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 specification of the IETF/W3C XML Signature Working Group follows the XML Signature Last Call and attempts to address all last call comments sent to the list and those issues discussed at the April meeting. This is the version being forward to the IESG and W3C Director for consideration as a Proposed Draft and Candidate Recommendation. During this phase we will hope to gain implementation experience over the following
Please send comments to the editors and cc: the list <email@example.com>. Publication as a Working Draft does not imply endorsement by the W3C membership or IESG. It is inappropriate to cite W3C Drafts as other than "work in progress." A list of current W3C working drafts can be found at http://www.w3.org/TR/. Current IETF drafts can be found at http://www.ietf.org/1id-abstracts.html.
Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page.
This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element.
This specification also defines other useful types including methods of referencing collections of resources, algorithms, and keying information and management.
For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types.Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See section 8.3:Check the Security Model.)
This specification uses both XML Schemas [XML-schema] and DTDs [XML]. (Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's " Declaring Elements and Attributes in an XML DTD" [Bourret].) The schema definition is presently normative.
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 [KEYWORDS]:
"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 keywords 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 XML-namespace specification [XML-ns] is described as "REQUIRED."
The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature-RD].
No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace The XML namespace [XML-ns] URI that MUST be used by implementations of this (dated) specification is:
This namespace is also used as the prefix for algorithm identifiers used by this specification. While applications MUST support XML and XML-namespaces, the use of internal entities [XML] or our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL; we use these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics. The URI in the namespace declaration above is also used as a prefix for URIs under the control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN] or Uniform Resource Locators [URL] defined by its normative external specification. If an external specification has not allocated itself a Uniform Resource Identifier we allocate an identifier under our own namespace. For instance:
SignaturePropertiesis identified and defined by this specification's namespace
Finally, in order to provide for terse namespace declarations we sometimes use XML internal entities [XML] as macros within URIs. For instance:
<?xml version='1.0'?> <!DOCTYPE Signature SYSTEM "xmldsig-core-schema.dtd" [ <!ENTITY dsig "http://www.w3.org/2000/02/xmldsig#"> ]> <Signature xmlns="&dsig;" Id="MyFirstSignature"> <SignedInfo> ...
The contributions of the following working group members to this specification are gratefully acknowledged:
As are the last call comments from the following:
This section provides an overview and examples of XML digital signature syntax. The specific processing is given in section 3: Processing Rules. The formal syntax is found in section 4: Core Signature Syntax and section 5: Additional Signature Syntax.
In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.
XML Signatures are applied to arbitrary digital content (data
objects) via an indirection. Data objects are digested, the
resulting value is placed in an element (with other information)
and that element is then digested and cryptographically signed. XML
digital signatures are represented by the
element which has the following structure (where "?" denotes zero
or one occurrence; "+" denotes one or more occurrences; and "*"
denotes zero or more occurrences):
<Signature> <SignedInfo> (CanonicalizationMethod)? (SignatureMethod) <Reference (URI=)? > (Transforms)? (DigestMethod) (DigestValue) (</Reference>)+ </SignedInfo> (SignatureValue) (KeyInfo)? (Object)* </Signature>
The content that is signed was, at the time of signature creation, referred to as an identified resource to which the specified transforms were applied.
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
Detached signatures are over external network resources or
local data objects that resides 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
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 [XML].
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/2000/02/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000119"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/02/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000119"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/02/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</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
element is the information that is actually signed. Core validation of
SignedInfo consists of two mandatory processes: validation of the
SignedInfo and validation of each
Reference digest within
Note that the algorithms used in calculating the
SignatureValue are also included in the signed information
SignatureValue element is outside
the algorithm that is used to canonicalize the
SignedInfo element before it is digested as part of the
signature operation. In the absence of a
CanonicalizationMethod element, no canonicalization is
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 and the signature design permits
arbitrary user algorithm specification.
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
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.
[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000119"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/02/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</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
Signature. (This limitation is imposed in order
to ensure that references and objects may be matched
[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
[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 and XPath. 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 we specify mandatory (and optional)
canonicalization and decoding algorithms, user specified transforms
[s09-10] DigestMethod is the algorithm applied to
the data after
Transforms is applied (if specified) to
DigestValue. The signing of the
DigestValue is what binds a resources content to the
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 (message
authentication, integrity, and/or signer authentication).
Applications that wish to represent other semantics must rely upon
other technologies, such as [XML, RDF]. For instance, an application might
foo:assuredby attribute within its own markup to
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
assurdby 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
Reference for the
SignedInfo. While the
signing application should be very careful about what it signs (it
should understand what is in the
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
Object that includes a
SignatureProperty element. (Such a signature would not only
[p02] but enveloping
[ ] ... [p01] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI=" #AMadeUpTimeStamp " [p04] Type="http://www.w3.org/2000/02/xmldsig#SignatureProperty"> [p05] <DigestMethod Algorithm="http://www.w3.org/2000/02/xmldsig#sha1"/> [p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference> [p08] </SignedInfo> [p09] ... [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp" Target=" #MySecondSignature "> [p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</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
Manifest element. This can be used by applications
to initiate special processing of some
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
Type, if given, SHOULD
Object. Note that
advisory and no action based on it or checking of its correctness
is required by core behavior.
