Copyright © 2010 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.
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 second Last Call Working Draft of XML Signature 1.1. The only substantive changes since the previous Last Call Working Draft are:
The addition of the KeyInfoReference
element in section
4.5.10 ("The KeyInfoReference
Element"),
also referred to in section
4.5.3 ("The RetrievalMethod
Element"),
A correction to replace Agreement
with
DerivedKey
in section 4.5.8 ("XML Encryption
EncryptedKey
and DerivedKey
Elements"),
Updates to references.
A disposition of comments is available.
At the time of this publication, the most recent W3C Recommendation of XML Signature 1 is the 10 June 2008 XML Signature (Second Edition) Recommendation. Please review differences between the previous and this Working Draft, and differences between the previous XML Signature Recommendation and this Working Draft; a detailed explanation of changes is also available.
Conformance-affecting changes against this previous recommendation mainly affect the set of mandatory to implement cryptographic algorithms, including Elliptic Curve DSA (and mark-up for corresponding key material), and additional hash algorithms.
This Last Call Working Draft 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. The Working
Group may request transition
to Candidate Recommendation with this feature marked as "at
risk". If issues about deployment of this feature are raised
during Candidate Recommendation, the group may elect to make
this feature optional.
The Working Group is, in parallel to this work, developing requirements and designs for a more radically different version 2 of XML Signature.
This document was published by the XML Security Working Group as a Last Call Working Draft. 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). The Last Call period ends 10 June 2010. All feedback is welcome.
Publication as a Working Draft 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 is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.
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.
ds:CryptoBinary
Simple TypeSignature
elementSignatureValue
ElementSignedInfo
Element
KeyInfo
Element
KeyName
ElementKeyValue
Element
RetrievalMethod
ElementX509Data
Element
PGPData
ElementSPKIData
ElementMgmtData
ElementEncryptedKey
and
DerivedKey
ElementsDEREncodedKeyValue
ElementKeyInfoReference
ElementObject
ElementThis 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 Security Considerations (section 8).
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 Check the Security Model, section 8.2.)
This specification provides a normative XML Schema [XMLSCHEMA-1], [XMLSCHEMA-2]. The full normative grammar is defined by the XSD schema and the normative text in this specification. The standalone XSD schema file is authoritative in case there is any disagreement between it 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] and the XML Security 1.1 Requirements document [XMLSEC11-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#"> |
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
The http://www.w3.org/2000/09/xmldsig#
(dsig:
) namespace was introduced in the first
edition of this specification. This version does not coin any
new elements or algorithm identifiers in that namespace;
instead, the http://www.w3.org/2009/xmldsig11#
(dsig11:
) 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 (Staff contact, Editor), Ed Simon, Chris Solc, John Wray, Kelvin Yiu (Editor).
The Working Group thanks Makoto Murata for assistance with the RELAX NG schemas.
This section provides an overview and examples of XML digital signature syntax. The specific processing is given in Processing Rules (section 3). The formal syntax is found in Core Signature Syntax (section 4) and Additional Signature Syntax (section 5).
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].
Signature
, SignedInfo
,
Methods
, and Reference
s)The following example is a detached signature of the content of the HTML4 in XML specification.
[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
[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 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".)
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 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>
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
.
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 3.1.2 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.
SignedInfo
element with
SignatureMethod
,
CanonicalizationMethod
and
Reference
(s).SignatureValue
over SignedInfo
based on algorithms specified in
SignedInfo
.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).
The required
steps of core
validation include (1) reference validation, the
verification of the digest contained in each
Reference
in SignedInfo
, and (2)
the cryptographic signature validation of the
signature calculated over 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.
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 (section 4.4.1)) and that it Sees
What is Signed, which is the canonical form.
Note, After a Signature
element has been
created in 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.
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.
The general structure of an XML signature is described in Signature Overview (section 2). 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 [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">
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 could 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 the
canonicalization algorithm, a signature algorithm, and one or
more references. The SignedInfo
element may
contain an optional ID attribute that will allow 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 Algorithm Identifiers and
Implementation Requirements (section 6.1).
Implementations must
support the required canonicalization algorithms.
Alternatives to the required canonicalization algorithms (section 6.5), such as Canonical XML with Comments (section 6.5.1) 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 XML Canonicalization and Syntax Constraint Considerations, section 7). 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 8.1.2: 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 massively 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.
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>
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 6.1: 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.
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 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 the Reference Validation section (section 3.2.1) for further information on reference processing.)
If the URI
attribute is omitted
altogether, 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. This attribute may be omitted from at most one
Reference
in any particular
SignedInfo
, or Manifest
.
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 is 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 Reference Generation Model (section 3.1.1) 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 Transforms (section 4.4.3.4).)
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
Same-Document
URI-References (section 4.4.3.3).) 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 The Reference Processing Model (section 4.4.3.2).
Transforms
ElementThe optional Transforms
element contains
an ordered list of Transform
elements; these
describe how the signer obtained the data object that was
digested. 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. When transforms are
applied the signer is not signing the native (original)
document but the resulting (transformed) document. (See
Only What is Signed is Secure
(section 8.1.1).)
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. (See Algorithm Identifiers
and Implementation Requirements (section 6.1).)
