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RFC 3275 - (Extensible Markup Language) XML-Signature Syntax and


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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
                                                               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

   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

   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).

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#"

   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>
        ...

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/

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].

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.

   [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.)

   [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

   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.

   [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.

    [   ] ...
    [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.

   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.

   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.

      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 ''>

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>

      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.

      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.

      *  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 >

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.

      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

   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.

   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

   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)

   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).)

   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>

      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].

      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.)

   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 >

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;)* >

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.

      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>

   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.

      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 X509SubjectName element, which contains an X.509 subject
         distinguished name that SHOULD be compliant with RFC 2253
         [LDAP-DN],
      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].

   Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
   appear MUST refer to the certificate or certificates containing the
   validation key.  All such elements that refer to a particular
   individual certificate MUST be grouped inside a single X509Data
   element and if the certificate to which they refer appears, it MUST
   also be in that X509Data element.

   Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
   relate to the same key but different certificates MUST be grouped
   within a single KeyInfo but MAY occur in multiple X509Data elements.

   All certificates appearing in an X509Data element MUST relate to the
   validation key by either containing it or being part of a
   certification chain that terminates in a certificate containing the
   validation key.

   No ordering is implied by the above constraints.  The comments in the
   following instance demonstrate these constraints:

   <KeyInfo>
     <X509Data> <!-- two pointers to certificate-A -->
       <X509IssuerSerial>
         <X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
           L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
         <X509SerialNumber>12345678</X509SerialNumber>
       </X509IssuerSerial>
       <X509SKI>31d97bd7</X509SKI>
     </X509Data>
     <X509Data><!-- single pointer to certificate-B -->
       <X509SubjectName>Subject of Certificate B</X509SubjectName>
     </X509Data>
     <X509Data> <!-- certificate chain -->
       <!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
       <X509Certificate>MIICXTCCA..</X509Certificate>
       <!-- Intermediate cert subject CN=arbolCA,OU=FVT,O=IBM,C=US
            issuer CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
       <X509Certificate>MIICPzCCA...</X509Certificate>
       <!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
       <X509Certificate>MIICSTCCA...</X509Certificate>
     </X509Data>
   </KeyInfo>

   Note, there is no direct provision for a PKCS#7 encoded "bag" of
   certificates or CRLs.  However, a set of certificates and CRLs can
   occur within an X509Data element and multiple X509Data elements can
   occur in a KeyInfo.  Whenever multiple certificates occur in an
   X509Data element, at least one such certificate must contain the
   public key which verifies the signature.

   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">
          <choice>
            <element name="X509IssuerSerial"
                     type="ds:X509IssuerSerialType"/>
            <element name="X509SKI" type="base64Binary"/>
            <element name="X509SubjectName" type="string"/>
            <element name="X509Certificate" type="base64Binary"/>
            <element name="X509CRL" type="base64Binary"/>
            <any namespace="##other" processContents="lax"/>
          </choice>
        </sequence>
      </complexType>
      <complexType name="X509IssuerSerialType">
        <sequence>
          <element name="X509IssuerName" type="string"/>
          <element name="X509SerialNumber" type="integer"/>
        </sequence>
      </complexType>

      DTD:

      <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName
                           | X509Certificate | X509CRL)+ %X509.ANY;)>
      <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
      <!ELEMENT X509IssuerName (#PCDATA) >
      <!ELEMENT X509SubjectName (#PCDATA) >
      <!ELEMENT X509SerialNumber (#PCDATA) >
      <!ELEMENT X509SKI (#PCDATA) >
      <!ELEMENT X509Certificate (#PCDATA) >
      <!ELEMENT X509CRL (#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

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

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

      Schema Definition:

