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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 3075

XML Digital Signatures Working Group               D. Eastlake,
INTERNET-DRAFT                                     Motorola
draft-ietf-xmldsig-core-10                         J. Reagle,
Expires March 12, 2001                             W3C/MIT
                                                   D. Solo,
                                                   Citigroup

                    XML-Signature Syntax and Processing

Copyright Notice

   Copyright (c) 2000 The Internet Society & W3C (MIT, INRIA, Keio), All
   Rights Reserved.

IETF Status of this Memo

   This document is an Internet-Draft and is in full conformance with all
   provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering Task
   Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

W3C Status of this document

   This document is a production of the joint IETF/W3C XML Signature
   Working Group.

   http://www.w3.org/Signature

   The comparable html draft of this version may be found at

   http://www.w3.org/TR/2000/WD-xmldsig-core-20001012/

   This specification from the IETF/W3C XML Signature Working Group is a
   update to the second last with an abbreviated W3C last call termination
   date of October 20th (5 weeks in total). This update includes minor
   editorial changes, reference to the latest Canonical XML, as well as
   an adoption of the latest Schema specification. We ask Working Group
   members and other readers to review our approach to all issues raised
   by the first last call and, more substantively, consider changes
   resulting from the recent Canonical XML last call upon which this
   specification is dependent. Barring substantive comment (we expect
   little), we will request Candidate Recommendation status as soon as

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   possible (following the Canonical XML request). However, we do wish to
   ensure that readers are aware of following three substantive changes
   in the second last call:
    1. We've changed the Reference Processing Model (section 4.3.3.1). to
       permit the presentation and acceptance of XML node-sets between
       Transforms (and resulting from some URI References) when
       appropriate.
         1. We accomplish this by heavily relying upon the XPath
            specification but still do NOT require a conformant XPath
            implementation.
    2. We've revised the treatment of pre-pended algorithm object
       identifier within the encoded RSA SignatureValue by the PKCS1
       algorithm (section 6.4.2).
    3. We've revised the X509Data element (section  4.4.4) to clarify the
       treatment of certificate "bags" and CRLs within that structure.

   Please send comments to the editors and cc: the list
   <w3c-ietf-xmldsig@w3.org>. Publication as a Working Draft does not
   imply endorsement by the W3C membership or IESG. It is inappropriate
   to cite W3C Drafts as other than "work in progress." A list of current
   W3C working drafts can be found at http://www.w3.org/TR/. Current IETF
   drafts can be found at http://www.ietf.org/1id-abstracts.html.

   Patent disclosures relevant to this specification may be found on the
   Working Group's patent disclosure page in conformance with W3C policy,
   and the IETF Page of Intellectual Property Rights Notices in
   conformance with IETF policy.

Abstract

   This document specifies XML digital signature processing rules and
   syntax. XML Signatures provide integrity, message authentication,
   and/or signer authentication services for data of any type, whether
   located within the XML that includes the signature or elsewhere.

Table of Contents

    1. Introduction
         1. Editorial Conventions
         2. Design Philosophy
         3. Versions, Namespaces and Identifiers
         4. Acknowledgements
    2. Signature Overview and Examples
         1. Simple Example (Signature, SignedInfo, Methods, and
            References)
              1. More on Reference
         2. Extended Example (Object and SignatureProperty)
         3. Extended Example (Object and Manifest)
    3. Processing Rules
         1. Signature Generation
         2. Signature Validation
    4. Core Signature Syntax
         1. The Signature element

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         2. The SignatureValue Element
         3. The SignedInfo Element
              1. The CanonicalizationMethod Element
              2. The SignatureMethod Element
              3. The Reference Element
                   1. The URI Attribute
                   2. The Reference Processing Model
                   3. Same-Document URI-References
                   4. The Transforms Element
                   5. The DigestMethod Element
                   6. The DigestValue Element
         4. The KeyInfo Element
         5. The Object Element
    5. Additional Signature Syntax
         1. The Manifest Element
         2. The SignatureProperties Element
         3. Processing Instructions
         4. Comments in dsig Elements
    6. Algorithms
         1. Algorithm Identifiers and Implementation Requirements
         2. Message Digests
         3. Message Authentication Codes
         4. Signature Algorithms
         5. Canonicalization Algorithms
         6. Transform Algorithms
              1. Canonicalization
              2. Base64
              3. XPath Filtering
              4. Enveloped Signature Transform
              5. XSLT Transform
    7. XML Canonicalization and Syntax Constraint Considerations
         1. XML 1.0, Syntax Constraints, and Canonicalization
         2. DOM/SAX Processing and Canonicalization
    8. Security Considerations
         1. Transforms
              1. Only What is Signed is Secure
              2. Only What is "Seen" Should be Signed
              3. "See" What is Signed
         2. Check the Security Model
         3. Algorithms, Key Lengths, Etc.
    9. Schema, DTD, Data Model,and Valid Examples
   10. Definitions
   11. References
   12. Authors' Address
     _________________________________________________________________

1.0 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

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   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 uses both XML Schemas [XML-schema] and DTDs [XML].
   (Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's
   " Declaring Elements and Attributes in an XML DTD" [Bourret].) The
   schema definition is presently normative.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   specification are to be interpreted as described in RFC2119
   [KEYWORDS]:

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

   Consequently, we use these capitalized keywords to unambiguously
   specify requirements over protocol and application features and
   behavior that affect the interoperability and security of
   implementations. These key words are not used (capitalized) to
   describe XML grammar; schema definitions unambiguously describe such
   requirements and we wish to reserve the prominence of these terms for
   the natural language descriptions of protocols and features. For
   instance, an XML attribute might be described as being "optional."

