in the processing of any "executable content" that might be provided to such algorithms as parameters, such as XSLT transforms. The algorithms specified in this document will usually be implemented via a trusted library but even there perverse parameters might cause unacceptable processing or memory demand. Even more care may be warranted with application defined algorithms.
The security of an overall system will also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. All the factors listed in this section are important to the overall security of a system; however, most are beyond the scope of this specification.
This section is non-normative.
Object
designates a specific XML element. Occasionally we refer to a data
object as a document or as a resource's content. The term element
content is used to describe the data between XML start and end
tags [XML10]. The term XML document is used to describe
data objects which conform to the XML specification [XML10].
Object
element is merely one type of digital data (or document)
that can be signed via a Reference
.Signature
element type and its children (including SignatureValue
)
and the specified algorithms.Signature
element, and can be identified via a URI
or transform.
Consequently, the signature is "detached" from the content it signs.
This definition typically applies to separate data objects, but it also
includes the instance where the Signature
and data object
reside within the same XML document but are sibling elements.Object
element of the signature itself. The Object
(or its
content) is identified via a Reference
(via a URI
fragment identifier or transform).SignatureValue
.SignedInfo
reference validation. Reference
, matches its specified DigestValue
.SignatureValue
matches the result of processing
SignedInfo
with CanonicalizationMethod
and SignatureMethod
as specified in section 4.3 Core Validation.Use of the "Compatibility Mode" described in this section enables the XML Signature 1.x model to be used where necessary, to enable backward compatibility.
The following examples are for a detached signature of the content of the HTML4 in XML specification.
This example uses "Compatibility Mode".
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [s10] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s02-12]
The required SignedInfo
element
is the information that is actually signed. Core validation of
SignedInfo
consists of two mandatory processes: validation of the
signature over SignedInfo
and validation of each Reference
digest within SignedInfo
. Note that the algorithms used
in calculating the SignatureValue
are also included in
the signed information while the SignatureValue
element
is outside SignedInfo
.
[s03]
The CanonicalizationMethod
is the
algorithm that is used to canonicalize the SignedInfo
element before it is digested as part of the signature operation. Note
that this example is not in canonical form. (None of the examples in
this specification are in canonical form.)
[s04]
The SignatureMethod
is the
algorithm that is used to convert the canonicalized SignedInfo
into the SignatureValue
. It is a combination of a digest
algorithm and a key dependent algorithm and possibly other algorithms
such as padding, for example RSA-SHA1. The algorithm names are signed
to resist attacks based on substituting a weaker algorithm. To promote
application interoperability we specify a set of signature algorithms
that must be implemented, though their use is at the discretion of the
signature creator. We specify additional algorithms as recommended or
optional for implementation; the design also permits arbitrary user
specified algorithms.
[s05-11]
Each Reference
element includes
the digest method and resulting digest value calculated over the
identified data object. It also may include transformations that
produced the input to the digest operation. A data object is signed by
computing its digest value and a signature over that value. The
signature is later checked via reference and signature validation.
[s14-16]
KeyInfo
indicates the key to be
used to validate the signature. Possible forms for identification
include certificates, key names, and key agreement algorithms and
information -- we define only a few. KeyInfo
is optional
for two reasons. First, the signer may not wish to reveal key
information to all document processing parties. Second, the information
may be known within the application's context and need not be
represented explicitly. Since KeyInfo
is outside of
SignedInfo
, if the signer wishes to bind the keying information
to the signature, a Reference
can easily identify and
include the KeyInfo
as part of the signature.
Use of KeyInfo
is optional, however note that senders and
receivers
must agree on how it will be used through a mechanism out of scope for
this specification.
Reference
These section explaining the lines [s05]
to [s11]
of the previous example. This signature is in "compatibility mode".
[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [s10] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [s11] </Reference>
[s05]
The optional URI
attribute of Reference
identifies the data object to be signed. This attribute may be omitted
on at most one Reference
in a Signature
.
(This limitation is imposed in order to ensure that references and
objects may be matched unambiguously.)
[s05-08]
This identification, along with the
transforms, is a description provided by the signer on how they
obtained the signed data object in the form it was digested (i.e. the
digested content). The verifier may obtain the digested content in
another method so long as the digest verifies. In particular, the
verifier may obtain the content from a different location such as a
local store than that specified in the URI
.
[s06-08] Transforms
is an optional ordered list of
processing steps that were applied to the resource's content before it
was digested. Transforms can include operations such as
canonicalization, encoding/decoding (including compression/inflation),
XSLT, XPath, XML schema validation, or XInclude. XPath transforms
permit the signer to derive an XML document that omits portions of the
source document. Consequently those excluded portions can change
without affecting signature validity. For example, if the resource
being signed encloses the signature itself, such a transform must be
used to exclude the signature value from its own computation. If no Transforms
element is present, the resource's content is digested directly. While
the Working Group has specified mandatory (and optional)
canonicalization and decoding algorithms, user specified transforms are
permitted.
