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Network Working Group D. Eastlake 3rd Request for Comments: 3275 Motorola Obsoletes: 3075 J. Reagle Category: Standards Track W3C

                                                               D. Solo
                                                            March 2002
  (Extensible Markup Language) XML-Signature Syntax and Processing

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.
 Copyright Notice
 Copyright (c) 2002 The Internet Society & W3C (MIT, INRIA, Keio), All
 Rights Reserved.


 This document specifies XML (Extensible Markup Language) 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.

Table of Contents

 1. Introduction...................................................  3
 1.1 Editorial and Conformance Conventions.........................  4
 1.2 Design Philosophy.............................................  4
 1.3 Versions, Namespaces and Identifiers..........................  4
 1.4 Acknowledgements..............................................  6
 1.5 W3C Status....................................................  6
 2. Signature Overview and Examples................................  7
 2.1 Simple Example (Signature, SignedInfo, Methods, and References) 8
 2.1.1 More on Reference...........................................  9
 2.2 Extended Example (Object and SignatureProperty)............... 10
 2.3 Extended Example (Object and Manifest)........................ 12
 3.0 Processing Rules.............................................. 13
 3.1 Core Generation............................................... 13
 3.1.1 Reference Generation........................................ 13

Eastlake, et al. Standards Track [Page 1] RFC 3275 XML-Signature Syntax and Processing March 2002

 3.1.2 Signature Generation........................................ 13
 3.2 Core Validation............................................... 14
 3.2.1 Reference Validation........................................ 14
 3.2.2 Signature Validation........................................ 15
 4.0 Core Signature Syntax......................................... 15
 4.0.1 The ds:CryptoBinary Simple Type............................. 17
 4.1 The Signature element......................................... 17
 4.2 The SignatureValue Element.................................... 18
 4.3 The SignedInfo Element........................................ 18
 4.3.1 The CanonicalizationMethod Element.......................... 19
 4.3.2 The SignatureMethod Element................................. 21
 4.3.3 The Reference Element....................................... 21 The URI Attribute......................................... 22 The Reference Processing Model............................ 23 Same-Document URI-References.............................. 25 The Transforms Element.................................... 26 The DigestMethod Element.................................. 28 The DigestValue Element................................... 28
 4.4 The KeyInfo Element........................................... 29
 4.4.1 The KeyName Element......................................... 31
 4.4.2 The KeyValue Element........................................ 31 The DSAKeyValue Element................................... 32 The RSAKeyValue Element................................... 33
 4.4.3 The RetrievalMethod Element................................. 34
 4.4.4 The X509Data Element........................................ 35
 4.4.5 The PGPData Element......................................... 38
 4.4.6 The SPKIData Element........................................ 39
 4.4.7 The MgmtData Element........................................ 40
 4.5 The Object Element............................................ 40
 5.0 Additional Signature Syntax................................... 42
 5.1 The Manifest Element.......................................... 42
 5.2 The SignatureProperties Element............................... 43
 5.3 Processing Instructions in Signature Elements................. 44
 5.4 Comments in Signature Elements................................ 44
 6.0 Algorithms.................................................... 44
 6.1 Algorithm Identifiers and Implementation Requirements......... 44
 6.2 Message Digests............................................... 46
 6.2.1 SHA-1....................................................... 46
 6.3 Message Authentication Codes.................................. 46
 6.3.1 HMAC........................................................ 46
 6.4 Signature Algorithms.......................................... 47
 6.4.1 DSA......................................................... 47
 6.4.2 PKCS1 (RSA-SHA1)............................................ 48
 6.5 Canonicalization Algorithms................................... 49
 6.5.1 Canonical XML............................................... 49
 6.6 Transform Algorithms.......................................... 50
 6.6.1 Canonicalization............................................ 50
 6.6.2 Base64...................................................... 50

Eastlake, et al. Standards Track [Page 2] RFC 3275 XML-Signature Syntax and Processing March 2002

 6.6.3 XPath Filtering............................................. 51
 6.6.4 Enveloped Signature Transform............................... 54
 6.6.5 XSLT Transform.............................................. 54
 7. XML Canonicalization and Syntax Constraint Considerations...... 55
 7.1 XML 1.0, Syntax Constraints, and Canonicalization............. 56
 7.2 DOM/SAX Processing and Canonicalization....................... 57
 7.3 Namespace Context and Portable Signatures..................... 58
 8.0 Security Considerations....................................... 59
 8.1 Transforms.................................................... 59
 8.1.1 Only What is Signed is Secure............................... 60
 8.1.2 Only What is 'Seen' Should be Signed........................ 60
 8.1.3 'See' What is Signed........................................ 61
 8.2 Check the Security Model...................................... 62
 8.3 Algorithms, Key Lengths, Certificates, Etc.................... 62
 9. Schema, DTD, Data Model, and Valid Examples.................... 63
 10. Definitions................................................... 63
 Appendix: Changes from RFC 3075................................... 67
 References........................................................ 67
 Authors' Addresses................................................ 72
 Full Copyright Statement.......................................... 73

1. Introduction

 This document specifies XML syntax and processing rules for creating
 and representing digital signatures.  XML Signatures can be applied
 to any digital content (data object), including XML.  An XML
 Signature may be applied to the content of one or more resources.
 Enveloped or enveloping signatures are over data within the same XML
 document as the signature; detached signatures are over data external
 to the signature element.  More specifically, this specification
 defines an XML signature element type and an XML signature
 application; conformance requirements for each are specified by way
 of schema definitions and prose respectively.  This specification
 also includes other useful types that identify methods for
 referencing collections of resources, algorithms, and keying and
 management information.
 The XML Signature is a method of associating a key with referenced
 data (octets); it does not normatively specify how keys are
 associated with persons or institutions, nor the meaning of the data
 being referenced and signed.  Consequently, while this specification
 is an important component of secure XML applications, it itself is
 not sufficient to address all application security/trust concerns,
 particularly with respect to using signed XML (or other data formats)
 as a basis of human-to-human communication and agreement.  Such an
 application must specify additional key, algorithm, processing and
 rendering requirements.  For further information, please see Security
 Considerations (section 8).

Eastlake, et al. Standards Track [Page 3] RFC 3275 XML-Signature Syntax and Processing March 2002

1.1 Editorial and Conformance Conventions

 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 8.3.)
 This specification provides an XML Schema [XML-schema] and DTD [XML].
 The schema definition is normative.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 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-ns] is
 described as "REQUIRED."

1.2 Design Philosophy

 The design philosophy and requirements of this specification are
 addressed in the XML-Signature Requirements document [XML-Signature-

1.3 Versions, Namespaces and Identifiers

 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:

Eastlake, et al. Standards Track [Page 4] RFC 3275 XML-Signature Syntax and Processing March 2002

 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:
 SignatureProperties is identified and defined by this specification's
 XSLT is identified and defined by an external URI
 SHA1 is identified via this specification's namespace and defined via
 a normative reference
    FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
    Commerce/National Institute of Standards and Technology.
 Finally, in order to provide for terse namespace declarations we
 sometimes use XML internal entities [XML] within URIs.  For instance:
    <?xml version='1.0'?>
    <!DOCTYPE Signature SYSTEM
      "xmldsig-core-schema.dtd" [ <!ENTITY dsig
      ""> ]>
    <Signature xmlns="&dsig;" Id="MyFirstSignature">

Eastlake, et al. Standards Track [Page 5] RFC 3275 XML-Signature Syntax and Processing March 2002

1.4 Acknowledgements

 The contributions of the following Working Group members to this
 specification are gratefully acknowledged:
  • Mark Bartel, Accelio (Author)
  • John Boyer, PureEdge (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, W3C (Chair, Author/Editor)
  • Ed Simon, XMLsec (Author)
  • David Solo, Citigroup (Author/Editor)
  • Petteri Stenius, DONE Information, Ltd
  • Raghavan Srinivas, Sun
  • Kent Tamura, IBM
  • Winchel Todd Vincent III, GSU
  • Carl Wallace, Corsec Security, Inc.
  • Greg Whitehead, Signio Inc.
 As are the Last Call comments from the following:
  • Dan Connolly, W3C
  • Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG.
  • Martin J. Duerst, W3C; and Masahiro Sekiguchi, Fujitsu; on

behalf of the Internationalization WG/IG.