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   Compliance with the XML-namespace 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/PR-xslt-19991008

   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">

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     <SignedInfo>
     ...

  1.4  Acknowledgements

   The contributions of the following working group members to this
   specification are gratefully acknowledged:
     * Mark Bartel, JetForm Corporation (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
     * Peter Lipp, IAIK TU Graz
     * Joseph Reagle, W3C (Chair, Author/Editor)
     * Ed Simon, Entrust Technologies Inc. (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.

2.0 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

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   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>
     <SignedInfo>
       (CanonicalizationMethod)
       (SignatureMethod)
       (<Reference (URI=)? >
         (Transforms)?
         (DigestMethod)
         (DigestValue)
       </Reference>)+
     </SignedInfo>
     (SignatureValue)
    (KeyInfo)?
    (Object)*
   </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 resides
   within the same XML document as sibling elements; in this case, the
   signature is neither enveloping (signature is parent) nor enveloped
   (signature is child). Since a Signature element (and its Id attribute
   value/name) may co-exist or be combined with other elements (and their
   IDs) within a single XML document, care should be taken in choosing
   names such that there are no subsequent collisions that violate the ID
   uniqueness validity constraint [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/xmld
sig#">
   [s02]   <SignedInfo>
   [s03]   <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-
c14n-20001011"/>
   [s04]   <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sh
a1"/>
   [s05]   <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
   [s06]     <Transforms>
   [s07]       <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-2000
1011"/>
   [s08]     </Transforms>
   [s09]     <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>

   [s10]     <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
   [s11]   </Reference>

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   [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.

   [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 and the signature design
   permits arbitrary user algorithm specification.

   [s05-11] Each Reference element includes the digest method and
   resulting digest value calculated over the identified data object. It
   also may include transformations that produced the input to the digest
   operation. A data object is signed by computing its digest value and a
   signature over that value. The signature is later checked via
   reference and signature validation.

   [s14-16] KeyInfo indicates the key to be used to validate the
   signature. Possible forms for identification include certificates, key
   names, and key agreement algorithms and information -- we define only
   a few. KeyInfo is optional for two reasons. First, the signer may not
   wish to reveal key information to all document processing parties.
   Second, the information may be known within the application's context
   and need not be represented explicitly. Since KeyInfo is outside of
   SignedInfo, if the signer wishes to bind the keying information to the
   signature, a Reference can easily identify and include the KeyInfo as
   part of the signature.

  2.1.1 More on Reference

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   [s05]   <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
   [s06]     <Transforms>
   [s07]       <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-2000
1011"/>
   [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 than that
   specified in the URI.

   [s06-08] Transforms is an optional ordered list of processing steps
   that were applied to the resource's content before it was digested.
   Transforms can include operations such as canonicalization,
   encoding/decoding (including compression/inflation), XSLT and XPath.
   XPath transforms permit the signer to derive an XML document that
   omits portions of the source document. Consequently those excluded
   portions can change without affecting signature validity. For example,
   if the resource being signed encloses the signature itself, such a
   transform must be used to exclude the signature value from its own
   computation. If no Transforms element is present, the resource's
   content is digested directly. While we specify mandatory (and
   optional) canonicalization and decoding algorithms, user specified
   transforms 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 (message authentication,
   integrity, and/or signer authentication). Applications that wish to
   represent other semantics must rely upon other technologies, such as
   [XML, RDF]. For instance, an application might 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

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   to make trust decisions given the validity of the signature and the
   meaning of assurdby syntax. We also define a SignatureProperties
   element type for the inclusion of assertions about the signature
   itself (e.g., signature semantics, the time of signing or the serial
   number of hardware used in cryptographic processes). Such assertions
   may be signed by 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#SignatureProperty">
   [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="#MySecondSignatur
e">
   [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

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   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 follows.

   First, applications frequently need to efficiently sign multiple data
   objects even where the signature operation itself is an expensive
   public key signature. This requirement can be met by 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>

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   [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.

    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.

  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

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   execute specified algorithms, or inability or unwillingness to
   dereference specified URIs (some URI schemes may cause undesirable
   side effects), etc.

    3.2.1 Reference Validation

   For each Reference in SignedInfo:
    1. Canonicalize the SignedInfo element based on the
       CanonicalizationMethod in SignedInfo.
    2. Obtain the data object to be digested. (The signature application
       may rely upon the identification (URI) and Transforms provided by
       the signer in the Reference element, or it may obtain the content
       through other means such as a local cache.)
    3. Digest the resulting data object using the DigestMethod specified
       in its Reference specification.
    4. 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 to ensure the application
   Sees What is Signed, which is the canonical form. For instance, if the
   CanonicalizationMethod rewrote the URIs (e.g., absolutizing relative
   URIs) the signature processing must be cognizant of this.

