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

Abstract

   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



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   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
   4.3.3.1 The URI Attribute......................................... 22
   4.3.3.2 The Reference Processing Model............................ 23
   4.3.3.3 Same-Document URI-References.............................. 25
   4.3.3.4 The Transforms Element.................................... 26
   4.3.3.5 The DigestMethod Element.................................. 28
   4.3.3.6 The DigestValue Element................................... 28
   4.4 The KeyInfo Element........................................... 29
   4.4.1 The KeyName Element......................................... 31
   4.4.2 The KeyValue Element........................................ 31
   4.4.2.1 The DSAKeyValue Element................................... 32
   4.4.2.2 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



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   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).



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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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   specification are to be interpreted as described in RFC2119
   [KEYWORDS]:

      "they MUST only be used where it is actually required for
      interoperation or to limit behavior which has potential for
      causing harm (e.g., limiting retransmissions)"

   Consequently, we use these capitalized 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-
   RD].

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:

      xmlns="http://www.w3.org/2000/09/xmldsig#"



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   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
   namespace:
      http://www.w3.org/2000/09/xmldsig#SignatureProperties

   XSLT is identified and defined by an external URI
      http://www.w3.org/TR/1999/REC-xslt-19991116

   SHA1 is identified via this specification's namespace and defined via
   a normative reference
      http://www.w3.org/2000/09/xmldsig#sha1
      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
        "http://www.w3.org/2000/09/xmldsig#"> ]>
      <Signature xmlns="&dsig;" Id="MyFirstSignature">
        <SignedInfo>
        ...












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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:

      http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/







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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
   occurrences):

      <Signature ID?>
         <SignedInfo>
           <CanonicalizationMethod/>
           <SignatureMethod/>
           (<Reference URI? >
             (<Transforms>)?
             <DigestMethod>
             <DigestValue>
           </Reference>)+
         </SignedInfo>
         <SignatureValue>
        (<KeyInfo>)?
        (<Object ID?>)*
       </Signature>

   Signatures are related to data objects via URIs [URI].  Within an XML
   document, signatures are related to local data objects via fragment
   identifiers.  Such local data can be included within an enveloping
   signature or can enclose an enveloped signature.  Detached signatures
   are over external network resources or local data objects that reside
   within the same XML document as sibling elements; in this case, the
   signature is neither enveloping (signature is parent) nor enveloped
   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].



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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"
   xmlns="http://www.w3.org/2000/09/xmldsig#">
    [s02]   <SignedInfo>
    [s03]   <CanonicalizationMethod
   Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
    [s04]   <SignatureMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
    [s05]   <Reference
   URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
    [s06]     <Transforms>
    [s07]       <Transform
   Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
    [s08]     </Transforms>
    [s09]     <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    [s10]     <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
    [s11]   </Reference>
    [s12] </SignedInfo>
    [s13]   <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue>
    [s14]   <KeyInfo>
    [s15a]    <KeyValue>
    [s15b]      <DSAKeyValue>
    [s15c]        <P>...</P><Q>...</Q><G>...</G><Y>...</Y>
    [s15d]      </DSAKeyValue>
    [s15e]    </KeyValue>
    [s16]   </KeyInfo>
    [s17] </Signature>

   [s02-12] The required SignedInfo element is the information that is
   actually signed.  Core validation of SignedInfo consists of two
   mandatory processes: validation of the signature over SignedInfo and
   validation of each Reference digest within SignedInfo.  Note that the
   algorithms used in calculating the SignatureValue are also included
   in the signed information while the SignatureValue element is outside
   SignedInfo.

   [s03] The CanonicalizationMethod is the algorithm that is used to
   canonicalize the SignedInfo element before it is digested as part of
   the signature operation.  Note that this example, and all examples in
   this specification, are not in canonical form.






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   [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
   URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
    [s06]     <Transforms>
    [s07]       <Transform
   Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
    [s08]     </Transforms>
    [s09]     <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    [s10]     <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
    [s11]   </Reference>

   [s05] The optional URI attribute of Reference identifies the data
   object to be signed.  This attribute may be omitted on at most one
   Reference in a Signature.  (This limitation is imposed in order to
   ensure that references and objects may be matched unambiguously.)





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   [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



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   trust engine may wish to).  Any content about the signature
   generation may be located within the SignatureProperty element.  The
   mandatory Target attribute references the Signature element to which
   the property applies.

   Consider the preceding example with an additional reference to a
   local Object that includes a SignatureProperty element.  (Such a
   signature would not only be detached [p02] but enveloping [p03].)