[s09-10] DigestMethod
is the algorithm applied to the
data after Transforms
is applied (if specified) to yield
the DigestValue
. The signing of the DigestValue
is what binds the content of a resource to the signer's key.
Object
and SignatureProperty
)This specification does not address mechanisms for making statements
or assertions. Instead, this document defines what it means for
something to be signed by an XML Signature (integrity, message authentication, and/or signer authentication).
Applications that wish to represent other semantics must rely upon
other technologies, such as [XML10], [RDF-PRIMER]. For instance,
an application might use a foo:assuredby
attribute within
its own markup to reference a Signature
element.
Consequently, it's the application that must understand and know how to
make trust decisions given the validity of the signature and the
meaning of assuredby
syntax. We also define a SignatureProperties
element type for the inclusion of assertions about the signature itself
(e.g., signature semantics, the time of signing or the serial number of
hardware used in cryptographic processes). Such assertions may be
signed by including a Reference
for the SignatureProperties
in SignedInfo
. While the signing application should be
very careful about what it signs (it should understand what is in the SignatureProperty
)
a receiving application has no obligation to understand that semantic
(though its parent trust engine may wish to). Any content about the
signature generation may be located within the SignatureProperty
element. The mandatory Target
attribute references the Signature
element to which the property applies.
Consider the preceding example (in "Compatibility Mode") with an
additional reference to a local Object
that includes a SignatureProperty
element. (Such a signature would not only be detached [p02]
but enveloping
[p03]
.)
[ ] <Signature Id="MySecondSignature" ...> [p01] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI="#AMadeUpTimeStamp" [p04] Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties"> [p05] <Transforms> [p06] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [p07] </Transforms> [p08] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [p09] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [p10] </Reference> [p11] </SignedInfo> [p12] ... [p13] <Object> [p14] <SignatureProperties> [p15] <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> [p16] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p17] <date>19990914</date> [p18] <time>14:34:34:34</time> [p19] </timestamp> [p20] </SignatureProperty> [p21] </SignatureProperties> [p22] </Object> [p23]</Signature>
[p04]
The optional Type
attribute of Reference
provides information about the resource identified by the URI
.
In particular, it can indicate that it is an Object
, SignatureProperty
,
or Manifest
element. This can be used by applications to
initiate special processing of some Reference
elements.
References to an XML data element within an Object
element should identify the actual element pointed to. Where the
element content is not XML (perhaps it is binary or encoded data) the
reference should identify the Object
and the Reference
Type
, if given, should indicate Object
.
Note that Type
is advisory and no action based on it or
checking of its correctness is required by core behavior.
[p13]
Object
is an optional element for
including data objects within the signature element or elsewhere. The Object
can be optionally typed and/or encoded.
[p14-21]
Signature properties, such as time of signing,
can be optionally signed by identifying them from within a Reference
.
(These properties are traditionally called signature "attributes"
although that term has no relationship to the XML term "attribute".)
This is the same example in "2.0 Mode". Only the Reference
content is different.
[ ] ... [p03] <Reference> [p04] [p05] <Transforms> [p06] <Transform Algorithm="http://www.w3.org/2010/xmldsig2#transform"> [s06a] <dsig2:Selection type="http://www.w3.org/2010/xmldsig2#xml" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#" URI="#AMadeUpTimeStamp" > [p06b] </dsig2:Selection> [p06c] <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/> [p06d] </Transform> [p07] </Transforms> [p08] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [p09] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [p10] </Reference> [ ] ...
Object
and Manifest
)The Manifest
element is provided to meet additional
requirements not directly addressed by the mandatory parts of this
specification. Two requirements and the way the Manifest
satisfies them follow.
First, applications frequently need to efficiently sign multiple
data objects even where the signature operation itself is an expensive
public key signature. This requirement can be met by including multiple
Reference
elements within SignedInfo
since
the inclusion of each digest secures the data digested. However, some
applications may not want the core validation behavior associated
with this approach because it requires every Reference
within SignedInfo
to undergo reference
validation -- the DigestValue
elements are checked.
These applications may wish to reserve reference validation decision
logic to themselves. For example, an application might receive a signature valid SignedInfo
element that includes three Reference
elements. If a
single Reference
fails (the identified data object when
digested does not yield the specified DigestValue
) the
signature would fail core
validation. However, the application may wish to treat the
signature over the two valid Reference
elements as valid
or take different actions depending on which fails. To accomplish
this, SignedInfo
would reference a Manifest
element that contains one or more Reference
elements
(with the same structure as those in SignedInfo
). Then,
reference validation of the Manifest
is under application
control.