    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 validate 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, internal entity, and simpleType:
   Schema Definition:


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   <!DOCTYPE schema
      PUBLIC "-//W3C//DTD XMLSCHEMA 200010//EN" "http://www.w3.org/2000/10/XMLS
chema.dtd"
     [
      <!ATTLIST schema
        xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
      <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
     ]>


   <schema xmlns="http://www.w3.org/2000/10/XMLSchema"
         xmlns:ds="&dsig;"
         targetNamespace="&dsig;"
         version="0.1"
         elementFormDefault="qualified">

   <!-- Basic Types Defined for Signatures -->

   <simpleType name="CryptoBinary">
     <restriction base="binary">
      <encoding value="base64"/>
     </restriction>
   </simpleType>
   DTD:

   <!-- These entity declarations permit the flexible parts of Signature
        content model to be easily expanded -->

   <!ENTITY % Object.ANY '(#PCDATA|Signature|SignatureProperties|Manifest)*'>
   <!ENTITY % Method.ANY '(#PCDATA|HMACOutputLength)*'>
   <!ENTITY % Transform.ANY '(#PCDATA|XPath|XSLT)'>
   <!ENTITY % SignatureProperty.ANY '(#PCDATA)*'>
   <!ENTITY % Key.ANY '(#PCDATA|KeyName|KeyValue|RetrievalMethod|
              X509Data|PGPData|MgmtData|DSAKeyValue|RSAKeyValue)*'>

  4.1 The Signature element

   The Signature element is the root element of an XML Signature.
   Signature elements MUST be laxly schema valid [XML-schema] with
   respect to the following schema definition:
   Schema Definition:

   <element name="Signature">
     <complexType>
       <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>
   </element>

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   DTD:

   <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*)  >
   <!ATTLIST Signature
             xmlns  CDATA   #FIXED 'http://www.w3.org/2000/09/xmldsig#'
             Id     ID  #IMPLIED >

  4.2 The SignatureValue Element

   The SignatureValue element contains the actual value of the digital
   signature; it is always encoded using Base64 [MIME].While we specify a
   mandatory and optional to implement SignatureMethod algorithms, user
   specified algorithms are permitted.
   Schema Definition:

   <element name="SignatureValue" type="ds:CryptoBinary"/>
   DTD:

   <!ELEMENT SignatureValue (#PCDATA) >

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

   <!ELEMENT SignedInfo (CanonicalizationMethod,
             SignatureMethod,  Reference+)  >
   <!ATTLIST SignedInfo
             Id  ID      #IMPLIED>

    4.3.1 The CanonicalizationMethod Element


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   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 Canonical XML [XML-C14N] method.

   Alternatives to the REQUIRED Canonical XML algorithm (section 6.5.2),
   such as Canonical XML with Comments (section 6.5.2) and Minimal
   Canonicalization (the CRLF and charset normalization specified in
   section 6.5.1), may be explicitly specified but are NOT REQUIRED.
   Consequently, their use may not interoperate with other applications
   that do no support the specified algorithm (see XML Canonicalization
   and Syntax Constraint Considerations, section 7). Security issues may
   also arise in the treatment of entity processing and comments if
   minimal or other non-XML aware canonicalization algorithms are not
   properly constrained (see section 8.2: Only What is "Seen" Should be
   Signed).

   The way in which the SignedInfo element is presented to the
   canonicalization method is dependent on that method. The following
   applies to the two types of algorithms specified by this document:
     * Canonical XML [XML-C14N] (with or without comments) implementation
       MUST be provided with an 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 (such that the namespace
       context and similar ancestor information of the SignedInfo is
       preserved).
     * Minimal canonicalization implementations MUST be provided with the
       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.

   We RECOMMEND that resource constrained applications that do not
   implement the Canonical XML [XML-C14N] algorithm and instead choose
   minimal canonicalization (or some other form) are implemented to
   generate Canonical XML as their output serialization so as to easily
   mitigate some of these interoperability and security concerns. (While
   a result might not be the canonical form of the original, it can still
   be in canonical form.) For instance, such an implementation SHOULD (at
   least) generate standalone XML instances [XML].
   Schema Definition:

   <element name="CanonicalizationMethod">
     <complexType>
       <sequence>
         <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
       </sequence>
       <attribute name="Algorithm" type="uriReference" use="required"/>
     </complexType>

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   </element>
   DTD:

   <!ELEMENT CanonicalizationMethod %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">
     <complexType>
       <sequence>
         <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
       </sequence>
       <attribute name="Algorithm" type="uriReference" use="required"/>
      </complexType>
   </element>
   DTD:

   <!ELEMENT SignatureMethod %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">
     <complexType>
       <sequence>
         <element ref="ds:Transforms" minOccurs="0"/>
         <element ref="ds:DigestMethod"/>
         <element ref="ds:DigestValue"/>

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       </sequence>
       <attribute name="Id" type="ID" use="optional"/>
       <attribute name="URI" type="uriReference" use="optional"/>
       <attribute name="Type" type="uriReference" use="optional"/>
     </complexType>
   </element>
   DTD:

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

    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
       bytes.
    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 byte 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 a 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

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   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 nodesets" 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 a an octet stream and the next Transform
       requires a node-set, the signature application MUST attempt to
       parse the octets.
     * 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 the REQUIRED canonicalization
       algorithm [XML-C14N].

   Users may specify alternative transforms that over-ride 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.