Object is an optional element
for including data objects within the signature element or
Object can be optionally typed and/or
[p11-18] 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".)
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 follows.
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
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
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
element that includes three
Reference elements. If a
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
Reference elements as valid or take
different actions depending on which fails. To accomplish
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
References); this is wasteful and redundant. A
more efficient solution is to include many references in a single
Manifest that is then referenced from multiple
The example below includes a
Reference that signs a
Manifest found within the
[ ] ... [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/02/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/02/xmldsig#sha1"/> [m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] ... [m06] <Object> [m07] <Manifest Id="MyFirstManifest"> [m08] <Reference> [m09] ... [m10] </Reference> [m11] <Reference> [m12] ... [m13] </Reference> [m14] </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
For each data object being signed:
Transforms, as determined by the application, to the data object.
Referenceelement, including the (optional) identification of the data object, any (optional) transform elements, the digest algorithm and the
CanonicalizationMethodif required, and
SignedInfobased on algorithms specified in
Signatureelement that includes
Object(s) (if desired, encoding may be different than that used for signing),
KeyInfo(if required), and
The REQUIRED steps of core validation include (1) reference
validation, the verification of the digest contained in each
SignedInfo, and (2) the
signature validation of the signature calculated over
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.
Transformsprovided by the signer in the
Referenceelement, or it may obtain the content through other means such as a local cache.)
DigestMethodspecified in its
Reference; if there is any mismatch, validation fails.
SignedInfoelement based on the
KeyInfoor from an external source.
SignatureMethodto validate the
SignatureValueover the (optionally canonicalized)
The general structure of an XML signature is described in section 2: Signature Overview. This section provides detailed syntax of the core signature features and actual examples. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with the following XML preamble, declaration, internal entity, and simpleType:
Schema Definition: <?xml version='1.0'?> <!DOCTYPE schema SYSTEM 'http://www.w3.org/1999/XMLSchema.dtd' [ <!ENTITY dsig 'http://www.w3.org/2000/02/xmldsig#'> ]> <schema targetNamespace='&dsig;' version='0.1' xmlns='http://www.w3.org/1999/XMLSchema' xmlns:ds='&dsig;' elementFormDefault='qualified'> <!-- Basic Types Defined for Signatures --> <simpleType name='CryptoBinary' base='binary'> <encoding value='base64'/> </simpleType>
DTD: <!-- These entity declarations permit the flexible parts of Signature content model to be easily expanded --> <!ENTITY % Object.ANY '(#PCDATA|SignatureProperties|Manifest)*'> <!ENTITY % Method.ANY '(#PCDATA|HMACOutputLength)*'> <!ENTITY % Transform.ANY '(#PCDATA|XPath|XSLT)*'> <!ENTITY % Key.ANY '(#PCDATA|KeyName|KeyValue|RetrievalMethod| X509Data|PGPData|MgmtData|DSAKeyValue|RSAKeyValue)*'>
Signature element is the root element of a XML
Signature. A simple example of a complete signature follows:
Schema Definition: <element name='Signature'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:SignedInfo' minOccurs='1' maxOccurs='1'/> <element ref='ds:SignatureValue' minOccurs='1' maxOccurs='1'/> <element ref='ds:KeyInfo' minOccurs='0' maxOccurs='1'/> <element ref='ds:Object' minOccurs='0' maxOccurs='unbounded'/> </sequence> <attribute name='Id' type='ID' use='optional'/> </complexType> </element>
DTD: <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*) > <!ATTLIST Signature xmlns CDATA #FIXED 'http://www.w3.org/2000/02/xmldsig#' Id ID #IMPLIED >
SignatureValue element contains the actual
value of the digital signature; it is encoded according to the
identifier specified in
SignatureMethod. Base64 [MIME] is the encoding method for all
SignatureMethods specified within this specification. While
we specify a mandatory and optional to implement
SignatureMethod algorithms, user specified algorithms (with
their own encodings) are permitted.
Schema Definition: <element name='SignatureValue' type='ds:CryptoBinary'/>
DTD: <!ELEMENT SignatureValue (#PCDATA) >
The structure of
SignedInfo includes the
canonicalization algorithm, a signature algorithm, and one or more
SignedInfo element may contain an
optional ID attribute that will allow it to be referenced by other
signatures and objects.
Schema Definition: <element name='SignedInfo'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:CanonicalizationMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:SignatureMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/> </sequence> <attribute name='Id' type='ID' use='optional'/> </complexType> </element>
DTD: <!ELEMENT SignedInfo (CanonicalizationMethod, SignatureMethod, Reference+) > <!ATTLIST SignedInfo Id ID #IMPLIED>
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
CanonicalizationMethod is a required element that specifies the
canonicalization algorithm applied to the
element prior to performing signature calculations. This element
uses the general structure for algorithms described in section 6.1:
Algorithm Identifiers and Implementation
Requirements. The default canonicalization algorithm (applied
if this element is omitted) is Canonical XML [XML-C14N].