  • Jonathan Marsh, Microsoft, on behalf of the Extensible

Stylesheet Language WG.

1.5 W3C Status

 The World Wide Web Consortium Recommendation corresponding to
 this RFC is at:

Eastlake, et al. Standards Track [Page 6] RFC 3275 XML-Signature Syntax and Processing March 2002

2. Signature Overview and Examples

 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
    <Signature ID?>
         (<Reference URI? >
      (<Object ID?>)*
 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
 attribute (signature is child).  Since a Signature element (and its
 Id 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].

Eastlake, et al. Standards Track [Page 7] RFC 3275 XML-Signature Syntax and Processing March 2002

2.1 Simple Example (Signature, SignedInfo, Methods, and References)

 The following example is a detached signature of the content of the
 HTML4 in XML specification.
  [s01] <Signature Id="MyFirstSignature"
  [s02]   <SignedInfo>
  [s03]   <CanonicalizationMethod
  [s04]   <SignatureMethod
  [s05]   <Reference
  [s06]     <Transforms>
  [s07]       <Transform
  [s08]     </Transforms>
  [s09]     <DigestMethod
  [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 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
 [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, and all examples in
 this specification, are not in canonical form.

Eastlake, et al. Standards Track [Page 8] RFC 3275 XML-Signature Syntax and Processing March 2002

 [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 may also 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.

2.1.1 More on Reference

  [s05]   <Reference
  [s06]     <Transforms>
  [s07]       <Transform
  [s08]     </Transforms>
  [s09]     <DigestMethod
  [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
 Reference in a Signature.  (This limitation is imposed in order to
 ensure that references and objects may be matched unambiguously.)

Eastlake, et al. Standards Track [Page 9] RFC 3275 XML-Signature Syntax and Processing March 2002

 [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, as
 opposed to 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 a resources content to the
 signer's key.

2.2 Extended Example (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 [XML, RDF].  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

Eastlake, et al. Standards Track [Page 10] RFC 3275 XML-Signature Syntax and Processing March 2002

 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="">
  [   ]   ...
  [p03]   <Reference URI="#AMadeUpTimeStamp"
  [p05]    <DigestMethod
  [p06]    <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue>
  [p07]   </Reference>
  [p08]  </SignedInfo>
  [p09]  ...
  [p10]  <Object>
  [p11]   <SignatureProperties>
  [p12]     <SignatureProperty Id="AMadeUpTimeStamp"
  [p13]        <timestamp xmlns="">
  [p14]          <date>19990908</date>
  [p15]          <time>14:34:34:34</time>
  [p16]        </timestamp>
  [p17]     </SignatureProperty>
  [p18]   </SignatureProperties>
  [p19]  </Object>
 [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.

Eastlake, et al. Standards Track [Page 11] RFC 3275 XML-Signature Syntax and Processing March 2002

 [p10] Object is an optional element for including data objects within
 the signature element or elsewhere.  The Object can be optionally
 typed and/or encoded.
 [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".)

2.3 Extended Example (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
 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 Signature elements.
 The example below includes a Reference that signs a Manifest found
 within the Object element.

Eastlake, et al. Standards Track [Page 12] RFC 3275 XML-Signature Syntax and Processing March 2002

  [   ] ...
  [m01]   <Reference URI="#MyFirstManifest"
  [m02]     Type="">
  [m03]     <DigestMethod
  [m04]     <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
  [m05]   </Reference>
  [   ] ...
  [m06] <Object>
  [m07]   <Manifest Id="MyFirstManifest">
  [m08]     <Reference>
  [m09]     ...
  [m10]     </Reference>
  [m11]     <Reference>
  [m12]     ...
  [m13]     </Reference>
  [m14]   </Manifest>
  [m15] </Object>

3.0 Processing Rules

 The sections below describe the operations to be performed as part of
 signature generation and validation.

3.1 Core Generation

 The REQUIRED steps include the generation of Reference elements and
 the SignatureValue over SignedInfo.

3.1.1 Reference Generation

 For each data object being signed:
 1. Apply the Transforms, as determined by the application, to the
    data object.
 2. Calculate the digest value over the resulting data object.
 3. Create a 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.)

3.1.2 Signature Generation

 1. Create SignedInfo element with SignatureMethod,
    CanonicalizationMethod and Reference(s).
 2. Canonicalize and then calculate the SignatureValue over SignedInfo
    based on algorithms specified in SignedInfo.

Eastlake, et al. Standards Track [Page 13] RFC 3275 XML-Signature Syntax and Processing March 2002

 3. Construct the 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, [XML] or
 [XML-schema] 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).

3.2 Core Validation

 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 values in reference and signature validation are 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.

3.2.1 Reference Validation

 1. Canonicalize the SignedInfo element based on the
    CanonicalizationMethod in SignedInfo.
 2. For each Reference in SignedInfo:
    2.1 Obtain the data object to be digested.  (For example, the
        signature application may dereference the 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
    2.2 Digest the resulting data object using the DigestMethod
        specified in its Reference specification.
    2.3 Compare the generated digest value against DigestValue in the
        SignedInfo Reference; if there is any mismatch, validation

Eastlake, et al. Standards Track [Page 14] RFC 3275 XML-Signature Syntax and Processing March 2002

 Note, SignedInfo is canonicalized in step 1.  The application must
 ensure that the CanonicalizationMethod has no dangerous side affects,
 such as rewriting URIs, (see CanonicalizationMethod (section 4.3))
 and that it Sees What is Signed, which is the canonical form.

3.2.2 Signature Validation

 1. Obtain the keying information from KeyInfo or from an external
 2. Obtain the canonical form of the SignatureMethod using the
    CanonicalizationMethod and use the result (and previously obtained
    KeyInfo) to confirm the SignatureValue over the SignedInfo
 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.

4.0 Core Signature Syntax

 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 DTDs and [XML-Schema] with the following XML preamble,
 declaration, and internal entity.

Eastlake, et al. Standards Track [Page 15] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE schema
      PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
       <!ATTLIST schema
         xmlns:ds CDATA #FIXED "">
       <!ENTITY dsig ''>
       <!ENTITY % p ''>
       <!ENTITY % s ''>
    <schema xmlns=""
            version="0.1" elementFormDefault="qualified">
    The following entity declarations enable external/flexible content
    in the Signature content model.
    #PCDATA emulates schema:string; when combined with element types
    it emulates schema mixed="true".
    %foo.ANY permits the user to include their own element types from
    other namespaces, for example:
      <!ENTITY % KeyValue.ANY '| ecds:ECDSAKeyValue'>
      <!ELEMENT ecds:ECDSAKeyValue (#PCDATA)  >
    <!ENTITY % Object.ANY ''>
    <!ENTITY % Method.ANY ''>
    <!ENTITY % Transform.ANY ''>
    <!ENTITY % SignatureProperty.ANY ''>
    <!ENTITY % KeyInfo.ANY ''>
    <!ENTITY % KeyValue.ANY ''>
    <!ENTITY % PGPData.ANY ''>
    <!ENTITY % X509Data.ANY ''>
    <!ENTITY % SPKIData.ANY ''>

Eastlake, et al. Standards Track [Page 16] RFC 3275 XML-Signature Syntax and Processing March 2002

4.0.1 The ds:CryptoBinary Simple Type

 This 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 [MIME]
 encoded.  (The conversion from integer to octet string is equivalent
 to IEEE 1363's I2OSP [1363] 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">

4.1 The Signature element

 The Signature element is the root element of an XML Signature.
 Implementation MUST generate laxly schema valid [XML-schema]
 Signature elements as specified by the following schema:
    Schema Definition:
    <element name="Signature" type="ds:SignatureType"/>
    <complexType name="SignatureType">
        <element ref="ds:SignedInfo"/>
        <element ref="ds:SignatureValue"/>
        <element ref="ds:KeyInfo" minOccurs="0"/>
        <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/>
      <attribute name="Id" type="ID" use="optional"/>

Eastlake, et al. Standards Track [Page 17] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?,
 Object*)  >
    <!ATTLIST Signature
     xmlns   CDATA   #FIXED ''
     Id      ID  #IMPLIED >

4.2 The SignatureValue Element

 The SignatureValue element contains the actual value of the digital
 signature; it is always encoded using base64 [MIME].  While we
 identify two SignatureMethod algorithms, one mandatory and one
 optional to implement, user specified algorithms may be used as well.
    Schema Definition:
    <element name="SignatureValue" type="ds:SignatureValueType"/>
    <complexType name="SignatureValueType">
        <extension base="base64Binary">
          <attribute name="Id" type="ID" use="optional"/>
    <!ELEMENT SignatureValue (#PCDATA) >
    <!ATTLIST SignatureValue
              Id  ID      #IMPLIED>

4.3 The SignedInfo Element

 The 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.