Second, consider an application where many signatures (using
different keys) are applied to a large number of documents. An
inefficient solution is to have a separate signature (per key)
repeatedly applied to a large SignedInfo
element (with
many Reference
s); this is wasteful and redundant. A more
efficient solution is to include many references in a single Manifest
that is then referenced from multiple Signature
elements.
The example (in "Compatibility Mode") below includes a Reference
that signs a Manifest
found within the Object
element.
[ ] ... [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest"> [m03] <Transforms> [m04] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [m05] </Transforms> [m06] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [m07] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...=</DigestValue> [m08] </Reference> [ ] ... [m09] <Object> [m10] <Manifest Id="MyFirstManifest"> [m11] <Reference> [m12] ... [m13] </Reference> [m14] <Reference> [m15] ... [m16] </Reference> [m17] </Manifest> [m18] </Object>
Here is the modified Reference
in "2.0 Mode"
[m01] <Reference [m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest"> [m03] <Transforms> [m04] <Transform Algorithm="http://www.w3.org/2010/xmldsig2#transform"> [m04a] <dsig2:Selection type="http://www.w3.org/2010/xmldsig2#xml" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#" URI="#MyFirstManifest"> [m04b] </dsig2:Selection> [m04c] <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/> [m04d] </Transform> [m05] </Transforms> [m06] <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> [m07] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...=</DigestValue> [m08] </Reference>
For each data object being signed:
Transforms
, as determined by the
application, to the data object.Reference
element, including the
(optional) identification of the data object, any (optional) transform
elements, the digest algorithm and the DigestValue
.
(Note, it is the canonical form of these references that are signed in
3.1.3 and validated in 3.2.1 .)
Transform
elements is a node-set. We RECOMMEND that, when
generating signatures, signature applications do not rely on this
default behavior, but explicitly identify the transformation that is
applied to perform this mapping. In cases in which inclusive
canonicalization is desired, we RECOMMEND that Canonical XML 1.1
[XML-C14N11] be used. It is very important to check that the Reference
actually
includes the data that is expected to be signed. The [XMLDSIG-BESTPRACTICES] document
describes a number of attacks, where what is apparently being signed is not
actually signed.
One way to check the reference is to allow only certain combinations of transforms. For example [SAML2-CORE] and [EBXML-MSG] follow this approach.
Another option is for XML Signature libraries to return the pre-digest data to the application, so that application can inspect it to verify what is actually signed. This too may not be enough, for example in a Web Services scenario, if the reference is pointing to a soap:Body, it is not sufficient to just check the name of the "soap:Body" element, as it can lead to wrapping attacks [MCINTOSH-WRAP];Instead the application should check if this soap:Body is in the correct position, i.e. as a child of the top level soap:Envelope.
KeyInfo
or from an external source.SignatureMethod
using the CanonicalizationMethod
and use the result
(and previously obtained KeyInfo
) to confirm the SignatureValue
over the SignedInfo
element.Note, KeyInfo
(or some transformed version thereof) may be signed via a Reference
element. Transformation and validation of this reference (3.2.1) is
orthogonal to Signature Validation which uses the KeyInfo
as parsed.
Additionally, the SignatureMethod
URI may have been
altered by the canonicalization of SignedInfo
(e.g.,
absolutization of relative URIs) and it is the canonical form that must
be used. However, the required canonicalization [XML-C14N] of this
specification does not change URIs.
SignedInfo
element based on the
CanonicalizationMethod
in SignedInfo
.Reference
in SignedInfo
:
URI
and execute
Transforms
provided by the signer in the Reference
element, or it may obtain the content through other means such as a
local cache.)DigestMethod
specified in its Reference
specification.DigestValue
in the SignedInfo
Reference
; if there is
any mismatch, validation fails.Note, SignedInfo
is canonicalized in step 1. The
application must ensure that the CanonicalizationMethod has no
dangerous side effects, such as rewriting URIs, (see CanonicalizationMethod
Note in section B.3 Use of CanonicalizationMethod in "Compatibility Mode") and that it
"Sees What is
Signed", which is the canonical form (see section 12.1.3 "See" What is Signed ).
Note, After a Signature
element has been created during
Signature Generation for a signature with a same document reference, an
implementation can serialize the XML content with variations in that
serialization. This means that Reference Validation needs to
canonicalize the XML document before digesting in step 1 to avoid
issues related to variations in serialization.
CanonicalizationMethod
in "Compatibility Mode"Alternatives to the required section B.6 "Compatibility Mode" Canonicalization Algorithms, such as section B.6.1 Canonical XML 1.0 or a minimal canonicalization (such as CRLF and charset normalization), may be explicitly specified but are not required. Consequently, their use may not interoperate with other applications that do not support the specified algorithm (see section 11. XML Canonicalization and Syntax Constraint Considerations. Security issues may also arise in the treatment of entity processing and comments if non-XML aware canonicalization algorithms are not properly constrained (see section 12.1.2 Only What is "Seen" Should be Signed).