Alternatives, such as the minimal canonicalization algorithm (the CRLF and charset normalization specified in section 6.5.1: Minimal Canonicalization), may be explicitly specified but are NOT REQUIRED. Consequently, their use may not interoperate with other applications that do no support the specified algorithm (see section 7: XML Canonicalization and Syntax Constraint Considerations). Security issues may also arise in the treatment of entity processing and comments if minimal or other non-XML aware canonicalization algorithms are not properly constrained (see section 8.2: Only What is "Seen" Should be Signed).
We RECOMMEND that resource constrained applications that do not implement the Canonical XML [XML-C14N] transform and instead choose minimal canonicalization (or some other form) are implemented to generate Canonical XML as their output serialization to easily mitigate some of these interoperability and security concerns. For instance, such an implementation SHOULD (at least) generate standalone XML instances [XML].
Schema Definition: <element name='CanonicalizationMethod'> <complexType content='elementOnly'> <any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element>
DTD: <!ELEMENT CanonicalizationMethod %Method.ANY; > <!ATTLIST CanonicalizationMethod Algorithm CDATA #REQUIRED >
SignatureMethod 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
Schema Definition: <element name='SignatureMethod'> <complexType content='elementOnly'> <any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element>
DTD: <!ELEMENT SignatureMethod %Method.ANY; > <!ATTLIST SignatureMethod Algorithm CDATA #REQUIRED >
Reference 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
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
Schema Definition: <element name='Reference'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:Transforms' minOccurs='0' maxOccurs='1'/> <element ref='ds:DigestMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:DigestValue' minOccurs='1' maxOccurs='1'/> </sequence> <attribute name='Id' type='ID' use='optional'/> <attribute name='URI' type='uriReference' use='optional'/> <attribute name='Type' type='uriReference' use='optional'/> </complexType> </element>
DTD: <!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) > <!ATTLIST Reference Id ID #IMPLIED URI CDATA #IMPLIED Type CDATA #IMPLIED >
URI attribute identifies a data object using a
URI-Reference, as specified by RFC2396 [URI]. (Non-ASCII characters in a URI should be
represented in UTF-8 [UTF-8] as one or
more bytes, and then escaping these bytes with the URI escaping
mechanism. [XML]) Note that a null URI
URI="") is permitted and identifies the XML document
that the reference is contained within (the root element). XML
Signature applications MUST be able to parse URI syntax. We
RECOMMEND they be able to dereference URIs and null URIs in the
HTTP scheme. (See the section
3.2.1:Reference Validation for a further comment on URI
dereferencing.) Applications should be cognizant of the fact that
protocol parameter and state information, (such as a HTTP cookies,
HTML device profiles or content negotiation), may affect the
content yielded by dereferencing a URI.
[URI] permits identifiers that specify a fragment identifier via a separating number/pound symbol '#'. (The meaning of the fragment is defined by the resource's MIME type). XML Signature applications MUST support the XPointer 'bare name' [Xptr] shortcut after '#' so as to identify IDs within XML documents. The results are serialized as specified in section 6.6.3:XPath Filtering. For example,
Otherwise, support of other fragment/MIME types (e.g., PDF) or
XML addressing mechanisms (e.g., [XPath,
Xptr]) is OPTIONAL, though we RECOMMEND
support of [XPath]. Regardless, such
fragment identification and addressing SHOULD be given under
Transforms (not as part of the URI) so that they can be
fully identified and specified. For instance, one could reference a
fragment of a document that is encoded by using the
URI to identify the resource, and one
Transform to specify decoding, and a second to specify
an XPath selection.
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
The digest algorithm is applied to the data octets being
secured. Typically that is done by locating (possibly using the
URI if provided) the data and transforming it. If the
data is an XML document, the document is assumed to be unparsed
prior to the application of
Transforms. If there are
Transforms, then the data is passed to the digest
The optional Type attribute contains information about the type of object being signed. This is represented as a URI. For example:
The Type attribute applies to the item being pointed at, not its
contents. For example, a reference that identifies an Object
element containing a S
ignatureProperties element is
still of type
#Object. The type attribute is advisory.
No validation of the type information is required by this
Transforms element contains an ordered
Transform elements; these describe how the
signer obtained the data object that was digested. The output of
Transform (octets) serves as input to the next
Transform. The input to the first
Transform is the source data. 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 section 8.1: Only What is Signed is Secure).
Transform consists of an
Algorithm attribute and content parameters, if any,
appropriate for the given algorithm. The
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 input
resource, (see section 6.1: Algorithm
Identifiers and Implementation Requirements).
Transform may require explicit MimeType,
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
information. Such data characteristics are provided as parameters
Transform algorithm and should be described in
the specification for the algorithm.