Eastlake, et al. Standards Track [Page 18] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="SignedInfo" type="ds:SignedInfoType"/>
    <complexType name="SignedInfoType">
        <element ref="ds:CanonicalizationMethod"/>
        <element ref="ds:SignatureMethod"/>
        <element ref="ds:Reference" maxOccurs="unbounded"/>
      <attribute name="Id" type="ID" use="optional"/>
    <!ELEMENT SignedInfo (CanonicalizationMethod,
     SignatureMethod,  Reference+)  >
    <!ATTLIST SignedInfo
     Id   ID      #IMPLIED

4.3.1 The CanonicalizationMethod Element

 CanonicalizationMethod 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 8.2: 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:
  • XML based canonicalization implementations MUST be provided

with a [XPath] node-set originally formed from the document

       containing the SignedInfo and currently indicating the
       SignedInfo, its descendants, and the attribute and namespace
       nodes of SignedInfo and its descendant elements.

Eastlake, et al. Standards Track [Page 19] RFC 3275 XML-Signature Syntax and Processing March 2002

  • Text based canonicalization algorithms (such as CRLF and

charset normalization) should be provided with the UTF-8 octets

       that represent the well-formed 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
 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
 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 References
 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"
    <complexType name="CanonicalizationMethodType" mixed="true">
        <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
        <!-- (0,unbounded) elements from (1,1) namespace -->
      <attribute name="Algorithm" type="anyURI" use="required"/>
    <!ELEMENT CanonicalizationMethod (#PCDATA %Method.ANY;)* >
    <!ATTLIST CanonicalizationMethod
     Algorithm CDATA #REQUIRED >

Eastlake, et al. Standards Track [Page 20] RFC 3275 XML-Signature Syntax and Processing March 2002

4.3.2 The SignatureMethod Element

 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 signature values.
    Schema Definition:
    <element name="SignatureMethod" type="ds:SignatureMethodType"/>
    <complexType name="SignatureMethodType" mixed="true">
        <element name="HMACOutputLength" minOccurs="0"
        <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
        <!-- (0,unbounded) elements from (1,1) external namespace -->
     <attribute name="Algorithm" type="anyURI" use="required"/>
    <!ELEMENT SignatureMethod
              (#PCDATA|HMACOutputLength %Method.ANY;)* >
    <!ATTLIST SignatureMethod
     Algorithm CDATA #REQUIRED >

4.3.3 The Reference Element

 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 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.

Eastlake, et al. Standards Track [Page 21] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="Reference" type="ds:ReferenceType"/>
    <complexType name="ReferenceType">
        <element ref="ds:Transforms" minOccurs="0"/>
        <element ref="ds:DigestMethod"/>
        <element ref="ds:DigestValue"/>
      <attribute name="Id" type="ID" use="optional"/>
      <attribute name="URI" type="anyURI" use="optional"/>
      <attribute name="Type" type="anyURI" use="optional"/>
    <!ELEMENT Reference (Transforms?, DigestMethod, DigestValue)  >
    <!ATTLIST Reference
     Id  ID  #IMPLIED
     Type    CDATA   #IMPLIED> The URI Attribute

 The URI attribute identifies a data object using a URI-Reference, as
 specified by RFC2396 [URI].  The set of allowed characters for URI
 attributes is the same as for XML, namely [Unicode].  However, some
 Unicode characters are disallowed from URI references including all
 non-ASCII characters and the excluded characters listed in RFC2396
 [URI, section 2.4].  However, the number sign (#), percent sign (%),
 and square bracket characters re-allowed in RFC 2732 [URI-Literal]
 are permitted.  Disallowed characters must be escaped as follows:
 1. Each disallowed character is converted to [UTF-8] as one or more
 2. Any octets corresponding to a disallowed character are escaped
    with the URI escaping mechanism (that is, converted to %HH, where
    HH is the hexadecimal notation of the octet value).
 3. The original character is replaced by the resulting character
 XML signature applications MUST be able to parse URI syntax.  We
 RECOMMEND they be able to dereference URIs in the HTTP scheme.
 Dereferencing a URI in the HTTP scheme MUST comply with the Status
 Code Definitions of [HTTP] (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

Eastlake, et al. Standards Track [Page 22] RFC 3275 XML-Signature Syntax and Processing March 2002

 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.,
 pressrelease.html.en instead of
 pressrelease).  (See the Reference Validation (section 3.2.1) for a
 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.  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 SignatureProperties element is still of type #Object.
 The type attribute is advisory.  No validation of the type
 information is required by this specification. The Reference Processing Model

 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

Eastlake, et al. Standards Track [Page 23] RFC 3275 XML-Signature Syntax and Processing March 2002

 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:
  • If the data object is an octet stream and the next transform

requires a node-set, the signature application MUST attempt to

       parse the octets yielding the required node-set via [XML]
       well-formed processing.
    *  If the data object is a node-set and the next transform
       requires octets, the signature application MUST attempt to
       convert the node-set to an octet stream using Canonical XML
 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.
 Unless the URI-Reference is a 'same-document' reference as defined in
 [URI, Section 4.2], 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
 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.  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 that the URI attribute not include
 fragment identifiers and that such processing be specified as an
 additional XPath Transform.
 When a fragment is not preceded by a URI in the URI-Reference, XML
 signature applications MUST support the null URI and barename
 XPointer.  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.)  All other support for
 XPointers is OPTIONAL, especially all support for barename and other

Eastlake, et al. Standards Track [Page 24] RFC 3275 XML-Signature Syntax and Processing March 2002

 XPointers in external resources since the application may not have
 control over how the fragment is generated (leading to
 interoperability problems and validation failures).
 The following examples demonstrate what the URI attribute identifies
 and how it is dereferenced:
     Identifies the octets that represent the external resource
     '', that is probably an XML document
     given its file extension.
     Identifies the element with ID attribute value 'chapter1' of the
     external XML resource '', provided as
     an octet stream.  Again, for the sake of interoperability, the
     element identified as 'chapter1' should be obtained using an
     XPath transform rather than a URI fragment (barename XPointer
     resolution in external resources is not REQUIRED in this
     Identifies the node-set (minus any comment nodes) of the XML
     resource containing the signature
     Identifies a node-set containing the element with ID attribute
     value 'chapter1' of the XML resource containing the signature.
     XML Signature (and its applications) modify this node-set to
     include the element plus all descendents including namespaces and
     attributes -- but not comments. Same-Document URI-References

 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].
 When processing an XPointer, the application MUST behave as if the
 root node of the XML document containing the URI attribute were used
 to initialize the XPointer evaluation context.  The application MUST
 behave as if the result of XPointer processing were a node-set
 derived from the resultant location-set as follows:
 1. discard point nodes
 2. replace each range node with all XPath nodes having full or
    partial content within the range
 3. replace the root node with its children (if it is in the node-set)

Eastlake, et al. Standards Track [Page 25] RFC 3275 XML-Signature Syntax and Processing March 2002

 4. replace any element node E with E plus all descendants of E (text,
    comment, PI, element) and all namespace and attribute nodes of E
    and its descendant elements.
 5. if the URI is not a full XPointer, then delete all comment nodes
 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, barename XPointers and
 child sequence XPointers.  It's necessary because when [XML-C14N] is
 passed a node-set, it processes the node-set as is: with or without
 comments.  Only when it's 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 full
 XPointer.  To retain comments while selecting an element by an
 identifier ID, use the following full XPointer:
 URI='#xpointer(id('ID'))'.  To retain comments while selecting the
 entire document, use the following full XPointer: URI='#xpointer(/)'.
 This XPointer contains a simple XPath expression that includes the
 root node, which the second to last step above replaces with all
 nodes of the parse tree (all descendants, plus all attributes, plus
 all namespaces nodes). The Transforms Element

 The 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).)
 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).)