The way in which the SignedInfo
element is presented
to the canonicalization method is dependent on that method. The
following applies to algorithms which process XML as nodes or
characters:
SignedInfo
and currently indicating the SignedInfo
,
its descendants, and the attribute and namespace nodes of SignedInfo
and its descendant elements.SignedInfo
element, from the first
character to the last character of the XML representation, inclusive.
This includes the entire text of the start and end tags of the SignedInfo
element as well as all descendant markup
and character data (i.e., the text)
between those tags. Use of text based canonicalization of SignedInfo
is not recommended.We recommend applications that implement a text-based instead of XML-based canonicalization -- such as resource constrained apps -- generate canonicalized XML as their output serialization so as to mitigate interoperability and security concerns. For instance, such an implementation should (at least) generate standalone XML instances [XML10].
NOTE: The signature
application must exercise great care in accepting and executing an
arbitrary CanonicalizationMethod
. For example, the
canonicalization method could rewrite the URIs of the Reference
s
being validated. Or, the method could significantly transform SignedInfo
so that validation would always succeed (i.e., converting it to a
trivial signature with a known key over trivial data). Since CanonicalizationMethod
is inside SignedInfo
, in the resulting canonical form it
could erase itself from SignedInfo
or modify the SignedInfo
element so that it appears that a different canonicalization function
was used! Thus a Signature
which appears to authenticate
the desired data with the desired key, DigestMethod
, and SignatureMethod
,
can be meaningless if a capricious CanonicalizationMethod
is used.
URI
Attribute in "Compatibility Mode" If the URI
attribute is omitted for a "Compatibility Mode" signature, then the
receiving application is expected to know the identity of the object.
For example, a lightweight data protocol might omit this attribute
given the identity of the object is part of the application context.
In "Compatibility Mode", at most one Reference
element
without a URI
attribute may be present in any particular
SignedInfo
, or Manifest
.
The URI
attribute identifies a data object using a
URI-Reference [URI].
The mapping from this attribute's value to a URI reference must be performed as specified in section 3.2.17 of [XMLSCHEMA-2]. Additionally: Some existing implementations are known to verify the value of the URI attribute against the grammar in [URI]. It is therefore safest to perform any necessary escaping while generating the URI attribute.
We RECOMMEND XML Signature applications be able to dereference URIs in the HTTP scheme. Dereferencing a URI in the HTTP scheme must comply with the Status Code Definitions of [HTTP11] (e.g., 302, 305 and 307 redirects are followed to obtain the entity-body of a 200 status code response). Applications should also be cognizant of the fact that protocol parameter and state information, (such as HTTP cookies, HTML device profiles or content negotiation), may affect the content yielded by dereferencing a URI.
If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease). (See section 4.3 Core Validation for further information on reference processing.)
The optional Type attribute contains information about the type of
object being signed after all ds:Reference
transforms
have been applied. This is represented as a URI. For example:
Type="http://www.w3.org/2000/09/xmldsig#Object"
Type="http://www.w3.org/2000/09/xmldsig#Manifest"
The Type
attribute applies to the item being pointed
at, not its contents. For example, a reference that results in the
digesting of an Object
element containing a SignatureProperties
element is still of type #Object
. The Type
attribute is advisory. No validation of the type information is
required by this specification.
Note: XPath is recommended. Signature applications need not conform to [XPATH] specification in order to conform to this specification. However, the XPath data model, definitions (e.g., node-sets) and syntax are used within this document in order to describe functionality for those that want to process XML-as-XML (instead of octets) as part of signature generation. For those that want to use these features, a conformant [XPATH] implementation is one way to implement these features, but it is not required. Such applications could use a sufficiently functional replacement to a node-set and implement only those XPath expression behaviors required by this specification. However, for simplicity we generally will use XPath terminology without including this qualification on every point. Requirements over "XPath node-sets" can include a node-set functional equivalent. Requirements over XPath processing can include application behaviors that are equivalent to the corresponding XPath behavior.
The data-type of the result of URI dereferencing or subsequent Transforms is either an octet stream or an XPath node-set.
The Transforms
specified in this document are defined
with respect to the input they require. The following is the default
signature application behavior:
Users may specify alternative transforms that override these
defaults in transitions between transforms that expect different
inputs. The final octet stream contains the data octets being secured.
The digest algorithm specified by DigestMethod
is then
applied to these data octets, resulting in the DigestValue
.
Note: The section B.2.1 Reference Generation in "Compatibility Mode" includes further restrictions on the reliance upon defined default transformations when applications generate signatures.