Schema Definition: <element name='Transforms' > <complexType content='elementOnly'> <element ref='ds:Transform' minOccurs='1' maxOccurs='unbounded'/> </complexType> </element> <element name='Transform'> <complexType content='mixed'> <any minOccurs='0' maxOccurs='unbounded'/> <element name='Xpath' type='string'/> <element name='XSLT' type='string'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element>
DTD: <!ELEMENT Transforms (Transform+)> <!ELEMENT Transform %Transform.ANY; > <!ATTLIST Transform Algorithm CDATA #REQUIRED > <!ELEMENT XPath (#PCDATA) > <!ELEMENT XSLT (#PCDATA) >
Examples of transforms include but are not limited to base64
decoding [MIME], canonicalization [XML-C14N], XPath filtering [XPath], and XSLT [XSLT]. The generic definition of the
Transform element also allows application-specific transform
algorithms. For example, the transform could be a decompression
routine given by a Java class appearing as a base64 encoded
parameter to a Java
Transform algorithm. However,
applications should refrain from using application-specific
transforms if they wish their signatures to be verifiable outside
of their application domain. Section 6.6: Transform Algorithms defines the list of
DigestMethod is a required element that identifies the digest algorithm to be applied to the signed object. This element uses the general structure here for algorithms specified in section 6.1: Algorithm Identifiers and Implementation Requirements.
Schema Definition: <element name='DigestMethod'> <complexType content='elementOnly'> <any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element>
DTD: <!ELEMENT DigestMethod %Method.ANY; > <!ATTLIST DigestMethod Algorithm CDATA #REQUIRED >
DigestValue is an element that contains the encoded value of the digest. The digest is always encoded using base64 [MIME].
Schema Definition: <element name='DigestValue' type='ds:CryptoBinary'/>
DTD: <!ELEMENT DigestValue (#PCDATA) >
<!-- base64 encoded signature value -->
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 place their own key
identification and exchange semantics within this element type
through the XML-namespace facility. [XML-ns]
Schema Definition: <element name='KeyInfo'> <complexType content='elementOnly'> <choice minOccurs='1' maxOccurs='unbounded'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <element name='KeyName' type='string'/> <element ref='ds:KeyValue'/> <element ref='ds:RetrievalMethod'/> <element ref='ds:X509Data'/> <element ref='ds:PGPData'/> <element ref='ds:SPKIData'/> <element name='MgmtData' type='string' /> </choice> <attribute name='Id' type='ID' use='optional'/> </complexType> </element>
DTD: <!ELEMENT KeyInfo %Key.ANY; > <!ATTLIST KeyInfo Id ID #IMPLIED >
KeyInfo is an optional element that enables the
recipient(s) to obtain the key(s) needed to validate the signature.
If omitted, the recipient is expected to be able to identify the
key based on application context information. Multiple declarations
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 Section 4.4.2:
KeyValue and SHOULD implement Section 4.4.3:
KeyName element contains a string value which
may be used by the signer to communicate a key identifier to the
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
simple string names for keys, a key index, a distinguished name
(DN), an email address, etc.)
Schema Definition: <!-- type declared in KeyInfo -->
DTD: <!ELEMENT KeyName (#PCDATA) >
KeyValue element contains one or more public
keys that may be useful in validating the signature. Structured
formats for defining DSA (REQUIRED) and RSA (RECOMMENDED) public
keys are defined in Section 6.4:
Schema Definition: <element name='KeyValue'> <complexType content='mixed'> <choice minOccurs='1' maxOccurs='1'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <element ref='ds:DSAKeyValue'/> <element ref='ds:RSAKeyValue'/> </choice> </complexType > </element>
DTD: <!ELEMENT KeyValue %Key.ANY; >
RetrievalMethod element within
KeyInfo is used to convey a pointer to
KeyInfo-like information that is stored at a remote
location. For example, an X.509v3 certificate chain may be
published somewhere common to a number of documents; each document
can reference this chain using a single
RetrievalMethod element instead of including the entire
chain with a sequence of X509Certificate elements.
RetrievalMethod element contains three
Location contains a URI identifying
the actual object.
Method describes the process by
which the data retrieved from the
Location URI should
be converted into
KeyInfo sub-elements. The
Type sub-element describes the object type and encoding
format of the data stored at the
Schema Definition: <element name='RetrievalMethod'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element name='Location' type='uriReference' minOccurs='1' maxOccurs='1'/> <element name='Method' type='string' minOccurs='1' maxOccurs='1'/> <element ref='ds:Type' minOccurs='1' maxOccurs='1'/> </sequence> <attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element> <element name='Type' type='string'> <complexType content='mixed'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element>
DTD: <!ELEMENT RetrievalMethod (Location, Method, Type) > <!ELEMENT Location %Key.ANY; > <!ELEMENT Method %Key.ANY; > <!ELEMENT Type %Key.ANY; > <!ATTLIST Type Encoding CDATA #IMPLIED>
An X509Data element within
contains one or more identifiers of keys/X509 certificates that may
be useful for validation. Five types of
pointers are defined:
X509IssuerSerialelement, which contains an X.509 issuer distinguished name/serial number pair,
X509SubjectNameelement, which contains an X.509 subject distinguished name,
X509SKIelement, which contains an X.509 subject key identifier value.
X509Certificateelement, which contains a Base64-encoded X.509v3 certificate, and
X509CRLelement, which contains a Base64-encoded X.509v2 certificate revocation list (CRL).