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   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 Minimal Canonicalization or Canonical XML with
   Comments. (Otherwise URI="#foo" will automatically remove comments
   before the Canonical XML with Comments 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 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 nodeset (minus any comment nodes) of the XML
          resource containing the signature

   URI="#chapter1"
          Identifies a nodeset containing the element with ID attribute
          value 'chapter1' of the XML resource containing the signature.

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          XML Signature (and its applications) modify this nodeset 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. 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. 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

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   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 Transform may require explicit MimeType, Charset (IANA registered
   "character set"), or other such information concerning the data they
   are receiving from an earlier Transform or the source data, although
   no Transform algorithm specified in this document needs such 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) defines the list
   of standard transformations.
   Schema Definition:

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

     <element name="Transform">
       <complexType>
         <choice maxOccurs="unbounded">
           <any namespace="##other" processContents="lax" minOccurs="0" maxOccu
rs="unbounded"/>
           <element name="XSLT" type="string"/>
           <!-- should be an xsl:stylesheet element -->
           <element name="XPath" type="string"/>
         </choice>

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         <attribute name="Algorithm" type="uriReference" use="required"/>
       </complexType>
     </element>
   DTD:

   <!ELEMENT Transforms (Transform+)>

   <!ELEMENT Transform %Transform.ANY; >
   <!ATTLIST Transform
             Algorithm    CDATA    #REQUIRED >

   <!ELEMENT XPath (#PCDATA) >
   <!ELEMENT XSLT (#PCDATA) >

    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 Minimal
   Canonicalization or 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">
     <complexType>
       <sequence>
         <any namespace="##any" processContents="lax" minOccurs="0" maxOccurs="
unbounded"/>
       </sequence>
       <attribute name="Algorithm" type="uriReference" use="required"/>
     </complexType>
   </element>
   DTD:

   <!ELEMENT DigestMethod %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:


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   <element name="DigestValue" type="ds:CryptoBinary"/>
   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 place their own key
   identification and exchange semantics within this element type through
   the XML-namespace facility. [XML-ns]

   If KeyInfo is omitted, the recipient is expected to be able to
   identify the key based on application context information. 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 following list summarizes the KeyInfo types defined by this
   specification; these can be used within the RetrievalMethod Type
   attribute to describe the remote KeyInfo structure as represented as
   an octect stream.
     * 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 structures, we
   specify one additional type to indicate a binary X.509 Certificate
     * http://www.w3.org/2000/09/xmldsig#rawX509Certificate


   Schema Definition:

   <element name="KeyInfo">
     <complexType>
       <choice maxOccurs="unbounded">
         <any processContents="lax" namespace="##other" minOccurs="0" maxOccurs
="unbounded"/>
         <element name="KeyName" type="string"/>
         <element ref="ds:KeyValue"/>
         <element ref="ds:RetrievalMethod"/>
         <element ref="ds:X509Data"/>
         <element ref="ds:PGPData"/>
         <element ref="ds:SPKIData"/>
         <element name="MgmtData" type="string"/>
       </choice>

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       <attribute name="Id" type="ID" use="optional"/>
     </complexType>
   </element>
   DTD:

   <!ELEMENT KeyInfo %Key.ANY; >
   <!ATTLIST KeyInfo
             Id ID  #IMPLIED >

  4.4.1 The KeyName Element

   The KeyName element contains a string value 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:

   <!-- type declared in KeyInfo -->
   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).
   Schema Definition:

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

   DTD:

   <!ELEMENT KeyValue    %Key.ANY; >

  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

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   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. Note, if the
   result of dereferencing and transforming the specified URI  is a node
   set, then it may need to be to be canonicalized.All of the KeyInfo
   types defined by this specification (section 4.4) represent octets,
   consequently the Signature application is expected to attempt to
   canonicalize the nodeset via the The Reference Processing Model
   (section 4.3.3.2)

   Type is an optional identifier for the type of data to be retrieved.
   Schema Definition

   <element name="RetrievalMethod">
     <complexType>
       <sequence>
         <element ref="ds:Transforms" minOccurs="0"/>
       </sequence>
       <attribute name="URI" type="uriReference"/>
       <attribute name="Type" type="uriReference" use="optional"/>
     </complexType>
   </element>
   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 revocation
   lists). Five types of X509Data are defined
    1. The X509IssuerSerial element, which contains an X.509 issuer
       distinguished name/serial number pair that SHOULD be compliant
       with RFC2253 [LDAP-DN],
    2. The X509SubjectName element, which contains an X.509 subject
       distinguished name that SHOULD be compliant with RFC2253
       [LDAP-DN],
    3. The X509SKI element, which contains an X.509 subject key
       identifier value.
    4. The X509Certificate element, which contains a Base64-encoded

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       [X509v3] certificate, and
    5. The X509CRL element, which contains a Base64-encoded certificate
       revocation list (CRL) [X509v3].