In this specification, a 'same-document' reference is defined as a URI-Reference that consists of a hash sign ('#') followed by a fragment or alternatively consists of an empty URI [URI].
Unless the URI-Reference is such a 'same-document' reference, the result of dereferencing the URI-Reference must be an octet stream. In particular, an XML document identified by URI is not parsed by the signature application unless the URI is a same-document reference or unless a transform that requires XML parsing is applied. (See section 6.2 The Transforms Element.)
When a fragment is preceded by an absolute or relative URI in the
URI-Reference, the meaning of the fragment is defined by the resource's
MIME type [RFC2045]. Even for XML documents, URI dereferencing
(including the fragment processing) might be done for the signature
application by a proxy. Therefore, reference validation might fail if
fragment processing is not performed in a standard way (as defined in
the following section for same-document references). Consequently, we
RECOMMEND in this case that the URI
attribute not
include fragment identifiers and that such processing be specified as
an additional XPath Transform or XPath Filter
2 Transform [XMLDSIG-XPATH-FILTER2].
When a fragment is not preceded by a URI in the URI-Reference, XML
Signature applications must support the null URI and shortname XPointer
[XPTR-FRAMEWORK]. We RECOMMEND support for the same-document
XPointers '#xpointer(/)
' and '#xpointer(id('ID'))
'
if the application also intends to support any canonicalization that preserves comments. (Otherwise URI="#foo"
will automatically remove comments before the canonicalization can even
be invoked due to the processing defined in section B.4.2 "Compatibility Mode" Same-Document URI-References.) All other support
for XPointers is optional,
especially all support for shortname and other XPointers in external
resources since the application may not have control over how the
fragment is generated (leading to interoperability problems and
validation failures).
'#xpointer(/)
' must be interpreted to identify the root
node [XPATH] of the document that contains the URI
attribute.
'#xpointer(id('ID'))
' must be interpreted to
identify the element node identified by '#element(ID)
'
[XPTR-ELEMENT] when evaluated with respect to the document that
contains the URI
attribute.
The original edition of this specification [XMLDSIG-CORE]
referenced the XPointer Candidate Recommendation
[XPTR-XPOINTER-CR2001] and some implementations support it
optionally. That Candidate Recommendation has been superseded by the
[XPTR-FRAMEWORK], [XPTR-XMLNS] and [XPTR-ELEMENT]
Recommendations, and -- at the time of this edition -- the
[XPTR-XPOINTER] Working Draft. Therefore, the use of the
xpointer()
scheme [XPTR-XPOINTER] beyond the usage discussed
in this section is discouraged.
The following examples demonstrate what the URI attribute identifies and how it is dereferenced:
URI="http://example.com/bar.xml"
URI="http://example.com/bar.xml#chapter1"
URI=""
URI="#chapter1"
Dereferencing a same-document reference must result in an XPath
node-set suitable for use by Canonical XML [XML-C14N]. Specifically,
dereferencing a null URI (URI=""
) must result in an XPath
node-set that includes every non-comment node of the XML document
containing the URI
attribute. In a fragment URI, the
characters after the number sign ('#') character conform to the
XPointer syntax [XPTR-FRAMEWORK]. When processing an XPointer, the
application must behave as if the XPointer was evaluated with respect
to the XML document containing the URI
attribute . The
application must behave as if the result of XPointer processing
[XPTR-FRAMEWORK] were a node-set derived from the resultant
subresource as follows:
The second to last replacement is necessary because XPointer typically indicates a subtree of an XML document's parse tree using just the element node at the root of the subtree, whereas Canonical XML treats a node-set as a set of nodes in which absence of descendant nodes results in absence of their representative text from the canonical form.
The last step is performed for null URIs and shortname XPointers.
It is necessary because when [XML-C14N] or [XML-C14N11] is passed
a node-set, it processes the node-set as is: with or without comments.
Only when it is called with an octet stream does it invoke its own
XPath expressions (default or without comments). Therefore to retain
the default behavior of stripping comments when passed a node-set, they
are removed in the last step if the URI is not a scheme-based XPointer.
To retain comments while selecting an element by an identifier ID,
use the following scheme-based XPointer: URI='#xpointer(id('ID'))'
.
To retain comments while selecting the entire document, use the
following scheme-based XPointer: URI='#xpointer(/)'
.
The interpretation of these XPointers is defined in section B.4.1 The "Compatibility Mode" Reference Processing Model.
If the optional Transforms
element is present and
contains exactly one
Transform
element with an Algorithm
of "http://www.w3.org/2010/xmldsig2#transform"
then 2.0
processing is performed as described in
section 6.2 The Transforms Element
otherwise compatibility mode transform processing is
performed as described here.