Multiple declarations about a single certificate (e.g., a
MUST be grouped inside a single
multiple declarations about the same key but different certificates
(related to that single key) MUST be grouped within a single
KeyInfo element but multiple
For example, the following block contains two pointers to
certificate-A (issuer/serial number & SKI) and a single
reference to certificate-B (Subject Name):
<X509Data> <X509IssuerSerial> <X509IssuerName>My CA for Certificate A</X509IssuerName> <X509SerialNumber>12345678</X509SerialNumber> </X509IssuerSerial> <X509SKI>31d97bd7</X509SKI> </X509Data> <X509Data> <X509SubjectName>Subject of Certificate A</X509SubjectName> </X509Data>
Schema Definition: <element name='X509Data'> <complexType content='elementOnly'> <choice minOccurs='1' maxOccurs='unbounded'> <sequence minOccurs='1' maxOccurs='1'> <choice minOccurs='1' maxOccurs='1'> <element ref='ds:X509IssuerSerial'/> <element name='X509SKI' type='string'/> <!-- should this be binary? --> <element name='X509SubjectName' type='string'/> </choice> </sequence> <element name='X509Certificate' type='ds:CryptoBinary' minOccurs='0' maxOccurs='unbounded'/> <element name='X509CRL' type='ds:CryptoBinary' minOccurs='0' maxOccurs='unbounded'/> </choice> </complexType> </element> <element name='X509IssuerSerial'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element name='X509IssuerName' type='string' minOccurs='1' maxOccurs='1'/> <element name='X509SerialNumber' type='string' minOccurs='1' maxOccurs='1'/> </sequence> </complexType> </element>
DTD: <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName), X509Certificate*, X509CRL*)> <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) > <!ELEMENT X509IssuerName (#PCDATA) > <!ELEMENT X509SubjectName (#PCDATA) > <!ELEMENT X509SerialNumber (#PCDATA) > <!ELEMENT X509SKI (#PCDATA) > <!ELEMENT X509Certificate (#PCDATA) > <!ELEMENT X509CRL (#PCDATA) >
PGPData element within
used to convey information related to PGP public key pairs and
signatures on such keys. The
PGPKeyID's value is a
string containing a standard PGP public key identifier as defined
in Section 11.2 of [PGP]. The
PGPKeyPacket contains a base64-encoded Key Material Packet
as defined in Section 5.5 of [PGP]. Other
sub-types of the
PGPData element may be defined by the
OpenPGP working group.
Schema Definition: <element name='PGPData'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/> <element name='PGPKeyPacket' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </sequence> </complexType> </element>
DTD: <!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?) > <!ELEMENT PGPKeyPacket (#PCDATA) > <!ELEMENT PGPKeyID (#PCDATA) >
SPKIData element within
used to convey information related to SPKI public key pairs,
certificates and other SPKI data. The content of this element type
is open and can be defined elsewhere.
Schema Definition: <element name='SPKIData'> <complexType content='elementOnly'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> </complexType> </element>
DTD: <!ELEMENT SPKIData (#PCDATA) >
MgmtData element within
a string value used to convey in-band key distribution or agreement
data. For example, DH key exchange, RSA key encryption, etc.
Schema Definition: <!-- type declared in KeyInfo -->
DTD: <!ELEMENT MgmtData (#PCDATA)>
Type="http://www.w3.org/2000/02/xmldsig#Object"(this can be used within a
Referenceelement 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
MimeType attribute is an optional attribute
which describes the data within the
Object. This is a
string with values defined by [MIME]. For
example, if the
Object contains XML, the
MimeType could be text/xml. This attribute is purely
advisory; no validation of the
MimeType information is
required by this specification.
Id is commonly referenced
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' > <complexType content='mixed'> <element ref='ds:Manifest' minOccurs='1' maxOccurs='unbounded'/> <any namespace='##any' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Id' type='ID' use='optional'/> <attribute name='MimeType' type='string' use='optional'/> <!-- add a grep facet --> <attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element>
DTD: <!ELEMENT Object %Object.ANY; > <!ATTLIST Object Id ID #IMPLIED MimeType CDATA #IMPLIED Encoding CDATA #IMPLIED >
This section describes the optional to implement
SignatureProperties elements and
describes the handling of XML processing instructions and comments.
With respect to the elements
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
Type="http://www.w3.org/2000/02/xmldsig#Manifest"(this can be used within a
Referenceelement to identify the referent's type)
Manifest element provides a list of
References. 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
will be checked by the core signature validation behavior. The
digests within such a
Manifest are checked at
application discretion. If a
Manifest is referenced
Manifest, even the overall digest of this
two level deep
Manifest might not be checked.
Schema Definition: <element name='Manifest'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/> </sequence> <attribute name='Id' type='ID' use='optional'/> </complexType> </element>
DTD: <!ELEMENT Manifest (Reference+) > <!ATTLIST Manifest Id ID #IMPLIED >
Type="http://www.w3.org/2000/02/xmldsig#SignatureProperty"(this can be used within a
Referenceelement to identify the referent's type)
Additional information items concerning the generation of the
signature(s) can be placed in a
element (i.e., date/time stamp or the serial number of
cryptographic hardware used in signature generation).