      <element name="PGPData" type="ds:PGPDataType"/>
      <complexType name="PGPDataType">
        <choice>
          <sequence>
            <element name="PGPKeyID" type="base64Binary"/>
            <element name="PGPKeyPacket" type="base64Binary"
                     minOccurs="0"/>
            <any namespace="##other" processContents="lax" minOccurs="0"
             maxOccurs="unbounded"/>
          </sequence>
          <sequence>
            <element name="PGPKeyPacket" type="base64Binary"/>
            <any namespace="##other" processContents="lax" minOccurs="0"
             maxOccurs="unbounded"/>
          </sequence>
        </choice>
      </complexType>

      DTD:

      <!ELEMENT PGPData ((PGPKeyID, PGPKeyPacket?) | (PGPKeyPacket)
                        %PGPData.ANY;) >
      <!ELEMENT PGPKeyPacket  (#PCDATA)  >
      <!ELEMENT PGPKeyID  (#PCDATA)  >

4.4.6 The SPKIData Element

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

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

   Schema Definition:

   <element name="SPKIData" type="ds:SPKIDataType"/>
   <complexType name="SPKIDataType">
     <sequence maxOccurs="unbounded">
       <element name="SPKISexp" type="base64Binary"/>
       <any namespace="##other" processContents="lax" minOccurs="0"/>
     </sequence>
   </complexType>

   DTD:

   <!ELEMENT SPKIData (SPKISexp %SPKIData.ANY;)  >
   <!ELEMENT SPKISexp  (#PCDATA)  >

4.4.7 The MgmtData Element

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

      Schema Definition:

      <element name="MgmtData" type="string"/>

      DTD:

      <!ELEMENT MgmtData (#PCDATA)>

4.5 The Object Element

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

   Object is an optional element that may occur one or more times.  When
   present, this element may contain any data.  The Object element may
   include optional MIME type, ID, and encoding attributes.

   The Object's Encoding attributed may be used to provide a URI that
   identifies the method by which the object is encoded (e.g., a binary
   file).

   The MimeType attribute is an optional attribute which describes the
   data within the Object (independent of its encoding).  This is a
   string with values defined by [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"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
        <attribute name="MimeType" type="string" use="optional"/>
        <attribute name="Encoding" type="anyURI" use="optional"/>
      </complexType>

      DTD:

      <!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

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

   The Manifest element provides a list of 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">
        <sequence>
          <element ref="ds:Reference" maxOccurs="unbounded"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      DTD:

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

5.2 The SignatureProperties Element

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

   Additional information items concerning the generation of the
   signature(s) can be placed in a SignatureProperty element (i.e.,
   date/time stamp or the serial number of cryptographic hardware used
   in signature generation).

      Schema Definition:

      <element name="SignatureProperties"
               type="ds:SignaturePropertiesType"/>
      <complexType name="SignaturePropertiesType">
        <sequence>
          <element ref="ds:SignatureProperty" maxOccurs="unbounded"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      <element name="SignatureProperty"
               type="ds:SignaturePropertyType"/>
      <complexType name="SignaturePropertyType" mixed="true">
        <choice maxOccurs="unbounded">
          <any namespace="##other" processContents="lax"/>
          <!-- (1,1) elements from (1,unbounded) namespaces -->
        </choice>
        <attribute name="Target" type="anyURI" use="required"/>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      DTD:

      <!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

   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.

   Digest
      1. Required SHA1
         http://www.w3.org/2000/09/xmldsig#sha1
   Encoding
      1. Required base64
         http://www.w3.org/2000/09/xmldsig#base64
   MAC
      1. Required HMAC-SHA1
         http://www.w3.org/2000/09/xmldsig#hmac-sha1
   Signature
      1. Required DSAwithSHA1 (DSS)
         http://www.w3.org/2000/09/xmldsig#dsa-sha1
      2. Recommended RSAwithSHA1
         http://www.w3.org/2000/09/xmldsig#rsa-sha1
   Canonicalization
      1. Required Canonical XML (omits comments)
         http://www.w3.org/TR/2001/REC-xml-c14n-20010315
      2. Recommended Canonical XML with Comments
         http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments
   Transform
      1. Optional XSLT
         http://www.w3.org/TR/1999/REC-xslt-19991116
      2. Recommended XPath
         http://www.w3.org/TR/1999/REC-xpath-19991116
      3. Required Enveloped Signature*
         http://www.w3.org/2000/09/xmldsig#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.