   Multiple declarations about a single certificate (e.g., a
   X509SubjectName and X509IssuerSerial element) MUST be grouped inside a
   single X509Data element; multiple declarations about the same key but
   different certificates (related to that single key) MUST be grouped
   within a single KeyInfo element but MAY occur in multiple X509Data
   elements. For example, the following block contains two pointers to
   certificate-A (issuer/serial number and SKI) and a single reference to
   certificate-B (SubjectName) and also shows use of certificate elements
   <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> <!-- certificate chain -->
       <!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
       <X509Certificate>MIICXTCCA..</X509Certificate>
       <!-- Intermediate cert subject CN=arbolCA,OU=FVTO=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 or a CRL 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.
   Schema Definition

   <element name="X509Data">
      <complexType>
       <choice>
         <sequence maxOccurs="unbounded">
           <choice>
             <element ref="ds:X509IssuerSerial"/>
             <element name="X509SKI" type="ds:CryptoBinary"/>
             <element name="X509SubjectName" type="string"/>
             <element name="X509Certificate" type="ds:CryptoBinary"/>
           </choice>
         </sequence>

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         <element name="X509CRL" type="ds:CryptoBinary"/>
       </choice>
     </complexType>
   </element>

   <element name="X509IssuerSerial">
      <complexType>
       <sequence>
         <element name="X509IssuerName" type="string"/>
         <element name="X509SerialNumber" type="integer"/>
       </sequence>
      </complexType>
   </element>

   DTD

  <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName |
                      X509Certificate)+ | X509CRL)>
   <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
   <!ELEMENT X509IssuerName (#PCDATA) >
   <!ELEMENT X509SubjectName (#PCDATA) >
   <!ELEMENT X509SerialNumber (#PCDATA) >
   <!ELEMENT X509SKI (#PCDATA) >
   <!ELEMENT X509Certificate (#PCDATA) >
   <!ELEMENT X509CRL (#PCDATA) >

  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 string 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]. Other sub-types of the PGPData element may be defined by
   the OpenPGP working group.
   Schema Definition:

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

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   </element>


   DTD:

   <!ELEMENT PGPData (PGPKeyID, PGPKeyPacket)  >
   <!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.
   The content of this element type is expected to be a Canonical
   S-expression.
   Schema Definition:

   <element name="SPKIData" type="string"/>
   DTD:

   <!ELEMENT SPKIData (#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.
   Schema Definition:

   <!-- type declared in KeyInfo -->
   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

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   include optional MIME type, ID, and encoding attributes.

   The MimeType attribute is an optional attribute which describes the
   data within the Object. This is a string with values defined by
   [MIME]. For example, if the Object contains XML, the MimeType could be
   text/xml. This attribute is purely advisory; no validation of the
   MimeType information is required by this specification.

   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.

   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).

   Note, if the application wishes to exclude the <Object> tags from the
   digest calculation the Reference must identify the actual data object
   (easy for XML documents) or a transform must be used to remove the
   Object tags (likely where the data object is non-XML). Exclusion of
   the object tags may be desired for cases where one wants the signature
   to remain valid if the data object is moved from inside a signature to
   outside the signature (or vice-versa), or where the content of the
   Object is an encoding of an original binary document and it is desired
   to extract and decode so as to sign the original bitwise
   representation.
   Schema Definition:

   <element name="Object">
     <complexType mixed="true">
       <sequence maxOccurs="unbounded">
         <any namespace="##any" processContents="lax"/>
       </sequence>
       <attribute name="Id" type="ID" use="optional"/>
       <attribute name="MimeType" type="string" use="optional"/> <!-- add a gre
p facet -->
       <attribute name="Encoding" type="uriReference" use="optional"/>
     </complexType>
   </element>
   DTD:

   <!ELEMENT Object %Object.ANY; >
   <!ATTLIST Object
             Id ID  #IMPLIED
             MimeType   CDATA   #IMPLIED
             Encoding   CDATA   #IMPLIED >

5.0 Additional Signature Syntax

   This section describes the optional to implement Manifest and
   SignatureProperties elements and describes the handling of XML

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


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

      <element name="SignatureProperty">
        <complexType mixed="true">
          <choice minOccurs="0" maxOccurs="unbounded">
            <any namespace="##other" processContents="lax" minOccurs="0" maxOcc
urs="unbounded"/>
          </choice>
          <attribute name="Target" type="uriReference" use="required"/>
          <attribute name="Id" type="ID" use="optional"/>
          </complexType>
      </element>
   DTD:

   <!ELEMENT SignatureProperties (SignatureProperty+)  >
   <!ATTLIST SignatureProperties
             Id ID   #IMPLIED  >

   <!ELEMENT SignatureProperty %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 specified 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.

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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.

   (Note that the normative identifier is the complete URI in the table
   though they are sometimes abbreviated in XML syntax (e.g.,
   "&dsig;base64").)

   Algorithm Type Algorithm Requirements Algorithm URI
   Digest
     SHA1 REQUIRED http://www.w3.org/2000/09/xmldsig#sha1
   Encoding
     Base64 REQUIRED http://www.w3.org/2000/09/xmldsig#base64
   MAC
     HMAC-SHA1 REQUIRED http://www.w3.org/2000/09/xmldsig#hmac-sha1
   Signature
     DSAwithSHA1
   (DSS) REQUIRED http://www.w3.org/2000/09/xmldsig#dsa-sha1
     RSAwithSHA1 RECOMMENDED http://www.w3.org/2000/09/xmldsig#rsa-sha1
   Canonicalization
     minimal RECOMMENDED http://www.w3.org/2000/09/xmldsig#minimal
     Canonical XML with Comments RECOMMENDED
   http://www.w3.org/TR/2000/WD-xml-c14n-20001011#WithComments
     Canonical XML (omits comments) REQUIRED
   http://www.w3.org/TR/2000/WD-xml-c14n-20001011
   Transform
     XSLT OPTIONAL http://www.w3.org/TR/1999/REC-xslt-19991116
     XPath RECOMMENDED http://www.w3.org/TR/1999/REC-xpath-19991116