Schema Definition: <element name='SignatureProperties'> <complexType content='elementOnly'> <element ref='ds:SignatureProperty' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Id' type='ID' use='optional'/> </complexType> </element> <element name='SignatureProperty'> <complexType content='mixed'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Target' type='uriReference' use='required'/> <attribute name='Id' type='ID' use='optional'/> </complexType> </element>
DTD: <!ELEMENT SignatureProperties (SignatureProperty+) > <!ATTLIST SignatureProperties Id ID #IMPLIED > <!ELEMENT SignatureProperty %Object.ANY; > <!ATTLIST SignatureProperty Target CDATA #REQUIRED Id ID #IMPLIED >
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
CanonicalizationMethods specified within this specification
retain PIs. When a PI is part of content that is signed (e.g.,
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
SignedInfo or any other referenced
XML, they will be signed. Consequently, 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
This section identifies algorithms used with the XML digital
signature standard. 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
Algorithms are identified by URIs that appear as an attribute to
the element that identifies the algorithms' role
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
Explicit additional parameters to an algorithm appear as content
elements within the algorithm role element. Such parameter elements
have a descriptive element name, which is frequently algorithm
specific, and MUST be in the XML Signature namespace or an
algorithm specific namespace.
This specification defines a set of algorithms, their URIs, and requirements for implementation. Requirements are specified over implementation, not over requirements for signature use. Furthermore, the mechanism is extensible, alternative algorithms may be used by signature applications.
(Note that the normative identifier is the complete URI in the table though they are frequently abbreviated in XML syntax (e.g., "&dsig;base64").)
|Algorithm Type||Algorithm||Requirements||Algorithm URI|
Signatureelement from the calculation of the signature when the signature is within the document that it is being signed. This MAY be implemented via the RECOMMENDED XPath specification specified in 6.6.4: Enveloped Signature Transform; it MUST have the same effect as that specified by the XPath specification.
Only one digest algorithm is defined herein. However, it is expected that one or more additional strong digest algorithms will be developed in connection with the US Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptography have cast doubt on its strength.
The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:
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:
MAC algorithms take two implicit parameters, their keying
material determined from
KeyInfo and the octet stream
CanonicalizationMethod. MACs and signature
algorithms are syntactically identical but a MAC implies a shared
algorithm (RFC2104 [HMAC]) takes the
truncation length in bits as a parameter; if the parameter is not
specified then all the bits of the hash are output. An example of
<SignatureMethod Algorithm="&dsig;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
Schema Definition: <element name='HMACOutputLength' type='integer' minOccurs='0' maxOccurs='1'/>
DTD: <!ELEMENT HMACOutputLength (#PCDATA)>
Signature algorithms take two implicit parameters, their keying
material determined from
KeyInfo and the octet stream
CanonicalizationMethod. Signature and MAC
algorithms are syntactically identical but a signature implies
public key cryptography.
The DSA algorithm [DSS] takes no explicit
parameters. An example of a DSA
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. Integer to octet-stream conversion must be done according to the I2OSP operation defined in the RFC 2437 [PKCS1] specification with a k 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
DSA key values have the following set of fields: P, Q, G and Y are mandatory when appearing as a key value, J, seed and pgenCounter are optional but SHOULD be present. (The seed and pgenCounter fields MUST appear together or be absent). All parameters are encoded as base64 values.
Schema:<element name='DSAKeyValue'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element name='P' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Q' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='G' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Y' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='J' type='ds:CryptoBinary' minOccurs='0' maxOccurs='1'/> </sequence> <sequence minOccurs='0' maxOccurs='1'> <element name='Seed' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='PgenCounterQ' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </sequence> </complexType> </element>
DTD:<!ELEMENT DSAKeyValue (P, Q, G, Y, J?, (Seed, PgenCounter)?) > <!ELEMENT P (#PCDATA) > <!ELEMENT Q (#PCDATA) > <!ELEMENT G (#PCDATA) > <!ELEMENT Y (#PCDATA) > <!ELEMENT J (#PCDATA) > <!ELEMENT Seed (#PCDATA) > <!ELEMENT PgenCounter (#PCDATA) >
Arbitrary-length integers (e.g. "bignums" such as RSA modulii) are represented 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 even number of bytes). If the bitstring contains entire leading bytes that are zero, these are removed (so the high-order byte is always non-zero). This octet string is then Base64 encoded. (The conversion from integer to octet string is equivalent to IEEE P1363's I2OSP [P1363] with minimal length).
The expression "RSA algorithm" as used in this draft refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. (Note that support for PKCS1 Version 2 is planned as soon as that standard is finalized). The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:
SignatureValue content for an RSA signature
shall be the base64 encoding of the octet string. Signatures are
interpreted as unsigned integers. A signature MAY contain a
pre-pended algorithm object identifier, but the availability of an
ASN.1 parser and recognition of OIDs is not required of a signature
<SignatureValue>F8aupsHjmbIApjAH4AVYjcsmQkXChyjGYleVJe1KLAmmXWww 3PqkDPUMojithbwbVWVJJ0UhdT407nl0fBrohvkunDq8gzfGkjvO+zDJws1HkRtZ vl1IIBLVWf/qgcLJOgid/2A66niC20GwKcJgIp3o1L+6l7LlSKiZ/CkgDO4= </SignatureValue>
RSA key values have two fields: Modulus and Exponent
<RSAKeyValue> <Exponent>AQAB</Exponent> <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV 5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U= </Modulus> </RSAKeyValue>
Schema:<element name='RSAKeyValue'> <complexType content='elementOnly'> <element name='Modulus' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Exponent' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </complexType> </element>
DTD:<!ELEMENT RSAKeyValue (Modulus, Exponent) > <!ELEMENT Modulus (#PCDATA) > <!ELEMENT Exponent (#PCDATA) >
Canonicalization algorithms take one implicit parameter when
they appear as a
CanonicalizationMethod within the
An example of a minimal canonicalization element is:
The minimal canonicalization algorithm:
An example of an XML canonicalization element is:
The normative specification of Canonical XML is [XML-C14N].