Eastlake, et al. Standards Track [Page 26] RFC 3275 XML-Signature Syntax and Processing March 2002

 As described in The Reference Processing Model (section,
 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 Transforms 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
 [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.  Transform Algorithms
 (section 6.6) define the list of standard transformations.
    Schema Definition:
    <element name="Transforms" type="ds:TransformsType"/>
    <complexType name="TransformsType">
        <element ref="ds:Transform" maxOccurs="unbounded"/>
    <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"/>
      <attribute name="Algorithm" type="anyURI" use="required"/>

Eastlake, et al. Standards Track [Page 27] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT Transforms (Transform+)>
    <!ELEMENT Transform (#PCDATA|XPath %Transform.ANY;)* >
    <!ATTLIST Transform
     Algorithm    CDATA    #REQUIRED >
    <!ELEMENT XPath (#PCDATA) > The DigestMethod Element

 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 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  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">
        <any namespace="##other" processContents="lax"
             minOccurs="0" maxOccurs="unbounded"/>
      <attribute name="Algorithm" type="anyURI" use="required"/>
    <!ELEMENT DigestMethod (#PCDATA %Method.ANY;)* >
    <!ATTLIST DigestMethod
     Algorithm       CDATA   #REQUIRED > The DigestValue Element

 DigestValue is an element that contains the encoded value of the
 digest.  The digest is always encoded using base64 [MIME].

Eastlake, et al. Standards Track [Page 28] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="DigestValue" type="ds:DigestValueType"/>
    <simpleType name="DigestValueType">
      <restriction base="base64Binary"/>
    <!ELEMENT DigestValue  (#PCDATA)  >
    <!-- base64 encoded digest value -->

4.4 The KeyInfo Element

 KeyInfo 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-ns] 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 4.4.2) and SHOULD implement RetrievalMethod (section 4.4.3).
 The schema/DTD specifications 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 DTD that mixes their own and dsig
 qualified elements, or a schema that permits, includes, imports, or
 derives new types based on &dsig; elements.)

Eastlake, et al. Standards Track [Page 29] RFC 3275 XML-Signature Syntax and Processing March 2002

 The following list summarizes the KeyInfo types that are allocated to
 an identifier in the &dsig; namespace; these can be used within the
 RetrievalMethod Type attribute to describe a remote KeyInfo
 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
    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"/>
        <any processContents="lax" namespace="##other"/>
        <!-- (1,1) elements from (0,unbounded) namespaces -->
      <attribute name="Id" type="ID" use="optional"/>
    <!ELEMENT KeyInfo (#PCDATA|KeyName|KeyValue|RetrievalMethod|
                X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* >
    <!ATTLIST KeyInfo
     Id  ID   #IMPLIED >

Eastlake, et al. Standards Track [Page 30] RFC 3275 XML-Signature Syntax and Processing March 2002

4.4.1 The KeyName Element

 The 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"/>
    <!ELEMENT KeyName (#PCDATA) >

4.4.2 The KeyValue Element

 The KeyValue element contains a single public key that may be useful
 in validating the signature.  Structured formats for defining DSA
 (REQUIRED) and RSA (RECOMMENDED) public keys are defined in Signature
 Algorithms (section 6.4).  The KeyValue element may include
 externally defined public key values represented as PCDATA or element
 types from an external namespace.
    Schema Definition:
    <element name="KeyValue" type="ds:KeyValueType"/>
    <complexType name="KeyValueType" mixed="true">
       <element ref="ds:DSAKeyValue"/>
       <element ref="ds:RSAKeyValue"/>
       <any namespace="##other" processContents="lax"/>
    <!ELEMENT KeyValue (#PCDATA|DSAKeyValue|RSAKeyValue
                        %KeyValue.ANY;)* >

Eastlake, et al. Standards Track [Page 31] RFC 3275 XML-Signature Syntax and Processing March 2002 The DSAKeyValue Element

    Type="" (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 [DSS].  DSA
 public key values can have the following fields:
    a prime modulus meeting the [DSS] requirements
    an integer in the range 2**159 < Q < 2**160 which is a prime
    divisor of P-1
    an integer with certain properties with respect to P and Q
    G**X mod P (where X is part of the private key and not made
    (P - 1) / Q
    a DSA prime generation seed
    a DSA prime generation counter
 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 [DSS].  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 [MIME] values.
 Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
 represented in XML as octet strings as defined by the ds:CryptoBinary

Eastlake, et al. Standards Track [Page 32] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="DSAKeyValue" type="ds:DSAKeyValueType"/>
    <complexType name="DSAKeyValueType">
        <sequence minOccurs="0">
          <element name="P" type="ds:CryptoBinary"/>
          <element name="Q" type="ds:CryptoBinary"/>
        <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"/>
    DTD Definition:
    <!ELEMENT DSAKeyValue ((P, Q)?, G?, Y, J?, (Seed, PgenCounter)?) >
    <!ELEMENT Seed (#PCDATA) >
    <!ELEMENT PgenCounter (#PCDATA) > The RSAKeyValue Element

    Type="" (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.

Eastlake, et al. Standards Track [Page 33] RFC 3275 XML-Signature Syntax and Processing March 2002

 Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
 represented in XML as octet strings as defined by the ds:CryptoBinary
    Schema Definition:
    <element name="RSAKeyValue" type="ds:RSAKeyValueType"/>
    <complexType name="RSAKeyValueType">
        <element name="Modulus" type="ds:CryptoBinary"/>
        <element name="Exponent" type="ds:CryptoBinary"/>
    DTD Definition:
    <!ELEMENT RSAKeyValue (Modulus, Exponent) >
    <!ELEMENT Modulus (#PCDATA) >
    <!ELEMENT Exponent (#PCDATA) >

4.4.3 The RetrievalMethod Element

 A 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 and the Reference Processing Model
 (section except that there is no DigestMethod or DigestValue
 child elements and presence of the URI is mandatory.
 Type is an optional identifier for the type of data to be retrieved.
 The result of dereferencing a RetrievalMethod Reference for all
 KeyInfo types defined by this specification (section 4.4) 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.

Eastlake, et al. Standards Track [Page 34] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="RetrievalMethod" type="ds:RetrievalMethodType"/>
    <complexType name="RetrievalMethodType">
        <element ref="ds:Transforms" minOccurs="0"/>
      <attribute name="URI" type="anyURI"/>
      <attribute name="Type" type="anyURI" use="optional"/>
    <!ELEMENT RetrievalMethod (Transforms?) >
    <!ATTLIST RetrievalMethod
       Type  CDATA #IMPLIED >

4.4.4 The X509Data Element

    Type="" (this can be
    used within a 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:
 1. At least one element, from the following set of element types; any
    of these may appear together or more than once if (if and only if)
    each instance describes or is related to the same certificate:
    o  The X509IssuerSerial element, which contains an X.509 issuer
       distinguished name/serial number pair that SHOULD be compliant
       with RFC 2253 [LDAP-DN],
    o  The X509SubjectName element, which contains an X.509 subject
       distinguished name that SHOULD be compliant with RFC 2253
    o  The X509SKI element, which contains the base64 encoded plain
       (i.e., non-DER-encoded) value of a X509 V.3
       SubjectKeyIdentifier extension.
    o  The X509Certificate element, which contains a base64-encoded
       [X509v3] certificate, and
    o  Elements from an external namespace which
       accompanies/complements any of the elements above.
    o  The X509CRL element, which contains a base64-encoded
       certificate revocation list (CRL) [X509v3].

Eastlake, et al. Standards Track [Page 35] RFC 3275 XML-Signature Syntax and Processing March 2002

 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:
   <X509Data> <!-- two pointers to certificate-A -->
       <X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
         L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
   <X509Data><!-- single pointer to certificate-B -->
     <X509SubjectName>Subject of Certificate B</X509SubjectName>
   <X509Data> <!-- certificate chain -->
     <!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
     <!-- Intermediate cert subject CN=arbolCA,OU=FVT,O=IBM,C=US
          issuer CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
     <!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
 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.