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     Enveloped Signature* REQUIRED
   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 cryptography
   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="&dsig;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="&dsig;hmac-sha1">

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      <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:

   <element name="HMACOutputLength" type="integer"/>
   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="&dsig;dsa"/>

   The output of the DSA algorithm consists of a pair of integers usually
   referred by the pair (r, s). The signature value consists of the
   Base64 encoding of the concatenation of two octet-streams that
   respectively result from the octet-encoding of the values r and s.
   Integer to octet-stream conversion must be done according to the I2OSP
   operation defined in the RFC 2437 [PKCS1] specification with a k
   parameter equal to 20. For example, the SignatureValue element for a
   DSA signature (r, s) with values specified in hexadecimal:
   r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
   s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8

   from the example in Appendix 5 of the DSS standard would be
   <SignatureValue>
   i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>

   DSA key values have the following set of fields: P, Q, G and Y are
   mandatory when appearing as a key value, J, seed and pgenCounter are
   optional but should be present. (The seed and pgenCounter fields must

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   appear together or be absent). All parameters are encoded as Base64
   [MIME] values.
   Schema:

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

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

    6.4.2 PKCS1

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

   Arbitrary-length integers (e.g. "bignums" such as RSA modulii) are
   represented in XML as octet strings. The integer value is first
   converted to a "big endian" bitstring. The bitstring is then padded
   with leading zero bits so that the total number of bits == 0 mod 8 (so
   that there are an even number of bytes). If the bitstring contains
   entire leading bytes that are zero, these are removed (so the
   high-order byte is always non-zero). This octet string is then Base64
   [MIME] encoded. (The conversion from integer to octet string is
   equivalent to IEEE P1363's I2OSP [P1363] with minimal length).

   The expression "RSA algorithm" as used in this draft refers to the
   RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. The RSA
   algorithm takes no explicit parameters. An example of an RSA
   SignatureMethod element is:
   <SignatureMethod Algorithm="&dsig;rsa-sha1"/>


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   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 signaute 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 is 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 concatentation, "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>IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4
      t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
      </SignatureValue>

   RSA key values have two fields Modulus and Exponent

      <RSAKeyValue>

   <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W

   jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
         5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
         </Modulus>
         <Exponent>AQAB</Exponent>
      </RSAKeyValue>
   Schema:

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

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       </sequence>
     </complexType>
   </element>
   DTD:

   <!ELEMENT RSAKeyValue (Modulus, Exponent) >
   <!ELEMENT Modulus (#PCDATA) >
   <!ELEMENT Exponent (#PCDATA) >

  6.5 Canonicalization Algorithms

   If canonicalization is performed over octets, the Canonicalization
   algorithms take two implicit parameter: 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]. We RECOMMENDED 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 Minimal Canonicalization

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

   An example of a minimal canonicalization element is:
   <CanonicalizationMethod Algorithm="&dsig;minimal"/>

   The minimal canonicalization algorithm:
     * converts the character encoding to UTF-8 (without any byte order
       mark (BOM)). If an encoding is given in the XML declaration, it
       must be removed. Implementations MUST understand at least [UTF-8]
       and [UTF-16] as input encodings. Non-Unicode to Unicode
       transcoding MUST perform text normalization [NFC].
     * normalizes line endings as provided by [XML]. (See XML and
       Canonicalization and Syntactical Considerations (section 7).)

   This algorithm requires as input the octet stream of the resource to
   be processed; the algorithm outputs an octet stream. When used to
   canonicalize SignedInfo the algorithm MUST be provided with the octets
   that represent the well-formed SignedInfo element (and its children
   and content) as described in The CanonicalizationMethod Element
   (section 4.3.1).

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   If the signature application has a node set, then the signature
   application must convert it into octets as described in The Reference
   Processing Model (section 4.3.3.2). However, Minimal Canonicalization
   is NOT RECOMMENDED for processing XPath node-sets, the results of
   same-document URI references, and the output of other types of XML
   based transforms. It is only RECOMMENDED for simple character
   normalization of well formed XML that has no namespace or external
   entity complications.

    6.5.2 Canonical XML

   Identifier for REQUIRED Canonical XML (omits comments):
          http://www.w3.org/TR/2000/WD-xml-c14n-20001011

   Identifier for Canonical XML with Comments:
          http://www.w3.org/TR/2000/WD-xml-c14n-20001011#WithComments

   An example of an XML canonicalization element is:
   <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-200
01011"/>

   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 parameters: 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 base 64 decoding transforms is [MIME].

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   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 base-64 encoded
   character data, then this transform automatically strips away the
   start and end tags of the identified element[DEL: s :DEL] 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>

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   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
   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

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   evaluated.

   Note: The function definition for here() is intended to be consistent
   with its definition in XPointer. However, some minor differences are
   presently being discussed between the Working Groups.

