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rfc:rfc3075

Network Working Group D. Eastlake Request for Comments: 3075 Motorola Category: Standards Track J. Reagle

                                                               W3C/MIT
                                                               D. Solo
                                                             Citigroup
                                                            March 2001
                XML-Signature Syntax and Processing

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2001 The Internet Society & W3C (MIT, INRIA, Keio), All
 Rights Reserved.

Abstract

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

Table of Contents

 1.  Introduction ................................................  3
       1. Editorial Conventions ..................................  3
       2. Design Philosophy ......................................  4
       3. Versions, Namespaces and Identifiers ...................  4
       4. Acknowledgements .......................................  5
 2.  Signature Overview and Examples .............................  6
       1. Simple Example (Signature, SignedInfo, Methods, and
          References) ............................................  7
            1. More on Reference .................................  9
       2. Extended Example (Object and SignatureProperty) ........ 10
       3. Extended Example (Object and Manifest) ................. 11
 3.  Processing Rules ............................................ 13
       1. Core Generation .... ................................... 13
            1. Reference Generation .............................. 13
            2. Signature Generation .............................. 13

Eastlake, et al. Standards Track [Page 1] RFC 3075 XML-Signature Syntax and Processing March 2001

       2. Core Validation ........................................ 13
            1. Reference Validation .............................. 14
            2. Signature Validation .............................. 14
 4.  Core Signature Syntax ....................................... 14
       1. The Signature element .................................. 15
       2. The SignatureValue Element ............................. 16
       3. The SignedInfo Element ................................. 16
            1. The CanonicalizationMethod Element ................ 17
            2. The SignatureMethod Element ....................... 18
            3. The Reference Element ............................. 19
                 1. The URI Attribute ............................ 19
                 2. The Reference Processing Model ............... 21
                 3. Same-Document URI-References ................. 23
                 4. The Transforms Element ....................... 24
                 5. The DigestMethod Element ..................... 25
                 6. The DigestValue Element ...................... 26
       4. The KeyInfo Element .................................... 26
            1. The KeyName Element ............................... 27
            2. The KeyValue Element .............................. 28
            3. The RetrievalMethod Element ....................... 28
            4. The X509Data Element .............................. 29
            5. The PGPData Element ............................... 31
            6. The SPKIData Element .............................. 32
            7. The MgmtData Element .............................. 32
       5. The Object Element ..................................... 33
 5.  Additional Signature Syntax ................................. 34
       1. The Manifest Element ................................... 34
       2. The SignatureProperties Element ........................ 35
       3. Processing Instructions ................................ 36
       4. Comments in dsig Elements .............................. 36
 6.  Algorithms .................................................. 36
       1. Algorithm Identifiers and Implementation Requirements .. 36
       2. Message Digests ........................................ 38
            1. SHA-1 ............................................. 38
       3. Message Authentication Codes ........................... 38
            1. HMAC .............................................. 38
       4. Signature Algorithms ................................... 39
            1. DSA ............................................... 39
            2. PKCS1 ............................................. 40
       5. Canonicalization Algorithms ............................ 42
            1. Minimal Canonicalization .......................... 43
            2. Canonical XML ..................................... 43
       6. Transform Algorithms ................................... 44
            1. Canonicalization .................................. 44
            2. Base64 ............................................ 44
            3. XPath Filtering ................................... 45
            4. Enveloped Signature Transform ..................... 48
            5. XSLT Transform .................................... 48

Eastlake, et al. Standards Track [Page 2] RFC 3075 XML-Signature Syntax and Processing March 2001

 7.  XML Canonicalization and Syntax Constraint Considerations ... 49
       1. XML 1.0, Syntax Constraints, and Canonicalization  ..... 50
       2. DOM/SAX Processing and Canonicalization ................ 51
 8.  Security Considerations ..................................... 52
       1. Transforms ............................................. 52
            1. Only What is Signed is Secure ..................... 52
            2. Only What is "Seen" Should be Signed .............. 53
            3. "See" What is Signed .............................. 53
       2. Check the Security Model ............................... 54
       3. Algorithms, Key Lengths, Etc. .......................... 54
 9.  Schema, DTD, Data Model,and Valid Examples .................. 55
 10. Definitions ................................................. 56
 11. References .................................................. 58
 12. Authors' Addresses .......................................... 63
 13. Full Copyright Statement .................................... 64

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
 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