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

   Identifier:
       http://www.w3.org/2000/09/xmldsig#sha1

   The SHA-1 algorithm [SHA-1] takes no explicit parameters.  An example
   of an SHA-1 DigestAlg element is:

   <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>

   A SHA-1 digest is a 160-bit string.  The content of the DigestValue
   element shall be the base64 encoding of this bit string viewed as a
   20-octet octet stream.  For example, the DigestValue element for the
   message digest:

      A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D

   from Appendix A of the SHA-1 standard would be:

      <DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>

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

   Identifier:
      http://www.w3.org/2000/09/xmldsig#hmac-sha1

   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
   element:

      <SignatureMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1">
         <HMACOutputLength>128</HMACOutputLength>
      </SignatureMethod>

   The output of the HMAC algorithm is ultimately the output (possibly
   truncated) of the chosen digest algorithm.  This value shall be
   base64 encoded in the same straightforward fashion as the output of
   the digest algorithms.  Example: the SignatureValue element for the
   HMAC-SHA1 digest

      9294727A 3638BB1C 13F48EF8 158BFC9D

   from the test vectors in [HMAC] would be

      <SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>

      Schema Definition:

      <simpleType name="HMACOutputLengthType">
        <restriction base="integer"/>
      </simpleType>

      DTD:

      <!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
   cryptography.

6.4.1 DSA

   Identifier:
      http://www.w3.org/2000/09/xmldsig#dsa-sha1

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

      <SignatureMethod
       Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>

   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

      <SignatureValue>
       i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==
      </SignatureValue>

6.4.2 PKCS1 (RSA-SHA1)

   Identifier:
      http://www.w3.org/2000/09/xmldsig#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:

      <SignatureMethod
       Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>

   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

   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.,

      <SignatureValue>
       IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639
       In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
      </SignatureValue>

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
   configuration.

   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):
      http://www.w3.org/TR/2001/REC-xml-c14n-20010315

   Identifier for Canonical XML with Comments:
      http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments

   An example of an XML canonicalization element is:
      <CanonicalizationMethod
       Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>

   The normative specification of Canonical 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
   comments.

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

   Identifiers:
      http://www.w3.org/2000/09/xmldsig#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

   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
   stream.

6.6.3 XPath Filtering

   Identifier:
      http://www.w3.org/TR/1999/REC-xpath-19991116

   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
   stream.

   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
   node-set.

   Note: Even if the input node-set has had comments removed, the
   comment nodes still exist in the underlying parse tree and can
   separate text nodes.  For example, the markup <e>Hello, <!-- comment
   -->world!</e> contains two text nodes.  Therefore, the expression
   self::text()[string()="Hello, world!"] would fail.  Should this

   problem arise in the application, it can be solved by either
   canonicalizing the document before the XPath transform to physically
   remove the comments or by matching the node based on the parent
   element's string value (e.g., by using the expression
   self::text()[string(parent::e)="Hello, world!"]).

   The primary purpose of this transform is to ensure that only
   specifically defined changes to the input XML document are permitted
   after the signature is affixed.  This is done by omitting precisely
   those nodes that are allowed to change once the signature is affixed,
   and including all other input nodes in the output.  It is the
   responsibility of the XPath expression author to include all nodes
   whose change could affect the interpretation of the transform output
   in the application context.

   An important scenario would be a document requiring two enveloped
   signatures.  Each signature must omit itself from its own digest
   calculations, but it is also necessary to exclude the second
   signature element from the digest calculations of the first signature
   so that adding the second signature does not break the first
   signature.