   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="&dsig;">
        <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;">

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      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. 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]. The XSL
   stylesheet or transform to be evaluated appears as the character
   content of a transform parameter child element named XSLT. The root
   element of a XSLT stylesheet SHOULD be <xsl:stylesheet>.

   This transform requires an octet stream as input. If the actual input
   is an XPath node-set, then the signature application should attempt to
   covert 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

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   for the XSL stylesheet 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 perform
   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].

7.0 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 three categories. There are those related to the basic
   [XML], as described in 7.1 below. There are those related to [DOM],
   [SAX], or similar processing as described in 7.2 below. And, third,
   there is the possibility of coded character set conversion, such as

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   between UTF-8 and UTF-16, both of which all  [XML] compliant
   processors are required to support.

   Any canonicalization algorithm should yield output in a specific fixed
   coded character set. For both the minimal canonicalization defined in
   this specification and Canonical XML [XML-C14N] that coded character
   set is UTF-8 (without a byte order mark (BOM)).Neither the minimal
   canonicalization nor the Canonical XML [XML-C14N] algorithms provide
   character normalization. We RECOMMEND that signature applications
   create XML content (Signature elements and their descendents/content)
   in Normalized Form C [NFC] and check that any XML being consumed is in
   that form as well (if not, signatures may consequently fail to
   validate). 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
         A. replacing character and entity references as above,
         B. replacing occurrences of #x9, #xA, and #xD with #x20 (space)
            except that the sequence #xD#xA is replaced by a single
            space, and
         C. 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 (5C) 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 contraints 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,

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    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
   the DOM or SAX processing, a canonical method is needed to serialize
   the relevant part of a DOM tree or sequence of SAX events. XML
   canonicalization specifications, such as [XML-C14N], are based only on
   information which is preserved by DOM and SAX. For an XML Signature to
   be verifiable by an implementation using DOM or SAX, not only must the
   XML1.0 sytnax 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 octect
   stream that was signed.

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 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 tranforms,
   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 application may
   be satisfied with verifying an XML signature over a cached copy of

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   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 can not rely upon canonicalization to
   do this for them.

    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 an automated mechanism
   or person, then it is normally necessary to secure as exactly as
   practical the information that was presented to that mechanism or
   person. Note that this can be accomplished by literally signing what
   was presented, such as the screen images shown a user. However, this
   may result in data which is difficult for subsequent software to
   manipulate. Instead, one can sign the data along with whatever
   filters, style sheets, client profile or other information that
   affects its presentation.

    8.1.3 "See" What is Signed

   Just as a person or automatable mechanism should only sign what it
   "sees," persons and automated mechanisms 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 stylesheet [XSLT] it is the transformed
   document that that should be represented to the user and signed. To
   meet this recommendation where a document references an external style
   sheet, the content of that external resource should also be signed as
   via a signature Reference -- otherwise the content of that external
   content might change which alters the resulting document without
   invalidating the signature.

   Some applications might operate over the original or intermediary data
   but should be extremely careful about potential weaknesses introduced

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   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. Consequently, while
   we RECOMMEND all documents operated upon and generated by signature
   applications be in [NFC] (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,

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   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.

9.0 Schema, DTD, Data Model, and Valid Examples

   XML Signature Schema Instance
          xmldsig-core-schema.xsd
          Valid XML schema instance based on the 20000922 Schema/DTD
          [XML-Schema].

   XML Signature DTD
          xmldsig-core-schema.dtd

   RDF Data Model
          xmldsig-datamodel-20000112.gif

   XML Signature Object Example
          signature-example.xml
          A cryptographical invalid XML example that includes foreign
          content and validates under the schema. (It validates under the
          DTD when the foreign content is removed or the DTD is modified
          accordingly).

   RSA XML Signature Example
          signature-example-rsa.xml
          An XML Signature example with generated cryptographic values by
          Merlin Hughes and validated by Gregor Karlinger.

   DSA XML Signature Example
          signature-example-dsa.xml
          Similar to above but uses DSA.

10.0 Definitions

   Authentication Code
          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 (integrity) but not signer

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          authentication

   Authentication, Message
          "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
          "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]

   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 inability to change a message without also changing the
          signature value. See message authentication.

   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 being operated upon.


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   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 signer authentication and
          message authentication (integrity). (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 the structure of the Signature element type and its
          children (including SignatureValue) and mandatory to support
          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 idenitifier 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 octet stream from source content to derived
          content. 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.

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   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.

11.0 References

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

   Bourret
          Declaring Elements and Attributes in an XML DTD. Ron Bourret.
          http://www.informatik.tu-darmstadt.de/DVS1/staff/bourret/xml/xm
          ldtd.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 186-1. Digital Signature Standard (DSS). U.S.
          Department of Commerce/National Institute of Standards and
          Technology.
          http://csrc.nist.gov/fips/fips1861.pdf

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

   HTTP
          RFC 2616. Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys,
          J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee.
          June 1999.

   KEYWORDS
          RFC2119 Key words for use in RFCs to Indicate Requirement
          Levels. S. Bradner. March 1997.

   LDAP-DN
          RFC2253. Lightweight Directory Access Protocol (v3): UTF-8
          String Representation of Distinguished Names. M. Wahl, S.
          Kille, T. Howes. December 1997.

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   MD5
          RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. April
          1992.