The optional Transforms
element contains an ordered list of
Transform
elements; these describe how the signer obtained the data
object that was digested. Each Transform
consists of an
Algorithm
attribute and content parameters, if any, appropriate for the given
algorithm. The Algorithm
attribute value specifies the
name of the algorithm to be performed, and the Transform
content provides additional data to govern the algorithm's processing
of the transform
input.
The Transforms
element is optional and
its presence indicates that the signer is not signing the native
(original) document but the resulting (transformed) document. (See
section 12.1.1 Only What is Signed is Secure
).
The output of each Transform
serves as input to the
next Transform
. The input to the first Transform
is the result of dereferencing the URI
attribute of the Reference
element. The output from the last Transform
is the input
for the DigestMethod
algorithm.
As described in section B.4.1 The "Compatibility Mode" Reference Processing Model, some transforms take an XPath node-set as input, while others require an octet stream. If the actual input matches the input needs of the transform, then the transform operates on the unaltered input. If the transform input requirement differs from the format of the actual input, then the input must be converted.
Some Transform
s may require explicit MIME type,
charset (IANA registered "character set"), or other such information
concerning the data they are receiving from an earlier Transform
or the source data, although no Transform
algorithm
specified in this document needs such explicit information. Such data
characteristics are provided as parameters to the Transform
algorithm and should be described in the specification for the
algorithm.
Examples of transforms include but are not limited to base64
decoding [RFC2045], canonicalization [XML-C14N], XPath filtering
[XPATH], and XSLT [XSLT]. The generic definition of the Transform
element also allows application-specific transform algorithms. For
example, the transform could be a decompression routine given by a Java
class appearing as a base64 encoded parameter to a Java Transform
algorithm. However, applications should refrain from using application-specific
transforms if they wish their signatures to be verifiable outside of their
application domain. section B.7 "Compatibility Mode" Transform Algorithms
defines the list of standard "Compatibility Mode"
transformations.
If canonicalization is performed over octets, the canonicalization algorithms take two implicit parameters: the content and its charset. The charset is derived according to the rules of the transport protocols and media types (e.g, [XML-MEDIA-TYPES] defines the media types for XML). This information is necessary to correctly sign and verify documents and often requires careful server side configuration.
Various canonicalization algorithms require conversion to [UTF-8]. The algorithms below understand at least [UTF-8] and [UTF-16] as input encodings. We RECOMMEND that externally specified algorithms do the same. Knowledge of other encodings is optional.
Various canonicalization algorithms transcode from a non-Unicode encoding to Unicode. The output of these algorithms will be in NFC [NFC]. This is because the XML processor used to prepare the XPath data model input is required (by the Data Model) to use Normalization Form C when converting an XML document to the UCS character domain from any encoding that is not UCS-based.
We RECOMMEND that externally specified canonicalization algorithms do the same. (Note, there can be ambiguities in converting existing charsets to Unicode, for an example see the XML Japanese Profile Note [XML-Japanese].)
This specification REQUIRES implementation of Canonical XML 1.0 [XML-C14N], Canonical XML 1.1 [XML-C14N11]] and Exclusive XML Canonicalization [XML-EXC-C14N]. We RECOMMEND that applications that generate signatures choose Canonical XML 1.1 [XML-C14N11] when inclusive canonicalization is desired.
Note: Canonical XML 1.0 [XML-C14N] and Canonical XML 1.1 [XML-C14N11] specify a standard serialization of XML that, when applied to a subdocument, includes the subdocument's ancestor context including all of the namespace declarations and some attributes in the 'xml:' namespace. However, some applications require a method which, to the extent practical, excludes unused ancestor context from a canonicalized subdocument. The Exclusive XML Canonicalization Recommendation [XML-EXC-C14N] may be used to address requirements resulting from scenarios where a subdocument is moved between contexts.
An example of an XML canonicalization element is:
<CanonicalizationMethod Algorithm="
http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
The normative specification of Canonical XML1.0 is [XML-C14N]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML is easily parameterized (via an additional URI) to omit or retain comments.
The normative specification of Canonical XML 1.1 is [XML-C14N11]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML 1.1 is easily parameterized (via an additional URI) to omit or retain comments.
The normative specification of Exclusive XML Canonicalization 1.0 is [XML-EXC-C14N]].
Transform
AlgorithmsA Transform
algorithm has a single implicit parameter:
an octet stream from the Reference
or the output of an
earlier Transform
.
For implementation requirements, please see section 3.3 Compatibility Mode Conformance. Application developers are strongly encouraged to support all transforms that are listed as recommended unless the application environment has resource constraints that would make such support impractical. Compliance with this recommendation will maximize application interoperability and libraries should be available to enable support of these transforms in applications without extensive development.
Any canonicalization algorithm that can be used for CanonicalizationMethod
(such as those in section B.6 "Compatibility Mode" Canonicalization Algorithms) can
be used as a Transform
.