Eastlake, et al. Standards Track [Page 36] RFC 3275 XML-Signature Syntax and Processing March 2002

 Also, strings in DNames (X509IssuerSerial,X509SubjectName, and
 KeyNameif appropriate) should be encoded as follows:
  • Consider the string as consisting of Unicode characters.
  • Escape occurrences of the following special characters by

prefixing it with the "\" character: a "#" character occurring

       at the beginning of the string or one of the characters ",",
       "+", """, "\", "<", ">" or ";"
    *  Escape all occurrences of ASCII control characters (Unicode
       range \x00 - \x 1f) by replacing them with "\" followed by a
       two digit hex number showing its Unicode number.
    *  Escape any trailing white space by replacing "\ " with "\20".
    *  Since a 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">
          <element name="X509IssuerSerial"
          <element name="X509SKI" type="base64Binary"/>
          <element name="X509SubjectName" type="string"/>
          <element name="X509Certificate" type="base64Binary"/>
          <element name="X509CRL" type="base64Binary"/>
          <any namespace="##other" processContents="lax"/>
    <complexType name="X509IssuerSerialType">
        <element name="X509IssuerName" type="string"/>
        <element name="X509SerialNumber" type="integer"/>

Eastlake, et al. Standards Track [Page 37] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName
                         | X509Certificate | X509CRL)+ %X509.ANY;)>
    <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
    <!ELEMENT X509IssuerName (#PCDATA) >
    <!ELEMENT X509SubjectName (#PCDATA) >
    <!ELEMENT X509SerialNumber (#PCDATA) >
    <!ELEMENT X509Certificate (#PCDATA) >
 <!-- Note, this DTD and schema permit X509Data to be empty; this is
 precluded by the text in KeyInfo Element (section 4.4) 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. -->

4.4.5 The PGPData Element

    Type="" (this can be used
    within a RetrievalMethod or Reference element to identify the
    referent's type)
 The PGPData element within KeyInfo is used to convey information
 related to PGP public key pairs and signatures on such keys.  The
 PGPKeyID's value is a base64Binary sequence containing a standard PGP
 public key identifier as defined in [PGP, section 11.2].  The
 PGPKeyPacket contains a base64-encoded Key Material Packet as defined
 in [PGP, section 5.5].  These children element types can be
 complemented/extended by siblings from an external namespace within
 PGPData, or PGPData can be replaced all together with an alternative
 PGP XML structure as a child of KeyInfo.  PGPData must contain one
 PGPKeyID and/or one PGPKeyPacket and 0 or more elements from an
 external namespace.

Eastlake, et al. Standards Track [Page 38] RFC 3275 XML-Signature Syntax and Processing March 2002

    Schema Definition:
    <element name="PGPData" type="ds:PGPDataType"/>
    <complexType name="PGPDataType">
          <element name="PGPKeyID" type="base64Binary"/>
          <element name="PGPKeyPacket" type="base64Binary"
          <any namespace="##other" processContents="lax" minOccurs="0"
          <element name="PGPKeyPacket" type="base64Binary"/>
          <any namespace="##other" processContents="lax" minOccurs="0"
    <!ELEMENT PGPData ((PGPKeyID, PGPKeyPacket?) | (PGPKeyPacket)
                      %PGPData.ANY;) >
    <!ELEMENT PGPKeyPacket  (#PCDATA)  >

4.4.6 The SPKIData Element

    Type="" (this can be
    used within a RetrievalMethod or Reference element to identify the
    referent's type)
 The SPKIData element within KeyInfo is used to convey information
 related to SPKI public key pairs, certificates and other SPKI data.
 SPKISexp is the base64 encoding of a SPKI canonical S-expression.
 SPKIData must have at least one SPKISexp; SPKISexp can be
 complemented/extended by siblings from an external namespace within
 SPKIData, or SPKIData can be entirely replaced with an alternative
 SPKI XML structure as a child of KeyInfo.

Eastlake, et al. Standards Track [Page 39] RFC 3275 XML-Signature Syntax and Processing March 2002

 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"/>

4.4.7 The MgmtData Element

    Type="" (this can be
    used within a RetrievalMethod or Reference element to identify the
    referent's type)
 The MgmtData element within KeyInfo is a string value used to convey
 in-band key distribution or agreement data.  For example, DH key
 exchange, RSA key encryption, etc.  Use of this element is NOT
 RECOMMENDED.  It provides a syntactic hook where in-band key
 distribution or agreement data can be placed.  However, superior
 interoperable child elements of KeyInfo for the transmission of
 encrypted keys and for key agreement are being specified by the W3C
 XML Encryption Working Group and they should be used instead of
    Schema Definition:
    <element name="MgmtData" type="string"/>
    <!ELEMENT MgmtData (#PCDATA)>

4.5 The Object Element

    Type="" (this can be used
    within a Reference element to identify the referent's type)

Eastlake, et al. Standards Track [Page 40] RFC 3275 XML-Signature Syntax and Processing March 2002

 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
 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 [MIME].  For example, if the Object
 contains base64 encoded PNG, the Encoding may be specified as
 '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"/>
      <attribute name="Id" type="ID" use="optional"/>
      <attribute name="MimeType" type="string" use="optional"/>
      <attribute name="Encoding" type="anyURI" use="optional"/>

Eastlake, et al. Standards Track [Page 41] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT Object (#PCDATA|Signature|SignatureProperties|Manifest
                      %Object.ANY;)* >
    <!ATTLIST Object
     Id  ID  #IMPLIED
     MimeType    CDATA   #IMPLIED
     Encoding    CDATA   #IMPLIED >

5.0 Additional Signature Syntax

 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.

5.1 The Manifest Element

    Type="" (this can be
    used within a Reference element to identify the referent's type)
 The 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 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">
        <element ref="ds:Reference" maxOccurs="unbounded"/>
      <attribute name="Id" type="ID" use="optional"/>

Eastlake, et al. Standards Track [Page 42] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT Manifest (Reference+)  >
    <!ATTLIST Manifest
              Id ID  #IMPLIED >

5.2 The SignatureProperties Element

    Type="" (this
    can be used within a Reference element to identify the referent's
 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"
    <complexType name="SignaturePropertiesType">
        <element ref="ds:SignatureProperty" maxOccurs="unbounded"/>
      <attribute name="Id" type="ID" use="optional"/>
    <element name="SignatureProperty"
    <complexType name="SignaturePropertyType" mixed="true">
      <choice maxOccurs="unbounded">
        <any namespace="##other" processContents="lax"/>
        <!-- (1,1) elements from (1,unbounded) namespaces -->
      <attribute name="Target" type="anyURI" use="required"/>
      <attribute name="Id" type="ID" use="optional"/>

Eastlake, et al. Standards Track [Page 43] RFC 3275 XML-Signature Syntax and Processing March 2002

    <!ELEMENT SignatureProperties (SignatureProperty+)  >
    <!ATTLIST SignatureProperties
              Id     ID      #IMPLIED  >
    <!ELEMENT SignatureProperty (#PCDATA %SignatureProperty.ANY;)* >
    <!ATTLIST SignatureProperty
              Target CDATA   #REQUIRED
              Id     ID      #IMPLIED  >

5.3 Processing Instructions in Signature Elements

 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 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.

5.4 Comments in Signature Elements

 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.

6.0 Algorithms

 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.

6.1 Algorithm Identifiers and Implementation Requirements

 Algorithms are identified by URIs that appear as an attribute to the
 element that identifies the algorithms' role (DigestMethod,
 Transform, SignatureMethod, or CanonicalizationMethod).  All

Eastlake, et al. Standards Track [Page 44] RFC 3275 XML-Signature Syntax and Processing March 2002

 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.
    1. Required SHA1
    1. Required base64
    1. Required HMAC-SHA1
    1. Required DSAwithSHA1 (DSS)
    2. Recommended RSAwithSHA1
    1. Required Canonical XML (omits comments)
    2. Recommended Canonical XML with Comments
    1. Optional XSLT
    2. Recommended XPath
    3. Required Enveloped Signature*
  • 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
 RECOMMENDED XPath specification specified in 6.6.4: Enveloped
 Signature Transform; it MUST have the same effect as that specified
 by the XPath Transform.