As described in The Reference Processing Model (section 4.4.3.2), 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. Transform
Algorithms (section 6.6) defines the list of standard
transformations.
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>
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 Algorithm Identifiers and Implementation
Requirements (section 6.1).
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 the Reference Processing Model (section 4.4.3.2). If the result of URI dereference and application of transforms is an octet stream, then no conversion occurs (comments might be present if the Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream.
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>
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.
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 4.5.2) and
should implement
RetrievalMethod
(section
4.5.3).
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 Signature Algorithms (section 6.4). 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 is calculatable 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>
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 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 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
PublicKey
content to preserve printed page
width.
Domain parameters can be encoded explicitly using the
ECParameters element or by reference using the NamedCurve
element. A named curve is specified through the
URN
attribute. For named curves that are
identified by OIDs, such as those defined in [RFC3279][RFC4055], and
[SECG1], the OID should be encoded according to
[URN-OID]. Conformant applications
must support the
NamedCurve element and the 256-bit prime field curve as
identified by the OID
1.2.840.10045.3.1.7
.
The PublicKey element contains a 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 ECParameters element consists of the following subelements. Note these definitions are based on the those described in [RFC3279].
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>
Prime fields are described by a single subelement 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 Reference
's URI (section 4.4.3.1)
and The Reference
Processing Model (section 4.4.3.2) 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 4.5) 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. In such cases,
use of KeyInfoReference
is encouraged instead,
see section 4.5.10.
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:
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:OCSPResponse
element
contains a base64-encoded OCSP response in DER
encoding. [OCSP].Any X509IssuerSerial
, X509SKI
,
and X509SubjectName
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
,
and X509SubjectName
elements that relate to
the same key but different certificates must be grouped within a single
KeyInfo
but may occur in multiple X509Data
elements.
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. Such deployments should avoid
schema-validating the X509IssuerSerial
element
as XML Schema validators may not support integer types with
decimal data exceeding 18 decimal digits [XML-schema].
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"/> --> <!-- OCSPResponse 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 permitsX509Data
to be empty; this is precluded by the text inKeyInfo
Element (section 4.5) 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" />
PGPData
ElementType="http://www.w3.org/2000/09/xmldsig#PGPData
"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
"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
"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.
Section 4.5.8 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.
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 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 4.5.2 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
DEREncodedKeyValue
element is not a child of
KeyValue
. The DEREncodedKeyValue
element is also not a child of the X509Data
element, as the keys represented by
DEREncodedKeyValue
may not have X.509
certificates associated with them (a requirement for
X509Data
).
KeyInfoReference
ElementA 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 KeyInfoReference
element instead of
including the entire chain with a sequence of
X509Certificate
elements repeated in multiple
places.
KeyInfoReference
uses the same syntax and
dereferencing behavior as Reference
's
URI
(section 4.4.3.1) and the Reference
Processing Model (section 4.4.3.2) except that there are no
child elements and the presence of the URI
attribute is mandatory.
The result of dereferencing a
KeyInfoReference
must be a KeyInfo
element, or an
XML document with a KeyInfo
element as the
root.
Note: The 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
which require normative type and encoding information for
signature validation should specify Transforms
with
well defined resulting types and/or encodings.
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 (easy for XML documents) or a transform must be used
to remove the Object
tags (likely where the data
object is non-XML). 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
CanonicalizationMethod
s 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 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 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.
ECDSAwithSHA256
marked as "at risk". If issues
about deployment of this feature are raised during
Candidate Recommendation, the group may elect to make this
feature optional.
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.
* 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.
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.
Note: Use of SHA-256 is strongly recommended over SHA-1 because recent advances in cryptanalysis (see e.g. [SHA-1-Analysis], [SHA-1-Collisions] ) have cast doubt on the long-term collision resistance of SHA-1.
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>
In Special Publication SP 800-57 Part 1 [SP800-57], NIST recommends using at least 2048-bit public keys for securing information beyond 2010 (and 3072-bit keys for securing information beyond 2030). All conforming implementations of XML Signature 1.1 must support RSA signature generation and verification with public keys at least 2048 bits in length. RSA public keys of 1024 bits or less should not be used for signatures that will be verified beyond 2010. XML Signature 1.1 implementations should use at least 2048-bit keys for all signatures, and should use at least 3072-bit keys for signatures that will be verified beyond 2030.
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.
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 Algorithm Identifiers and Implementation Requirements. 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
Canonicalization Algorithms
(section 6.5)) 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) taking the string-value
of the node-set. 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].
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 the XML 1.0 syntax constraints given in the previous section 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.
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 contain namespace declarations from its
ancestor context. In addition, the Canonical XML and
Canonical XML with Comments algorithms import all XML
namespace attributes (such as xml:lang
) from the
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="&foo;"> <B xmlns:n2="&bar;"> <Signature xmlns="&dsig;"> ... <Reference URI="#signme"/> ... </Signature> <C ID="signme" xmlns="&baz;"/> </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="&bar;"> <Signature xmlns="&dsig;"> ... </Signature> <C ID="signme" xmlns="&baz;"/> </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:
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.
Non-normative RELAX NG schema [RELAXNG-SCHEMA] information is available in a separate document [XMLSEC-RELAXNG].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 Core Validation (section 3.2).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.