The normative specification for base64 decoding transforms is
[RFC2045]. The base64 Transform
element has no
content. The input is decoded by the algorithms. This transform is
useful if an application needs to sign the raw data associated with the
encoded content of an element.
This transform accepts either an octet-stream or a node-set as
input. If an octet-string is given as input, then this octet-stream is
processed directly. If an XPath node-set (or sufficiently functional
alternative) is given as input, then it is converted to an octet stream
by performing operations logically equivalent to 1) applying an XPath
transform with expression self::text()
, then 2)
sorting the nodeset by document order, then concatenating the string-value
of each of the nodes into one long string. Thus, if an XML element is identified
by a shortname XPointer in the Reference
URI, and its
content consists solely of base64 encoded character data, then this
transform automatically strips away the start and end tags of the
identified element and any of its descendant elements as well as any
descendant comments and processing instructions. The output of this
transform is an octet stream.
The normative specification for XPath expression evaluation is
[XPATH]. The XPath expression to be evaluated appears as the
character content of a transform parameter child element named XPath
.
The input required by this transform is an XPath node-set or an octet-stream. Note that if the actual input is an XPath node-set resulting from a null URI or shortname XPointer dereference, then comment nodes will have been omitted. If the actual input is an octet stream, then the application must convert the octet stream to an XPath node-set suitable for use by Canonical XML with Comments. (A subsequent application of the required Canonical XML algorithm would strip away these comments.) In other words, the input node-set should be equivalent to the one that would be created by the following process:
(//. | //@* | //namespace::*)
The evaluation of this expression includes all of the document's nodes (including comments) in the node-set representing the octet stream.
The transform output is always an XPath node-set. The XPath
expression appearing in the XPath
parameter is evaluated
once for each node in the input node-set. The result is converted to a
boolean. If the boolean is true, then the node is included in the
output node-set. If the boolean is false, then the node is omitted from
the output node-set.
Note: Even if the input node-set has had comments
removed, the comment nodes still exist in the underlying parse tree and
can separate text nodes. For example, the markup <e>Hello,
<!-- comment -->world!</e>
contains two text nodes.
Therefore, the expression self::text()[string()="Hello, world!"]
would fail. Should this problem arise in the application, it can be
solved by either canonicalizing the document before the XPath transform
to physically remove the comments or by matching the node based on the
parent element's string value (e.g. by using the expression self::text()[string(parent::e)="Hello,
world!"]
).
The primary purpose of this transform is to ensure that only specifically defined changes to the input XML document are permitted after the signature is affixed. This is done by omitting precisely those nodes that are allowed to change once the signature is affixed, and including all other input nodes in the output. It is the responsibility of the XPath expression author to include all nodes whose change could affect the interpretation of the transform output in the application context.
Note that the XML-Signature XPath Filter 2.0 Recommendation [XMLDSIG-XPATH-FILTER2] may be used for this purpose. That recommendation defines an XPath transform that permits the easy specification of subtree selection and omission that can be efficiently implemented.
An important scenario would be a document requiring two enveloped signatures. Each signature must omit itself from its own digest calculations, but it is also necessary to exclude the second signature element from the digest calculations of the first signature so that adding the second signature does not break the first signature.
The XPath transform establishes the following evaluation context for each node of the input node-set:
As a result of the context node setting, the XPath expressions
appearing in this transform will be quite similar to those used in used
in [XSLT], except that the size and position are always 1 to reflect
the fact that the transform is automatically visiting every node (in
XSLT, one recursively calls the command apply-templates
to visit the nodes of the input tree).
The function here()
is defined as follows:
The here function returns a node-set containing the attribute or processing instruction node or the parent element of the text node that directly bears the XPath expression. This expression results in an error if the containing XPath expression does not appear in the same XML document against which the XPath expression is being evaluated.
As an example, consider creating an enveloped signature (a Signature
element that is a descendant of an element being signed). Although the
signed content should not be changed after signing, the elements within
the Signature
element are changing (e.g. the digest value
must be put inside the DigestValue
and the SignatureValue
must be subsequently calculated). One way to prevent these changes from
invalidating the digest value in DigestValue
is to add an
XPath Transform
that omits all Signature
elements and their descendants. For example,
<Document> ... <Signature xmlns="http://www.w3.org/2000/09/xmldsig#"> <SignedInfo> ... <Reference URI=""> <Transforms> <Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116"> <XPath xmlns:dsig="&dsig;"> not(ancestor-or-self::dsig:Signature) </XPath> </Transform> </Transforms> <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> <DigestValue></DigestValue> </Reference> </SignedInfo> <SignatureValue></SignatureValue> </Signature> ... </Document>
Due to the null Reference
URI in this example, the
XPath transform input node-set contains all nodes in the entire parse
tree starting at the root node (except the comment nodes). For each
node in this node-set, the node is included in the output node-set
except if the node or one of its ancestors has a tag of Signature
that is in the namespace given by the replacement text for the entity &dsig;
.