Eastlake, et al. Standards Track [Page 45] RFC 3275 XML-Signature Syntax and Processing March 2002

6.2 Message Digests

 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 cryptanalysis have cast doubt on its strength.

6.2.1 SHA-1

 The SHA-1 algorithm [SHA-1] takes no explicit parameters.  An example
 of an SHA-1 DigestAlg element is:
 <DigestMethod Algorithm=""/>
 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:

6.3 Message Authentication Codes

 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.

6.3.1 HMAC

 The HMAC 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 an HMAC SignatureMethod

Eastlake, et al. Standards Track [Page 46] RFC 3275 XML-Signature Syntax and Processing March 2002

 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:
    <simpleType name="HMACOutputLengthType">
      <restriction base="integer"/>
    <!ELEMENT HMACOutputLength (#PCDATA)>

6.4 Signature Algorithms

 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

6.4.1 DSA

 The DSA algorithm [DSS] takes no explicit parameters.  An example of
 a DSA SignatureMethod element is:

Eastlake, et al. Standards Track [Page 47] RFC 3275 XML-Signature Syntax and Processing March 2002

 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 2437 [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

6.4.2 PKCS1 (RSA-SHA1)

 The expression "RSA algorithm" as used in this document refers to the
 RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1].  The RSA
 algorithm takes no explicit parameters.  An example of an RSA
 SignatureMethod element is:
 The SignatureValue content for an RSA signature is the base64 [MIME]
 encoding of the octet string computed as per RFC 2437 [PKCS1, section
 8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature
 scheme].  As specified in the EMSA-PKCS1-V1_5-ENCODE function RFC
 2437 [PKCS1, section 9.2.1], the value input to the signature
 function MUST contain a pre-pended algorithm object identifier for
 the hash function, but the availability of an ASN.1 parser and
 recognition of OIDs are not required of a signature verifier.  The
 PKCS#1 v1.5 representation appears as:
    CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
 Note that the padded ASN.1 will be of the following form:
    01 | FF* | 00 | prefix | hash

Eastlake, et al. Standards Track [Page 48] RFC 3275 XML-Signature Syntax and Processing March 2002

 where "|" is concatenation, "01", "FF", and "00" are fixed octets of
 the corresponding hexadecimal value, "hash" is the SHA1 digest of the
 data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix
 required in PKCS1 [RFC 2437], that is,
    hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
 This prefix is included to make it easier to use standard
 cryptographic libraries.  The FF octet MUST be repeated the maximum
 number of times such that the value of the quantity being CRYPTed is
 one octet shorter than the RSA modulus.
 The resulting base64 [MIME] string is the value of the child text
 node of the SignatureValue element, e.g.,

6.5 Canonicalization Algorithms

 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., RFC2376 [XML-MT] defines the media
 types for XML).  This information is necessary to correctly sign and
 verify documents and often requires careful server side
 Various canonicalization algorithms require conversion to [UTF-8].
 The two 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 two algorithms below perform text
 normalization during transcoding [NFC, NFC-Corrigendum].  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
 [XML-Japanese] Note.)

6.5.1 Canonical XML

 Identifier for REQUIRED Canonical XML (omits comments):

Eastlake, et al. Standards Track [Page 49] RFC 3275 XML-Signature Syntax and Processing March 2002

 Identifier for Canonical XML with Comments:
 An example of an XML canonicalization element is:
 The normative specification of Canonical XML 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

6.6 Transform Algorithms

 A Transform algorithm has a single implicit parameter: an octet
 stream from the Reference or the output of an earlier Transform.
 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.

6.6.1 Canonicalization

 Any canonicalization algorithm that can be used for
 CanonicalizationMethod (such as those in  Canonicalization Algorithms
 (section 6.5)) can be used as a Transform.

6.6.2 Base64

 The normative specification for base64 decoding transforms is [MIME].
 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 requires an octet stream for input.  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

Eastlake, et al. Standards Track [Page 50] RFC 3275 XML-Signature Syntax and Processing March 2002

 node-set.  Thus, if an XML element is identified by a barename
 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

6.6.3 XPath Filtering

 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.  Note that
 if the actual input is an XPath node-set resulting from a null URI or
 barename 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:
 1. Initialize an XPath evaluation context by setting the initial node
    equal to the input XML document's root node, and set the context
    position and size to 1.
 2. Evaluate the XPath expression (//. | //@* | //namespace::*)
 The evaluation of this expression includes all of the document's
 nodes (including comments) in the node-set representing the octet
 The transform output is also 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
 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

Eastlake, et al. Standards Track [Page 51] RFC 3275 XML-Signature Syntax and Processing March 2002

 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.
 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
 The XPath transform establishes the following evaluation context for
 each node of the input node-set:
  • A context node equal to a node of the input node-set.
  • A context position, initialized to 1.
  • A context size, initialized to 1.
  • A library of functions equal to the function set defined in

[XPath] plus a function named here.

  • A set of variable bindings. No means for initializing these is

defined. Thus, the set of variable bindings used when

       evaluating the XPath expression is empty, and use of a variable
       reference in the XPath expression results in an error.
    *  The set of namespace declarations in scope for the XPath
 As a result of the context node setting, the XPath expressions
 appearing in this transform will be quite similar to those 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:
 Function: node-set here()

Eastlake, et al. Standards Track [Page 52] RFC 3275 XML-Signature Syntax and Processing March 2002

 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
 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,
    <Signature xmlns="">
        <Reference URI="">
              <XPath xmlns:dsig="&dsig;">
 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;.

Eastlake, et al. Standards Track [Page 53] RFC 3275 XML-Signature Syntax and Processing March 2002

 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]) >
 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

6.6.4 Enveloped Signature Transform

 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]) >
 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.

6.6.5 XSLT Transform


Eastlake, et al. Standards Track [Page 54] RFC 3275 XML-Signature Syntax and Processing March 2002

 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 declaration 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.  If the actual
 input is an XPath node-set, then the signature application should
 attempt to convert it to octets (apply Canonical XML]) as described
 in the Reference Processing Model (section
 The output of this transform is an octet stream.  The processing
 rules for the XSL style sheet 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 [XHTML].

7. XML Canonicalization and Syntax Constraint Considerations

 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.

Eastlake, et al. Standards Track [Page 55] RFC 3275 XML-Signature Syntax and Processing March 2002

 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
 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
 [XML], as described in 7.1 below.  There are those related to [DOM],
 [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  [XML] 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
 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 descendents/content) in Normalization Form C [NFC, NFC-
 Corrigendum] 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.

7.1 XML 1.0, Syntax Constraints, and Canonicalization

 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,

Eastlake, et al. Standards Track [Page 56] RFC 3275 XML-Signature Syntax and Processing March 2002

 1. line endings are normalized to the single character #xA by
    dropping #xD characters if they are immediately followed by a #xA
    and replacing them with #xA in all other cases,
 2. missing attributes declared to have default values are provided to
    the application as if present with the default value,
 3. character references are replaced with the corresponding
 4. entity references are replaced with the corresponding declared
 5. attribute values are normalized by
    5.1 replacing character and entity references as above,
    5.2 replacing occurrences of #x9, #xA, and #xD with #x20 (space)
        except that the sequence #xD#xA is replaced by a single space,
    5.3 if the attribute is not declared to be CDATA, stripping all
        leading and trailing spaces and replacing all interior runs of
        spaces with a single space.
 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 [XML-schema], 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:
 1. attributes having default values be explicitly present,
 2. all entity references (except "amp", "lt", "gt", "apos", "quot",
    and other character entities not representable in the encoding
    chosen) be expanded,
 3. attribute value white space be normalized

7.2 DOM/SAX Processing and Canonicalization

 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.

Eastlake, et al. Standards Track [Page 57] RFC 3275 XML-Signature Syntax and Processing March 2002

 If an XML Signature is to be produced or verified on a system using
 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.