   MIME
          RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part
          One: Format of Internet Message Bodies. N. Freed & N.
          Borenstein. November 1996.

   NFC
          TR15. Unicode Normalization Forms. M. Davis, M. Dürst. Revision
          18: November 1999.

   PGP
          RFC 2440 OpenPGP Message Format. J. Callas, L. Donnerhacke, H.
          Finney, R. Thayer. November 1998.

   RANDOM
          RFC1750 Randomness Recommendations for Security. D. Eastlake,
          S. Crocker, J. Schiller. December 1994.

   RDF
          RDF Schema W3C Candidate Recommendation. D. Brickley, R.V.
          Guha. March 2000.
          http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
          RDF Model and Syntax W3C Recommendation. O. Lassila, R. Swick.
          February 1999.
          http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/

   P1363
          IEEE P1363: Standard Specifications for Public Key
          Cryptography.

   PKCS1
          RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0.
          B. Kaliski, J. Staddon. October 1998.

   SAX
          SAX: The Simple API for XML David Megginson et. al. May 1998.
          http://www.megginson.com/SAX/index.html

   SHA-1
          FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
          Commerce/National Institute of Standards and Technology.
          http://csrc.nist.gov/fips/fip180-1.pdf

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

   UTF-16
          RFC2781. UTF-16, an encoding of ISO 10646. P. Hoffman , F.

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          Yergeau. February 2000.

   UTF-8
          RFC2279. UTF-8, a transformation format of ISO 10646. F.
          Yergeau. Janaury 1998.

   URI
          RFC2396. Uniform Resource Identifiers (URI): Generic Syntax. T.
          Berners-Lee, R. Fielding, L. Masinter. August 1998

   URI-Literal
          RFC 2732. Format for Literal IPv6 Addresses in URL's. R.
          Hinden, B. Carpenter, L. Masinter. December 1999.

   URL
          RFC1738. Uniform Resource Locators (URL). Berners-Lee, T.,
          Masinter, L., and M. McCahill. December 1994.

   URN
          RFC 2141. URN Syntax. R. Moats. May 1997.
          RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D.
          van Gulik, R. Iannella, P. Falstrom. June 1999.

   [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
          Recommendation. S. Pemberton, D. Raggett, et. al. January 2000.
          http://www.w3.org/TR/2000/REC-xhtml1-20000126/


   XLink
          XML Linking Language.Working Draft. S. DeRose, D. Orchard, B.
          Trafford. July 1999.
          http://www.w3.org/1999/07/WD-xlink-19990726

   XML
          Extensible Markup Language (XML) 1.0 Recommendation. T. Bray,
          J. Paoli, C. M. Sperberg-McQueen. February 1998.
          http://www.w3.org/TR/1998/REC-xml-19980210

   XML-C14N
          Canonical XML. Working Draft. J. Boyer. September 2000.
          http://www.w3.org/TR/2000/WD-xml-c14n-20001011

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

   XML-MT

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          RFC 2376. XML Media Types. E. Whitehead, M. Murata. July 1998.

   XML-ns
          Namespaces in XML Recommendation. T. Bray, D. Hollander, A.
          Layman. Janaury 1999.
          http://www.w3.org/TR/1999/REC-xml-names-19990114

   XML-schema
          XML Schema Part 1: Structures Working Draft. D. Beech, M.
          Maloney, N. Mendelshohn. September 2000.
          http://www.w3.org/TR/2000/WD-xmlschema-1-20000922/
          XML Schema Part 2: Datatypes Working Draft. P. Biron, A.
          Malhotra. September 2000.
          http://www.w3.org/TR/2000/WD-xmlschema-2-20000922/

   XML-Signature-RD
          RFC 2807. XML Signature Requirements. J. Reagle, April 2000.
          http://www.w3.org/TR/1999/WD-xmldsig-requirements-19991014

   XPath
          XML Path Language (XPath)Version 1.0. Recommendation. J. Clark,
          S. DeRose. October 1999.
          http://www.w3.org/TR/1999/REC-xpath-19991116

   XPointer
          XML Pointer Language (XPointer). Candidate Recommendation. S.
          DeRose, R. Daniel, E. Maler.
          http://www.w3.org/TR/2000/CR-xptr-20000607

   XSL
          Extensible Stylesheet Language (XSL) Working Draft. S. Adler,
          A. Berglund, J. Caruso, S. Deach, P. Grosso, E. Gutentag, A.
          Milowski, S. Parnell, J. Richman, S. Zilles. March 2000.
          http://www.w3.org/TR/2000/WD-xsl-20000327/xslspec.html

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

12. Authors' Address

   Donald E. Eastlake 3rd
   Motorola, Mail Stop: M4-10
   20 Forbes Boulevard
   Mansfield, MA 02048 USA
   Phone: 1-508-261-5434
   Email: Donald.Eastlake@motorola.com

   Joseph M. Reagle Jr., W3C
   Massachusetts Institute of Technology
   Laboratory for Computer Science
   NE43-350, 545 Technology Square

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   Cambridge, MA 02139
   Phone: 1.617.258.7621
   Email: reagle@w3.org

   David Solo
   Citigroup
   666 Fifth Ave, 3rd Floor
   NY, NY 10103 USA
   Phone: +1-212-830-8118
   Email: dsolo@alum.mit.edu











































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