Transform algorithm has a single implicit
parameters: an octet stream from the
Reference or the
output of an earlier
Application developers are strongly encouraged to support all transforms listed in this section 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 can be used as a
The normative specification for base 64 decoding transforms is
[MIME]. The base64
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.
XPath transform output is the result of applying an XPath
expression to an input string. The XPath expression appears in a
parameter element named
XPath. The input string is
equivalent to the result of dereferencing the URI attribute of the
Reference element containing the XPath transform,
then, in sequence, applying all transforms that appear before the
XPath transform in the
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. The XPath expression can
be created such that it includes all elements except those meeting
specific criteria. It is the responsibility of the XPath expression
author to ensure that all necessary information has been included
in the output such that modification of the excluded information
does not affect the interpretation of the output in the application
context. One simple example of this is the omission of an enveloped
signature from a
The XPath transform establishes the following evaluation context
for the XPath expression given in the
The additional function
this() is defined as
Function: node-set this()
The this function returns a node-set containing the single node that directly bears the XPath expression. The node could be of any type capable of directly bearing text, especially text and attribute. This expression results in an error if the containing XPath expression does not appear in an XML document.
An XML processor is used to read the input XML document and produce a parse tree capable of being used as the initial context node for the XPath evaluation, as described in the previous section. If the input is not a well-formed XML document, then the XPath transform must throw an exception.
Validating and non-validating XML processors only behave in the same way (e.g. with respect to attribute value normalization and entity reference definition) until an external reference is encountered. If the XPath transform implementation uses a non-validating processor, and it encounters an external reference in the input document, then an exception must be thrown to indicate that the necessary algorithm is unavailable (The XPath transform cannot simply generate incorrect output since many applications distinguish an unverifiable signature from an invalid signature).
As a result of reading the input with an XML processor, linefeeds are normalized, attribute values are normalized, CDATA sections are replaced by their content, and entity references are recursively replaced by substitution text. In addition, consecutive characters are grouped into a single text node.
The XPath implementation is expected to convert the information in the input XML document and the XPath expression string to the UCS character domain prior to making any comparisons such that the result of evaluating the expression is equivalent regardless of the initial encoding of the input XML document and XPath expression.
The namespace prefix of each node appearing in the original document must be preserved by the XML processor used by the XPath transform implementation. This is necessary in order to produce the serialized result.
Although a node-set is unordered, based on the expression evaluation requirements of the XPath function library, the document order position of each node must be available, except for the attribute and namespace axes. The XPath transform imposes no order on attribute and namespace nodes during XPath expression evaluation, and expressions based on attribute or namespace node position are not interoperable. The XPath transform does define an order for namespace and attribute nodes during serialization.
For the purpose of serialization, the XPath transform imposes a document order on namespace and attribute nodes. An element's namespace and attribute nodes have a document order position greater than the element but less than any child node of the element. Namespace nodes have a lesser document order position than attribute nodes. An element's namespace nodes are sorted lexicographically by local name (the default namespace node, if one exists, has no local name and is therefore lexicographically least). An element's attribute nodes are sorted lexicographically with namespace URI as the primary key and local name as the secondary key (an empty namespace URI is lexicographically least). Lexicographic comparison is based on the UCS codepoint values, which is equivalent to lexical ordering based on UTF-8.
A node-set is converted into a string by generating the representative text for each node in the node-set in ascending document order. No node is processed more than once. Note that processing an element node E includes the processing of all members of the node-set for which E is an ancestor. Therefore, directly after the representative text for E is generated, E and all nodes for which E is an ancestor are removed from the node-set (or some logically equivalent operation occurs such that the node-set's next node in document order has not been processed).
The method of text generation is dependent on the node type and given in the following list:
xmlns="". Then, generate the representative text for each namespace node that is in the element's namespace axis and in the node-set, except omit the namespace node with local name
xml, which defines the
xmlprefix, if its string value is
&, all double quote characters with
", and all whitespace characters (#x9, #xA, #xD, and #x20) with character references, except for #x20 characters with no preceding #x20. When whitespace characters are replaced, the character references are written in uppercase hexadecimal with no leading zeroes (for example, #xD is represented by the character reference
&, all open angle brackets (<) are replaced by
<, and all #xD characters are replaced by
. If the string value is empty, then the leading space is not added.
The QName of a node is either the local name if the namespace prefix string is empty or the namespace prefix, a colon, then the local name of the element. The namespace prefix used in the QName MUST be the same one which appeared in the input document.