7.3 Namespace Context and Portable Signatures

 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 xmlns:n1="&foo;">
      <B xmlns:n2="&bar;">
        <Signature xmlns="&dsig;">   ...
          <Reference URI="#signme"/> ...
        <C ID="signme" xmlns="&baz;"/>
 when either the element B or the signed element C is moved into a
 [SOAP] envelope for transport:

Eastlake, et al. Standards Track [Page 58] RFC 3275 XML-Signature Syntax and Processing March 2002

        <B xmlns:n2="&bar;">
          <Signature xmlns="&dsig;">
          <C ID="signme" xmlns="&baz;"/>
 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:
 1. Rely upon the enveloping application to properly divorce its body
    (the signature payload) from the context (the envelope) before the
    signature is validated.  Or,
 2. Use a canonicalization method that "repels/excludes" instead of
    "attracts" ancestor context.  [XML-C14N] purposefully attracts
    such context.

8.0 Security Considerations

 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.

8.1 Transforms

 A requirement of this specification is to permit signatures to "apply
 to a part or totality of a XML document." (See [XML-Signature-RD,
 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 a 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

Eastlake, et al. Standards Track [Page 59] RFC 3275 XML-Signature Syntax and Processing March 2002

 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.

8.1.1 Only What is Signed is Secure

 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 cannot 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.

8.1.2 Only What is 'Seen' Should be 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.

Eastlake, et al. Standards Track [Page 60] RFC 3275 XML-Signature Syntax and Processing March 2002

8.1.3 'See' What is Signed

 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
 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:
  • All documents operated upon and generated by signature

applications MUST be in [NFC, NFC-Corrigendum] (otherwise

       intermediate processors might unintentionally break the
    *  Encoding normalizations SHOULD NOT be done as part of a
       signature transform, or (to state it another way) if
       normalization does occur, the application SHOULD always "see"
       (operate over) the normalized form.

Eastlake, et al. Standards Track [Page 61] RFC 3275 XML-Signature Syntax and Processing March 2002

8.2 Check the Security Model

 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
 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.

8.3 Algorithms, Key Lengths, Certificates, Etc.

 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.

Eastlake, et al. Standards Track [Page 62] RFC 3275 XML-Signature Syntax and Processing March 2002

 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.

9. Schema, DTD, Data Model, and Valid Examples

 XML Signature Schema Instance
 Valid XML schema instance based on the 20001024 Schema/DTD
 XML Signature DTD
 RDF Data Model
 XML Signature Object Example
 A cryptographical fabricated XML example that includes foreign
 content and validates under the schema, it also uses schemaLocation
 to aid automated schema fetching and validation.
 RSA XML Signature Example
 An XML Signature example with generated cryptographic values by
 Merlin Hughes and validated by Gregor Karlinger.
 DSA XML Signature Example
 Similar to above but uses DSA.

10. Definitions

 Authentication Code (Protected Checksum)
    A value generated from the application of a shared key to a
    message via a cryptographic algorithm such that it has the
    properties of message authentication (and integrity) but not
    signer authentication.  Equivalent to protected checksum, "A

Eastlake, et al. Standards Track [Page 63] RFC 3275 XML-Signature Syntax and Processing March 2002

    checksum that is computed for a data object by means that protect
    against active attacks that would attempt to change the checksum
    to make it match changes made to the data object."  [SEC]
 Authentication, Message
    The property, given an authentication code/protected checksum,
    that tampering with both the data and checksum, so as to introduce
    changes while seemingly preserving integrity, are still detected.
    "A signature should identify what is signed, making it
    impracticable to falsify or alter either the signed matter or the
    signature without detection." [Digital Signature Guidelines, ABA].
 Authentication, Signer
    The property of the identity of the signer is as claimed.  "A
    signature should indicate who signed a document, message or
    record, and should be difficult for another person to produce
    without authorization." [Digital Signature Guidelines, ABA] Note,
    signer authentication is an application decision (e.g., does the
    signing key actually correspond to a specific identity) that is
    supported by, but out of the scope of, this specification.
    "A value that (a) is computed by a function that is dependent on
    the contents of a data object and (b) is stored or transmitted
    together with the object, for the purpose of detecting changes in
    the data." [SEC]
    The syntax and processing defined by this specification, including
    core validation.  We use this term to distinguish other markup,
    processing, and applications semantics from our own.
 Data Object (Content/Document)
    The actual binary/octet data being operated on (transformed,
    digested, or signed) by an application -- frequently an HTTP
    entity [HTTP].  Note that the proper noun 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 [XML].
    The term XML document is used to describe data objects which
    conform to the XML specification [XML].
    "The property that data has not been changed, destroyed, or lost
    in an unauthorized or accidental manner." [SEC] A simple checksum
    can provide integrity from incidental changes in the data; message
    authentication is similar but also protects against an active
    attack to alter the data whereby a change in the checksum is
    introduced so as to match the change in the data.
    An XML Signature element wherein arbitrary (non-core) data may be
    placed.  An Object element is merely one type of digital data (or
    document) that can be signed via a Reference.

Eastlake, et al. Standards Track [Page 64] RFC 3275 XML-Signature Syntax and Processing March 2002

    "A resource can be anything that has identity.  Familiar examples
    include an electronic document, an image, a service (e.g.,
    'today's weather report for Los Angeles'), and a collection of
    other resources....  The resource is the conceptual mapping to an
    entity or set of entities, not necessarily the entity which
    corresponds to that mapping at any particular instance in time.
    Thus, a resource can remain constant even when its content---the
    entities to which it currently corresponds---changes over time,
    provided that the conceptual mapping is not changed in the
    process." [URI] In order to avoid a collision of the term entity
    within the URI and XML specifications, we use the term data
    object, content or document to refer to the actual bits/octets
    being operated upon.
    Formally speaking, a value generated from the application of a
    private key to a message via a cryptographic algorithm such that
    it has the properties of integrity, message authentication and/or
    signer authentication.  (However, we sometimes use the term
    signature generically such that it encompasses Authentication Code
    values as well, but we are careful to make the distinction when
    the property of signer authentication is relevant to the
    exposition.)  A signature may be (non-exclusively) described as
    detached, enveloping, or enveloped.
 Signature, Application
    An application that implements the MANDATORY (REQUIRED/MUST)
    portions of this specification; these conformance requirements are
    over application behavior, the structure of the Signature element
    type and its children (including SignatureValue) and the specified
 Signature, Detached
    The signature is over content external to the 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.
 Signature, Enveloping
    The signature is over content found within an Object element of
    the signature itself.  The Object (or its content) is identified
    via a Reference (via a URI fragment identifier or transform).
 Signature, Enveloped
    The signature is over the XML content that contains the signature
    as an element.  The content provides the root XML document
    element.  Obviously, enveloped signatures must take care not to
    include their own value in the calculation of the SignatureValue.

Eastlake, et al. Standards Track [Page 65] RFC 3275 XML-Signature Syntax and Processing March 2002

    The processing of a data from its source to its derived form.
    Typical transforms include XML Canonicalization, XPath, and XSLT.
 Validation, Core
    The core processing requirements of this specification requiring
    signature validation and SignedInfo reference validation.
 Validation, Reference
    The hash value of the identified and transformed content,
    specified by Reference, matches its specified DigestValue.
 Validation, Signature
    The SignatureValue matches the result of processing SignedInfo
    with CanonicalizationMethod and SignatureMethod as specified in
    Core Validation (section 3.2).
 Validation, Trust/Application
    The application determines that the semantics associated with a
    signature are valid.  For example, an application may validate the
    time stamps or the integrity of the signer key -- though this
    behavior is external to this core specification.

Eastlake, et al. Standards Track [Page 66] RFC 3275 XML-Signature Syntax and Processing March 2002

Appendix: Changes from RFC 3075

 Numerous minor editorial changes were made.  In addition, the
 following substantive changes have occurred based on interoperation
 experience or other considerations:
 1. Minor but incompatible changes in the representation of DSA keys.
    In particular, the optionality of several fields was changed and
    two fields were re-ordered.
 2. Minor change in the X509Data KeyInfo structure to allow multiple
    CRLs to be grouped with certificates and other X509 information.
    Previously CRLs had to occur singly and each in a separate
    X509Data structure.
 3. Incompatible change in the type of PGPKeyID, which had previously
    been string, to the more correct base64Binary since it is actually
    a binary quantity.
 4. Several warnings have been added.  Of particular note, because it
    reflects a problem actually encountered in use and is the only
    warning added that has its own little section, is the warning of
    canonicalization problems when the namespace context of signed
    material changes.