   The XPath transform establishes the following evaluation context for
   each node of the input node-set:

      *  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
         expression.

   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()

   The here function returns a node-set containing the attribute or
   processing instruction node or the parent element of the text node
   that directly bears the XPath expression.  This expression results in
   an error if the containing XPath expression does not appear in the
   same XML document against which the XPath expression is being
   evaluated.

   As an example, consider creating an enveloped signature (a Signature
   element that is a descendant of an element being signed).  Although
   the signed content should not be changed after signing, the elements
   within the Signature element are changing (e.g., the digest value
   must be put inside the DigestValue and the SignatureValue must be
   subsequently calculated).  One way to prevent these changes from
   invalidating the digest value in DigestValue is to add an XPath
   Transform that omits all Signature elements and their descendants.
   For example,

      <Document>
      ...
      <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
        <SignedInfo>
         ...
          <Reference URI="">
            <Transforms>
              <Transform
   Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
                <XPath xmlns:dsig="&dsig;">
                not(ancestor-or-self::dsig:Signature)
                </XPath>
              </Transform>
            </Transforms>
            <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <DigestValue></DigestValue>
          </Reference>
        </SignedInfo>
        <SignatureValue></SignatureValue>
       </Signature>
       ...
      </Document>

   Due to the null Reference URI in this example, the XPath transform
   input node-set contains all nodes in the entire parse tree starting
   at the root node (except the comment nodes).  For each node in this
   node-set, the node is included in the output node-set except if the
   node or one of its ancestors, has a tag of Signature that is in the
   namespace given by the replacement text for the entity &dsig;.

   A more elegant solution uses the here function to omit only the
   Signature containing the XPath Transform, thus allowing enveloped
   signatures to sign other signatures.  In the example above, use the
   XPath element:

      <XPath xmlns:dsig="&dsig;">
      count(ancestor-or-self::dsig:Signature |
      here()/ancestor::dsig:Signature[1]) >
      count(ancestor-or-self::dsig:Signature)</XPath>

   Since the XPath equality operator converts node sets to string values
   before comparison, we must instead use the XPath union operator (|).
   For each node of the document, the predicate expression is true if
   and only if the node-set containing the node and its Signature
   element ancestors does not include the enveloped Signature element
   containing the XPath expression (the union does not produce a larger
   set if the enveloped Signature element is in the node-set given by
   ancestor-or-self::Signature).

6.6.4 Enveloped Signature Transform

   Identifier:
      http://www.w3.org/2000/09/xmldsig#enveloped-signature

   An enveloped signature transform T removes the whole Signature
   element containing T from the digest calculation of the Reference
   element containing T.  The entire string of characters used by an XML
   processor to match the Signature with the XML production element is
   removed.  The output of the transform is equivalent to the output
   that would result from replacing T with an XPath transform containing
   the following XPath parameter element:

      <XPath xmlns:dsig="&dsig;">
      count(ancestor-or-self::dsig:Signature |
      here()/ancestor::dsig:Signature[1]) >
      count(ancestor-or-self::dsig:Signature)</XPath>

   The input and output requirements of this transform are identical to
   those of the XPath transform, but may only be applied to a node-set
   from its parent XML document.  Note that it is not necessary to use
   an XPath expression evaluator to create this transform.  However,
   this transform MUST produce output in exactly the same manner as the
   XPath transform parameterized by the XPath expression above.

6.6.5 XSLT Transform

   Identifier:
      http://www.w3.org/TR/1999/REC-xslt-19991116

   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 4.3.3.2).

   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.

   Throughout this specification we distinguish between the
   canonicalization of a Signature element and other signed XML data
   objects.  It is possible for an isolated XML document to be treated
   as if it were binary data so that no changes can occur.  In that
   case, the digest of the document will not change and it need not be
   canonicalized if it is signed and verified as such.   However, XML
   that is read and processed using standard XML parsing and processing
   techniques is frequently changed such that some of its surface
   representation information is lost or modified.  In particular, this
   will occur in many cases for the Signature and enclosed SignedInfo
   elements since they, and possibly an encompassing XML document, will
   be processed as XML.