    [   ]  <Signature Id="MySecondSignature" ...>
    [p01]  <SignedInfo>
    [   ]   ...
    [p02]   <Reference URI="http://www.w3.org/TR/xml-stylesheet/">
    [   ]   ...
    [p03]   <Reference URI="#AMadeUpTimeStamp"
    [p04]
   Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties">
    [p05]    <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    [p06]    <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue>
    [p07]   </Reference>
    [p08]  </SignedInfo>
    [p09]  ...
    [p10]  <Object>
    [p11]   <SignatureProperties>
    [p12]     <SignatureProperty Id="AMadeUpTimeStamp"
   Target="#MySecondSignature">
    [p13]        <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt">
    [p14]          <date>19990908</date>
    [p15]          <time>14:34:34:34</time>
    [p16]        </timestamp>
    [p17]     </SignatureProperty>
    [p18]   </SignatureProperties>
    [p19]  </Object>
    [p20]</Signature>

   [p04] The optional Type attribute of Reference provides information
   about the resource identified by the URI.  In particular, it can
   indicate that it is an 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.




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   [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.





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    [   ] ...
    [m01]   <Reference URI="#MyFirstManifest"
    [m02]     Type="http://www.w3.org/2000/09/xmldsig#Manifest">
    [m03]     <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    [m04]     <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
    [m05]   </Reference>
    [   ] ...
    [m06] <Object>
    [m07]   <Manifest Id="MyFirstManifest">
    [m08]     <Reference>
    [m09]     ...
    [m10]     </Reference>
    [m11]     <Reference>
    [m12]     ...
    [m13]     </Reference>
    [m14]   </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.



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   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
          cache.)
      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
          fails.




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   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
      source.
   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
      element.

   Note, KeyInfo (or some transformed version thereof) may be signed via
   a Reference element.  Transformation and validation of this reference
   (3.2.1) is orthogonal to Signature Validation which uses the KeyInfo
   as parsed.

   Additionally, the SignatureMethod URI may have been altered by the
   canonicalization of SignedInfo (e.g., absolutization of relative
   URIs) and it is the canonical form that MUST be used.  However, the
   required canonicalization [XML-C14N] of this specification does not
   change URIs.

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.


















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      Schema Definition:

      <?xml version="1.0" encoding="utf-8"?>
      <!DOCTYPE schema
        PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
   "http://www.w3.org/2001/XMLSchema.dtd"
       [
         <!ATTLIST schema
           xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
         <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
         <!ENTITY % p ''>
         <!ENTITY % s ''>
        ]>

      <schema xmlns="http://www.w3.org/2001/XMLSchema"
              xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
              targetNamespace="http://www.w3.org/2000/09/xmldsig#"
              version="0.1" elementFormDefault="qualified">

      DTD:

      <!--

      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 ''>





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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">
        </restriction>
      </simpleType>

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">
        <sequence>
          <element ref="ds:SignedInfo"/>
          <element ref="ds:SignatureValue"/>
          <element ref="ds:KeyInfo" minOccurs="0"/>
          <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>




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      DTD:

      <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?,
   Object*)  >
      <!ATTLIST Signature
       xmlns   CDATA   #FIXED 'http://www.w3.org/2000/09/xmldsig#'
       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">
        <simpleContent>
          <extension base="base64Binary">
            <attribute name="Id" type="ID" use="optional"/>
          </extension>
        </simpleContent>
      </complexType>

      DTD:

      <!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.







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      Schema Definition:

      <element name="SignedInfo" type="ds:SignedInfoType"/>
      <complexType name="SignedInfoType">
        <sequence>
          <element ref="ds:CanonicalizationMethod"/>
          <element ref="ds:SignatureMethod"/>
          <element ref="ds:Reference" maxOccurs="unbounded"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      DTD:

      <!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.



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      *  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
         is NOT RECOMMENDED.

   We recommend applications that implement a text-based instead of
   XML-based canonicalization -- such as resource constrained apps --
   generate canonicalized XML as their output serialization so as to
   mitigate interoperability and security concerns.  For instance, such
   an implementation SHOULD (at least) generate standalone XML instances
   [XML].

   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"
               type="ds:CanonicalizationMethodType"/>
      <complexType name="CanonicalizationMethodType" mixed="true">
        <sequence>
          <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
          <!-- (0,unbounded) elements from (1,1) namespace -->
        </sequence>
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>

      DTD:

      <!ELEMENT CanonicalizationMethod (#PCDATA %Method.ANY;)* >
      <!ATTLIST CanonicalizationMethod
       Algorithm CDATA #REQUIRED >



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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">
        <sequence>
          <element name="HMACOutputLength" minOccurs="0"
                   type="ds:HMACOutputLengthType"/>
          <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
          <!-- (0,unbounded) elements from (1,1) external namespace -->
         </sequence>
       <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>

      DTD:

      <!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.