 [ABA]              Digital Signature Guidelines.
 [DOM]              Document Object Model (DOM) Level 1 Specification.
                    W3C Recommendation.  V. Apparao, S. Byrne, M.
                    Champion, S. Isaacs, I.  Jacobs, A. Le Hors, G.
                    Nicol, J. Robie, R. Sutor, C. Wilson, L.  Wood.
                    October 1998.
 [DSS]              FIPS PUB 186-2 . Digital Signature Standard (DSS).
                    U.S.  Department of Commerce/National Institute of
                    Standards and Technology.
 [HMAC]             Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                    Keyed-Hashing for Message Authentication", RFC
                    2104, February 1997.

Eastlake, et al. Standards Track [Page 67] RFC 3275 XML-Signature Syntax and Processing March 2002

 [HTTP]             Fielding, R. Gettys, J., Mogul, J., Frystyk, H.,
                    Masinter, L., Leach, P. and T. Berners-Lee,
                    "Hypertext Transfer Protocol -- HTTP/1.1", RFC
                    2616, June 1999.
 [KEYWORDS]         Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [LDAP-DN]          Wahl, M., Kille, S. and T. Howes, "Lightweight
                    Directory Access Protocol (v3): UTF-8 String
                    Representation of Distinguished Names", RFC 2253,
                    December 1997.
 [MD5]              Rivest, R., "The MD5 Message-Digest Algorithm",
                    RFC 1321, April 1992.
 [MIME]             Freed, N. and N. Borenstein, "Multipurpose
                    Internet Mail Extensions (MIME) Part One: Format
                    of Internet Message Bodies", RFC 2045, November
 [NFC]              TR15, Unicode Normalization Forms. M. Davis, M.
                    Drst. Revision 18: November 1999.
                    18.html.  NFC-Corrigendum Normalization
                    Corrigendum. The Unicode Consortium.
 [PGP]              Callas, J., Donnerhacke, L., Finney, H. and R.
                    Thayer, "OpenPGP Message Format", RFC 2440,
                    November 1998.
 [RANDOM]           Eastlake, 3rd, D., Crocker, S. and J. Schiller,
                    "Randomness Recommendations for Security", RFC
                    1750, December 1994.
 [RDF]              Resource Description Framework (RDF) Schema
                    Specification 1.0. W3C Candidate Recommendation.
                    D. Brickley, R.V. Guha. March 2000.
                    Resource Description Framework (RDF) Model and
                    Syntax Specification.  W3C Recommendation. O.
                    Lassila, R. Swick. February 1999.

Eastlake, et al. Standards Track [Page 68] RFC 3275 XML-Signature Syntax and Processing March 2002

 [1363]             IEEE 1363: Standard Specifications for Public Key
                    Cryptography.  August 2000.
 [PKCS1]            Kaliski, B. and J. Staddon, "PKCS #1: RSA
                    Cryptography Specifications Version 2.0", RFC
                    2437, October 1998.
 [SAX]              SAX: The Simple API for XML. D. Megginson, et al.
                    May 1998.
                    (THIS PAGE OUT OF DATE; GO TO
 [SEC]              Shirey, R., "Internet Security Glossary", FYI 36,
                    RFC 2828, May 2000.
 [SHA-1]            FIPS PUB 180-1. Secure Hash Standard. U.S.
                    Department of Commerce/National Institute of
                    Standards and Technology.
 [SOAP]             Simple Object Access Protocol (SOAP) Version 1.1.
                    W3C Note. D. Box, D. Ehnebuske, G. Kakivaya, A.
                    Layman, N. Mendelsohn, H. Frystyk Nielsen, S.
                    Thatte, D. Winer. May 2001.
 [Unicode]          The Unicode Consortium. The Unicode Standard.
 [UTF-16]           Hoffman, P. and F. Yergeau, "UTF-16, an encoding
                    of ISO 10646", RFC 2781, February 2000.
 [UTF-8]            Yergeau, R., "UTF-8, a transformation format of
                    ISO 10646", RFC 2279, January 1998.
 [URI]              Berners-Lee, T., Fielding, R. and L. Masinter,
                    "Uniform Resource Identifiers (URI): Generic
                    Syntax", RFC 2396, August 1998.
 [URI-Literal]      Hinden, R., Carpenter, B. and L. Masinter, "Format
                    for Literal IPv6 Addresses in URL's", RFC 2732,
                    December 1999.

Eastlake, et al. Standards Track [Page 69] RFC 3275 XML-Signature Syntax and Processing March 2002

 [URL]              Berners-Lee, T., Masinter, L. and M. McCahill,
                    "Uniform Resource Locators (URL)", RFC 1738,
                    December 1994.
 [URN]              Moats, R., "URN Syntax", RFC 2141, May 1997.
 [X509v3]           ITU-T Recommendation X.509 version 3 (1997).
                    "Information Technology - Open Systems
                    Interconnection - The Directory Authentication
                    Framework" ISO/IEC 9594-8:1997.
 [XHTML 1.0]        XHTML(tm) 1.0: The Extensible Hypertext Markup
                    Language. W3C Recommendation. S. Pemberton, D.
                    Raggett, et al. January 2000.
 [XLink]            XML Linking Language. W3C Recommendation. S.
                    DeRose, E. Maler, D. Orchard. June 2001.
 [XML]              Extensible Markup Language (XML) 1.0 (Second
                    Edition). W3C Recommendation. T. Bray, E. Maler,
                    J. Paoli, C. M. Sperberg-McQueen.  October 2000.
 [XML-C14N]         Boyer, J., "Canonical XML Version 1.0", RFC 3076,
                    March 2001.
 [XML-Japanese]     XML Japanese Profile. W3C Note. M. Murata April
 [XML-MT]           Whitehead, E. and M. Murata, "XML Media Types",
                    RFC 2376, July 1998.
 [XML-ns]           Namespaces in XML. W3C Recommendation. T. Bray, D.
                    Hollander, A. Layman. January 1999.

Eastlake, et al. Standards Track [Page 70] RFC 3275 XML-Signature Syntax and Processing March 2002

 [XML-schema]       XML Schema Part 1: Structures. W3C Recommendation.
                    D. Beech, M. Maloney, N. Mendelsohn, H. Thompson.
                    May 2001.
                    xmlschema-1-20010502/ XML Schema Part 2: Datatypes
                    W3C Recommendation. P. Biron, A. Malhotra.  May
 [XML-Signature-RD] Reagle, J., "XML Signature Requirements", RFC
                    2807, July 2000.
 [XPath]            XML Path Language (XPath) Version 1.0. W3C
                    Recommendation. J. Clark, S. DeRose. October 1999.
 [XPointer]         XML Pointer Language (XPointer). W3C Working
                    Draft. S. DeRose, R. Daniel, E. Maler. January
 [XSL]              Extensible Stylesheet Language (XSL). W3C Proposed
                    Recommendation. S.  Adler, A. Berglund, J. Caruso,
                    S. Deach, P. Grosso, E. Gutentag, A. Milowski, S.
                    Parnell, J. Richman, S. Zilles. August 2001.
 [XSLT]             XSL Transforms (XSLT) Version 1.0. W3C
                    Recommendation. J. Clark. November 1999.

Eastlake, et al. Standards Track [Page 71] RFC 3275 XML-Signature Syntax and Processing March 2002

Authors' Addresses

 Donald E. Eastlake 3rd
 Motorola, 20 Forbes Boulevard
 Mansfield, MA 02048 USA
 Phone: 1-508-851-8280
 Joseph M. Reagle Jr., W3C
 Massachusetts Institute of Technology
 Laboratory for Computer Science
 NE43-350, 545 Technology Square
 Cambridge, MA 02139
 Phone: +1.617.258.7621
 David Solo
 909 Third Ave, 16th Floor
 NY, NY 10043 USA
 Phone +1-212-559-2900

Eastlake, et al. Standards Track [Page 72] RFC 3275 XML-Signature Syntax and Processing March 2002

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Eastlake, et al. Standards Track [Page 73]

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