A more elegant solution uses the here
function to omit only the Signature
containing the XPath
Transform, thus allowing enveloped signatures to sign other signatures.
In the example above, use the XPath
element:
<XPath xmlns:dsig="&dsig;"> count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) > count(ancestor-or-self::dsig:Signature)</XPath>
Since the XPath equality operator converts node sets to string
values before comparison, we must instead use the XPath union operator
(|). For each node of the document, the predicate expression is true if
and only if the node-set containing the node and its Signature
element ancestors does not include the enveloped Signature
element containing the XPath expression (the union does not produce a
larger set if the enveloped Signature
element is in the
node-set given by ancestor-or-self::Signature
).
An enveloped signature transform T
removes the whole Signature
element containing T
from the digest calculation of the Reference
element
containing T. The entire string of
characters used by an XML processor to match the Signature
with the XML production element
is removed. The output
of the transform is equivalent to the output that would result from
replacing T with an XPath transform
containing the following XPath
parameter element:
<XPath xmlns:dsig="&dsig;"> count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) > count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to those of the XPath transform, but may only be applied to a node-set from its parent XML document. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform must produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.
The normative specification for XSL Transformations is [XSLT].
Specification of a namespace-qualified stylesheet element, which must
be the sole child of the Transform
element, indicates
that the specified style sheet should be used. Whether this
instantiates in-line processing of local XSLT declarations within the
resource is determined by the XSLT processing model; the ordered
application of multiple stylesheet may require multiple Transforms
.
No special provision is made for the identification of a remote
stylesheet at a given URI because it can be communicated via an xsl:include
or xsl:import
within the stylesheet
child of the Transform
.
This transform requires an octet stream as input.
The output of this transform is an octet stream. The processing rules for the XSL style sheet [XSL10] or transform element are stated in the XSLT specification [XSLT].
We RECOMMEND that XSLT transform authors use an output method of xml
for XML and HTML. As XSLT implementations do not produce consistent
serializations of their output, we further RECOMMEND inserting a
transform after the XSLT transform to canonicalize the output. These
steps will help to ensure interoperability of the resulting signatures
among applications that support the XSLT transform. Note that if the
output is actually HTML, then the result of these steps is logically
equivalent [XHTML10].
In [XPATH] and consequently the Canonical XML data model an element has namespace nodes that correspond to those declarations within the element and its ancestors:
"Note: An element E has namespace nodes that represent its namespace declarations as well as any namespace declarations made by its ancestors that have not been overridden in E's declarations, the default namespace if it is non-empty, and the declaration of the prefix
xml
." [XML-C14N]
When serializing a Signature
element or signed XML
data that's the child of other elements using these data models, that Signature
element and its children may have in-scope namespaces inherited from its ancestral context.
In addition, the Canonical XML and Canonical XML with
Comments algorithms define special treatment for attributes in the XML namespace,
which can cause them to be part of the canonicalized XML even if they were outside
of the document subset. Simple inheritable attributes (i.e. attributes that have a value
that requires at most a simple redeclaration such as xml:lang and xml:space)
are inherited from nearest
ancestor in which they are declared to the apex node
of canonicalized XML unless they are already declared at that node.
This may frustrate the intent of the signer to create a signature in
one context which remains valid in another. For example, given a
signature which is a child of B
and a grandchild of A
:
<A xmlns:n1="http://foo.example"> <B xmlns:n2="http://bar.example"> <Signature xmlns="http://www.w3.org/2000/09/xmldsig#"> ... <Reference URI="#signme"/> ... </Signature> <C ID="signme" xmlns="http://baz.example"/> </B> </A>
when either the element B
or the signed element C
is moved into a [SOAP12-PART1] envelope for transport:
<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/"> ... <SOAP:Body> <B xmlns:n2="http://bar.example"> <Signature xmlns="http://www.w3.org/2000/09/xmldsig#"> ... </Signature> <C ID="signme" xmlns="http://baz.example"/> </B> </SOAP:Body> </SOAP:Envelope>
The canonical form of the signature in this context will contain new
namespace declarations from the SOAP:Envelope
context,
invalidating the signature. Also, the canonical form will lack
namespace declarations it may have originally had from element A
's
context, also invalidating the signature. To avoid these problems, the
application may:
Dated references below are to the latest known or appropriate edition of the referenced work. The referenced works may be subject to revision, and conformant implementations may follow, and are encouraged to investigate the appropriateness of following, some or all more recent editions or replacements of the works cited. It is in each case implementation-defined which editions are supported.