   Similarly, these considerations apply to Manifest, Object, and
   SignatureProperties elements if those elements have been digested,
   their DigestValue is to be checked, and they are being processed as
   XML.

   The kinds of changes in XML that may need to be canonicalized can be
   divided into four categories.  There are those related to the basic
   [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
   below.

   Any canonicalization algorithm should yield output in a specific
   fixed coded character set.  All canonicalization algorithms
   identified in this document use UTF-8 (without a byte order mark
   (BOM)) and do not provide character normalization.  We RECOMMEND that
   signature applications create XML content (Signature elements and
   their 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,

   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
      character,
   4. entity references are replaced with the corresponding declared
      entity,
   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,
          and
      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.

   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:

      <A xmlns:n1="&foo;">
        <B xmlns:n2="&bar;">
          <Signature xmlns="&dsig;">   ...
            <Reference URI="#signme"/> ...
          </Signature>
          <C ID="signme" xmlns="&baz;"/>
        </B>
      </A>

   when either the element B or the signed element C is moved into a
   [SOAP] envelope for transport:

      <SOAP:Envelope
   xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
        ...
        <SOAP:Body>
          <B xmlns:n2="&bar;">
            <Signature xmlns="&dsig;">
              ...
            </Signature>
            <C ID="signme" xmlns="&baz;"/>
          </B>
        </SOAP:Body>
      </SOAP:Envelope>

   The canonical form of the signature in this context will contain new
   namespace declarations from the SOAP:Envelope context, invalidating
   the signature.  Also, the canonical form will lack namespace
   declarations it may have originally had from element A's context,
   also invalidating the signature.  To avoid these problems, the
   application may:

   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
   sections.

   Note, core validation behavior does not confirm that the signed data
   was obtained by applying each step of the indicated transforms.
   (Though it does check that the digest of the resulting content
   matches that specified in the signature.)  For example, some
   applications may be satisfied with verifying an XML signature over a
   cached copy of already transformed data.  Other applications might
   require that content be freshly dereferenced and transformed.

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.

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
         signature)
      *  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.

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
   systems.

   Keyed hash authentication codes, based on secret keys, are typically
   much more efficient in terms of the computational effort required but
   have the characteristic that all verifiers need to have possession of
   the same key as the signer.  Thus any verifier can forge signatures.

   This specification permits user provided signature algorithms and
   keying information designators.  Such user provided algorithms may
   have different security models.  For example, methods involving
   biometrics usually depend on a physical characteristic of the
   authorized user that can not be changed the way public or secret keys
   can be and may have other security model differences.

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.

   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
   http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-core-
   schema.xsd
   Valid XML schema instance based on the 20001024 Schema/DTD
   [XML-Schema].

   XML Signature DTD
   http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-core-
   schema.dtd

   RDF Data Model
   http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-datamodel-
   20000112.gif

   XML Signature Object Example
   http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example.xml
   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
   http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example-
   rsa.xml
   An XML Signature example with generated cryptographic values by
   Merlin Hughes and validated by Gregor Karlinger.

   DSA XML Signature Example
   http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example-
   dsa.xml
   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

      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.
   Checksum
      "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]
   Core
      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].
   Integrity
      "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.
   Object
      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.

   Resource
      "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.
   Signature
      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
      algorithms.
   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.

   Transform
      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.

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.

References

   [ABA]              Digital Signature Guidelines.
                      http://www.abanet.org/scitech/ec/isc/dsgfree.html

   [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.
                      http://www.w3.org/TR/1998/REC-DOM-Level-1-
                      19981001/

   [DSS]              FIPS PUB 186-2 . Digital Signature Standard (DSS).
                      U.S.  Department of Commerce/National Institute of
                      Standards and Technology.
                      http://csrc.nist.gov/publications/fips/fips186-
                      2/fips186-2.pdf

   [HMAC]             Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                      Keyed-Hashing for Message Authentication", RFC
                      2104, February 1997.