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      Schema Definition:

      <element name="Reference" type="ds:ReferenceType"/>
      <complexType name="ReferenceType">
        <sequence>
          <element ref="ds:Transforms" minOccurs="0"/>
          <element ref="ds:DigestMethod"/>
          <element ref="ds:DigestValue"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
        <attribute name="URI" type="anyURI" use="optional"/>
        <attribute name="Type" type="anyURI" use="optional"/>
      </complexType>

      DTD:

      <!ELEMENT Reference (Transforms?, DigestMethod, DigestValue)  >
      <!ATTLIST Reference
       Id  ID  #IMPLIED
       URI CDATA   #IMPLIED
       Type    CDATA   #IMPLIED>

4.3.3.1 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
      octets.
   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
      sequence.

   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




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   parameter and state information, (such as HTTP cookies, HTML device
   profiles or content negotiation), may affect the content yielded by
   dereferencing a URI.

   If a resource is identified by more than one URI, the most specific
   should be used (e.g., http://www.w3.org/2000/06/interop-
   pressrelease.html.en instead of http://www.w3.org/2000/06/interop-
   pressrelease).  (See the Reference Validation (section 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:

   Type="http://www.w3.org/2000/09/xmldsig#Object"
   Type="http://www.w3.org/2000/09/xmldsig#Manifest"

   The Type attribute applies to the item being pointed at, not its
   contents.  For example, a reference that 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.

4.3.3.2 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
   behavior.




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   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
         [XML-C14N].

   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 4.3.3.1).)

   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



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   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:

   URI="http://example.com/bar.xml"
       Identifies the octets that represent the external resource
       'http://example.com/bar.xml', that is probably an XML document
       given its file extension.
   URI="http://example.com/bar.xml#chapter1"
       Identifies the element with ID attribute value 'chapter1' of the
       external XML resource 'http://example.com/bar.xml', 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
       specification).
   URI=""
       Identifies the node-set (minus any comment nodes) of the XML
       resource containing the signature
   URI="#chapter1"
       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.

4.3.3.3 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)




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   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).

4.3.3.4 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).)





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   As described in The Reference Processing Model (section  4.3.3.2),
   some transforms take an XPath node-set as input, while others require
   an octet stream.  If the actual input matches the input needs of the
   transform, then the transform operates on the unaltered input.  If
   the transform input requirement differs from the format of the actual
   input, then the input must be converted.

   Some 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">
        <sequence>
          <element ref="ds:Transform" maxOccurs="unbounded"/>
        </sequence>
      </complexType>

      <element name="Transform" type="ds:TransformType"/>
      <complexType name="TransformType" mixed="true">
        <choice minOccurs="0" maxOccurs="unbounded">
          <any namespace="##other" processContents="lax"/>
          <!-- (1,1) elements from (0,unbounded) namespaces -->
          <element name="XPath" type="string"/>
        </choice>
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>







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      DTD:

      <!ELEMENT Transforms (Transform+)>

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

      <!ELEMENT XPath (#PCDATA) >

4.3.3.5 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  4.3.3.2).  If the result of URI
   dereference and application of transforms is an octet stream, then no
   conversion occurs (comments might be present if the Canonical XML
   with Comments was specified in the Transforms).  The digest algorithm
   is applied to the data octets of the resulting octet stream.

      Schema Definition:

      <element name="DigestMethod" type="ds:DigestMethodType"/>
      <complexType name="DigestMethodType" mixed="true">
        <sequence>
          <any namespace="##other" processContents="lax"
               minOccurs="0" maxOccurs="unbounded"/>
        </sequence>
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>

      DTD:

      <!ELEMENT DigestMethod (#PCDATA %Method.ANY;)* >
      <!ATTLIST DigestMethod
       Algorithm       CDATA   #REQUIRED >

4.3.3.6 The DigestValue Element

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




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      Schema Definition:

      <element name="DigestValue" type="ds:DigestValueType"/>
      <simpleType name="DigestValueType">
        <restriction base="base64Binary"/>
      </simpleType>

      DTD:

      <!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.)





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   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
   structure.

      * http://www.w3.org/2000/09/xmldsig#DSAKeyValue
      * http://www.w3.org/2000/09/xmldsig#RSAKeyValue
      * http://www.w3.org/2000/09/xmldsig#X509Data
      * http://www.w3.org/2000/09/xmldsig#PGPData
      * http://www.w3.org/2000/09/xmldsig#SPKIData
      * http://www.w3.org/2000/09/xmldsig#MgmtData

   In addition to the types above for which we define an XML structure,
   we specify one additional type to indicate a binary (ASN.1 DER) X.509
   Certificate.