The result of the XPath expression is a string, boolean, number, or node-set. If the result of the XPath expression is a string, then the string converted to UTF-8 is the output of the XPath transform. If the result is a boolean or number, then the XPath transform output is computed by converting the boolean or number to a string as if by a call to the XPath string() function, then converting to UTF-8. If the result of the XPath expression is a node-set, then the XPath transform result is computed by serializing the node-set with a UTF-8 encoding.
As an example, consider creating an enveloped signature (a
Signature element that is a descendant of an element
being signed). However, the elements within the signature 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
Transform that omits all
Signature elements and their descendants. For example,
<Reference URI=""> <Transforms> <Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116"> <XPath>(//. | //@* | //namespace::*)[not(ancestor-or-self::Signature)] </XPath> </Transform> </Transforms> ... </Reference>
(//. | //@* | //namespace::*)
means that all nodes in the entire parse tree starting at the root
node are candidates for the result node-set. For each node
candidate, the node is included in the resultant node-set if and
only if the node test (the boolean expression in the square
brackets) evaluates to "true" for that node. The node test returns
true for all nodes except nodes that either have or have an
ancestor with a tag of
A more elegant solution uses the
this function to omit only the
containing the XPath Transform, thus allowing enveloped signatures
to sign other signatures. In the example above, use the following
expression as the content of the
(//. | //@* | //namespace::*)
count(ancestor-or-self::Signature | this()/ancestor::Signature) > count(ancestor-or-self::Signature)
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
Signature element ancestors does not include
Signature element containing the XPath
expression (the union does not produce a larger set if the
Signature element is in the node-set given
It is RECOMMENDED that the XPath be constructed such that the result of this operation is a well-formed XML document. This should be the case if root element of the input resource is included by the XPath (even if a number of its descendant nodes are omitted by the XPath expression). It is also RECOMMENDED that nodes should not be omitted from the input if they affect the interpretation of the output nodes in the application context. The XPath expression author is responsible for this since the XPath expression author knows the application context.
An enveloped signature transform T removes the
Signature element containing T
from the digest calculation of the
containing T. The entire string of characters used by
an XML processor to match the
Signature with the XML
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 expression:
(//. | //@* | //namespace::*)
count(ancestor-or-self::Signature | this()/ancestor::Signature) > count(ancestor-or-self::Signature)
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's serialization method.
The Transform element contains a single parameter child
XSLT, whose content MUST conform to the
XSL Transforms [XSLT]
language syntax. The processing rules for the XSLT transform are
stated in the XSLT specification [XSLT].
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 data object 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
since they, and possibly an encompassing XML document, will be
processed as XML.
Similarly, these considerations apply to
if those elements have been digested, their
DigestValue is to be checked, and they are being processed
The kinds of changes in XML that may need to be canonicalized can be divided into three categories. There are those related to the basic [XML], as described in 7.1 below. There are those related to [DOM], [SAX], or similar processing as described in 7.2 below. And, third, there is the possibility of coded character set conversion, such as between UTF-8 and UTF-16, both of which all [XML] compliant processors are required to support.
Any canonicalization algorithm should yield output in a specific fixed coded character set. For both the minimal canonicalization defined in this specification, the W3C Canonical XML [XML-C14N], and the 2000 Canonical XML [XML-C14N-a], that coded character set is UTF-8. 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. Neither the minimal canonicalization nor the 2000 Canonical XML [XML-C14N-a] algorithms provide character normalization. We RECOMMEND that signature applications produce XML content in Normalized Form C [NFC] and check that any XML being consumed is in that form as well (if not, signatures may consequently fail to validate).
XML 1.0 [XML] 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 (5C) depend on specific schema,
DTD, or similar declarations. In the general case, such
declarations will not be available to or used by the signature
verifier. Thus, to interoperate between different XML
implementations, the following syntax contraints MUST be observed
when generating any signed material to be processed as XML,
In addition to the canonicalization and syntax constraints discussed above, many XML applications use the Document Object Model [DOM] 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 syntax constraints given in section 7.1 be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octect stream that was signed.
The XML Signature specification provides a very flexible digital signature mechanism. Implementors must give consideration to their application threat models and to the following factors.
A requirement of this specification is to permit signatures to
"apply to a part or totality of a XML document." (See section
3.1.3 of [XML-Signature-RD].)
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 [XPath] to exclude those portions the user
needs to change.
Transforms may be arbitrarily
specified and may include canonicalization instructions or even
XSLT transformations. Of course, signatures over such a derived
document do not secure any information discarded by the
Furthermore, 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 application 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.
If signing is intended to convey the judgment or consent of an automated mechanism or person, then it is normally necessary to secure as exactly as practical the information that was presented to that mechanism or person. 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.
Also 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.
This standard specifies 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 standard 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 validaters 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.
Objectdesignates 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 [XML]. The term XML document is used to describe data objects which conform to the XML specification [XML].
Objectelement is merely one type of digital data (or document) that can be signed via a
Signatureelement, and can be identified via a
URIor 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
Signatureand data object reside within the same XML document but are sibling elements.
Objectelement of the signature itself. The
Object(or its content) is identified via a
URIfragment idenitifier or transform).
Reference, matches its specified
SignatureValuematches the result of processing
SignatureMethodas specified in section 3.2.
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Joseph M. Reagle Jr., W3C
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