   [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
                      1996.

   [NFC]              TR15, Unicode Normalization Forms. M. Davis, M.
                      Drst. Revision 18: November 1999.
                      http://www.unicode.org/unicode/reports/tr15/tr15-
                      18.html.  NFC-Corrigendum Normalization
                      Corrigendum. The Unicode Consortium.
                      http://www.unicode.org/unicode/uni2errata/
                      Normalization_Corrigendum.html.

   [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.
                      http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
                      Resource Description Framework (RDF) Model and
                      Syntax Specification.  W3C Recommendation. O.
                      Lassila, R. Swick. February 1999.
                      http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/

   [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.  http://www.megginson.com/SAX/index.html
                      (THIS PAGE OUT OF DATE; GO TO www.saxproject.org)

   [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.
                      http://csrc.nist.gov/publications/fips/fips180-
                      1/fip180-1.txt

   [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.
                      http://www.w3.org/TR/2000/NOTE-SOAP-20000508/

   [Unicode]          The Unicode Consortium. The Unicode Standard.
                      http://www.unicode.org/unicode/standard/
                      standard.html

   [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.

   [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.
                      http://www.w3.org/TR/2000/REC-xhtml1-20000126/

   [XLink]            XML Linking Language. W3C Recommendation. S.
                      DeRose, E. Maler, D. Orchard. June 2001.
                      http://www.w3.org/TR/2000/REC-xlink-20010627/

   [XML]              Extensible Markup Language (XML) 1.0 (Second
                      Edition). W3C Recommendation. T. Bray, E. Maler,
                      J. Paoli, C. M. Sperberg-McQueen.  October 2000.
                      http://www.w3.org/TR/2000/REC-xml-20001006

   [XML-C14N]         Boyer, J., "Canonical XML Version 1.0", RFC 3076,
                      March 2001.

   [XML-Japanese]     XML Japanese Profile. W3C Note. M. Murata April
                      2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-
                      20000414/

   [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.
                      http://www.w3.org/TR/1999/REC-xml-names-19990114

   [XML-schema]       XML Schema Part 1: Structures. W3C Recommendation.
                      D. Beech, M. Maloney, N. Mendelsohn, H. Thompson.
                      May 2001.  http://www.w3.org/TR/2001/REC-
                      xmlschema-1-20010502/ XML Schema Part 2: Datatypes
                      W3C Recommendation. P. Biron, A. Malhotra.  May
                      2001.  http://www.w3.org/TR/2001/REC-xmlschema-2-
                      20010502/

   [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.
                      http://www.w3.org/TR/1999/REC-xpath-19991116

   [XPointer]         XML Pointer Language (XPointer). W3C Working
                      Draft. S. DeRose, R. Daniel, E. Maler. January
                      2001.  http://www.w3.org/TR/2001/WD-xptr-20010108

   [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.
                      http://www.w3.org/TR/2001/PR-xsl-20010828/

   [XSLT]             XSL Transforms (XSLT) Version 1.0. W3C
                      Recommendation. J. Clark. November 1999.
                      http://www.w3.org/TR/1999/REC-xslt-19991116.html

Authors' Addresses

   Donald E. Eastlake 3rd
   Motorola, 20 Forbes Boulevard
   Mansfield, MA 02048 USA

   Phone: 1-508-851-8280
   EMail: Donald.Eastlake@motorola.com

   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
   EMail: reagle@w3.org

   David Solo
   Citigroup
   909 Third Ave, 16th Floor
   NY, NY 10043 USA

   Phone +1-212-559-2900
   EMail: dsolo@alum.mit.edu

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   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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