      * http://www.w3.org/2000/09/xmldsig#rawX509Certificate

      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 -->
        </choice>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      DTD:

      <!ELEMENT KeyInfo (#PCDATA|KeyName|KeyValue|RetrievalMethod|
                  X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* >
      <!ATTLIST KeyInfo
       Id  ID   #IMPLIED >









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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"/>

      DTD:

      <!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">
       <choice>
         <element ref="ds:DSAKeyValue"/>
         <element ref="ds:RSAKeyValue"/>
         <any namespace="##other" processContents="lax"/>
       </choice>
      </complexType>

      DTD:

      <!ELEMENT KeyValue (#PCDATA|DSAKeyValue|RSAKeyValue
                          %KeyValue.ANY;)* >








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4.4.2.1 The DSAKeyValue Element

   Identifier
      Type="http://www.w3.org/2000/09/xmldsig#DSAKeyValue" (this can be
      used within a RetrievalMethod or Reference element to identify the
      referent's type)

   DSA keys and the DSA signature algorithm are specified in [DSS].  DSA
   public key values can have the following fields:

   P
      a prime modulus meeting the [DSS] requirements
   Q
      an integer in the range 2**159 < Q < 2**160 which is a prime
      divisor of P-1
   G
      an integer with certain properties with respect to P and Q
   Y
      G**X mod P (where X is part of the private key and not made
      public)
    J
      (P - 1) / Q
   seed
      a DSA prime generation seed
   pgenCounter
      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
   type.







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      Schema Definition:

      <element name="DSAKeyValue" type="ds:DSAKeyValueType"/>
      <complexType name="DSAKeyValueType">
        <sequence>
          <sequence minOccurs="0">
            <element name="P" type="ds:CryptoBinary"/>
            <element name="Q" type="ds:CryptoBinary"/>
          </sequence>
          <element name="G" type="ds:CryptoBinary" minOccurs="0"/>
          <element name="Y" type="ds:CryptoBinary"/>
          <element name="J" type="ds:CryptoBinary" minOccurs="0"/>
          <sequence minOccurs="0">
            <element name="Seed" type="ds:CryptoBinary"/>
            <element name="PgenCounter" type="ds:CryptoBinary"/>
          </sequence>
        </sequence>
      </complexType>

      DTD Definition:

      <!ELEMENT DSAKeyValue ((P, Q)?, G?, Y, J?, (Seed, PgenCounter)?) >
      <!ELEMENT P (#PCDATA) >
      <!ELEMENT Q (#PCDATA) >
      <!ELEMENT G (#PCDATA) >
      <!ELEMENT Y (#PCDATA) >
      <!ELEMENT J (#PCDATA) >
      <!ELEMENT Seed (#PCDATA) >
      <!ELEMENT PgenCounter (#PCDATA) >

4.4.2.2 The RSAKeyValue Element

   Identifier
      Type="http://www.w3.org/2000/09/xmldsig#RSAKeyValue" (this can be
      used within a RetrievalMethod or Reference element to identify the
      referent's type)

   RSA key values have two fields: Modulus and Exponent.

      <RSAKeyValue>
        <Modulus>
         xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
         jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRg
         BUwUlV5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
        </Modulus>
        <Exponent>AQAB</Exponent>
      </RSAKeyValue>




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   Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
   represented in XML as octet strings as defined by the ds:CryptoBinary
   type.

      Schema Definition:

      <element name="RSAKeyValue" type="ds:RSAKeyValueType"/>
      <complexType name="RSAKeyValueType">
        <sequence>
          <element name="Modulus" type="ds:CryptoBinary"/>
          <element name="Exponent" type="ds:CryptoBinary"/>
        </sequence>
      </complexType>

      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 4.3.3.1) and the Reference Processing Model
   (section 4.3.3.2) 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.









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      Schema Definition:

      <element name="RetrievalMethod" type="ds:RetrievalMethodType"/>
      <complexType name="RetrievalMethodType">
        <sequence>
          <element ref="ds:Transforms" minOccurs="0"/>
        </sequence>
        <attribute name="URI" type="anyURI"/>
        <attribute name="Type" type="anyURI" use="optional"/>
      </complexType>

      DTD:

      <!ELEMENT RetrievalMethod (Transforms?) >
      <!ATTLIST RetrievalMethod
         URI   CDATA #REQUIRED
         Type  CDATA #IMPLIED >

4.4.4 The X509Data Element

   Identifier
      Type="http://www.w3.org/2000/09/xmldsig#X509Data" (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:
   2.
      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 X509Subject