Eastlake, et al. Standards Track [Page 3] RFC 3075 XML-Signature Syntax and Processing March 2001

 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."
 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#"

Eastlake, et al. Standards Track [Page 4] RFC 3075 XML-Signature Syntax and Processing March 2001

 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">
      <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

Eastlake, et al. Standards Track [Page 5] RFC 3075 XML-Signature Syntax and Processing March 2001

  • 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 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

Eastlake, et al. Standards Track [Page 6] RFC 3075 XML-Signature Syntax and Processing March 2001

 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/xmldsig#">

[s02] <SignedInfo> [s03] <CanonicalizationMethod

       Algorithm="http://www.w3.org/TR/2000/CR-xml-c14n-20001026"/>

[s04] <SignatureMethod

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

[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/

           CR-xml-c14n-20001026"/>

Eastlake, et al. Standards Track [Page 7] RFC 3075 XML-Signature Syntax and Processing March 2001

[s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/

         xmldsig#sha1"/>

[s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=…</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>…</P><Q>…</Q><G>…</G><Y>…</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>

 [s02-12] The required SignedInfo element is the information that is
 actually signed.  Core validation of SignedInfo consists of two
 mandatory processes: validation of the signature over SignedInfo and
 validation of each Reference digest within SignedInfo.  Note that the
 algorithms used in calculating the SignatureValue are also included
 in the signed information while the SignatureValue element is outside
 SignedInfo.
 [s03] The CanonicalizationMethod is the algorithm that is used to
 canonicalize the SignedInfo element before it is digested as part of
 the signature operation.
 [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.

Eastlake, et al. Standards Track [Page 8] RFC 3075 XML-Signature Syntax and Processing March 2001

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

2.1.1 More on Reference

[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform

           Algorithm="http://www.w3.org/TR/2000/
           CR-xml-c14n-20001026"/>

[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

Eastlake, et al. Standards Track [Page 9] RFC 3075 XML-Signature Syntax and Processing March 2001

 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
 to make trust decisions given the validity of the signature and the
 meaning of assuredby syntax.  We also define a SignatureProperties
 element type for the inclusion of assertions about the signature
 itself (e.g., signature semantics, the time of signing or the serial
 number of hardware used in cryptographic processes).  Such assertions
 may be signed by including a Reference for the SignatureProperties in
 SignedInfo.  While the signing application should be very careful
 about what it signs (it should understand what is in the
 SignatureProperty) a receiving application has no obligation to
 understand that semantic (though its parent trust engine may wish
 to).  Any content about the signature generation may be located
 within the SignatureProperty element.  The mandatory Target attribute
 references the Signature element to which the property applies.
 Consider the preceding example with an additional reference to a
 local Object that includes a SignatureProperty element.  (Such a
 signature would not only be detached [p02] but enveloping [p03].)

[ ] <Signature Id="MySecondSignature" …> [p01] <SignedInfo> [ ] … [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] … [p03] <Reference URI="#AMadeUpTimeStamp" [p04] Type="http://www.w3.org/2000/09/

                  xmldsig#SignatureProperties">

[p05] <DigestMethod Algorithm="http://www.w3.org/2000/09/

        xmldsig#sha1"/>

[p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference>

Eastlake, et al. Standards Track [Page 10] RFC 3075 XML-Signature Syntax and Processing March 2001

[p08] </SignedInfo> [p09] … [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp"

         Target="#MySecondSignature">

[p13] <timestamp xmlns="http://www.ietf.org/rfc3075.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</Signature>

 [p04] The optional Type attribute of Reference provides information
 about the resource identified by the URI.  In particular, it can
 indicate that it is an Object, SignatureProperty, or Manifest
 element.  This can be used by applications to initiate special
 processing of some Reference elements.  References to an XML data
 element within an Object element SHOULD identify the actual element
 pointed to.  Where the element content is not XML (perhaps it is
 binary or encoded data) the reference should identify the Object and
 the Reference Type, if given, SHOULD indicate Object.  Note that Type
 is advisory and no action based on it or checking of its correctness
 is required by core behavior.
 [p10] Object is an optional element for including data objects within
 the signature element or elsewhere.  The Object can be optionally
 typed and/or encoded.
 [p11-18] Signature properties, such as time of signing, can be
 optionally signed by identifying them from within a Reference.
 (These properties are traditionally called signature "attributes"
 although that term has no relationship to the XML term "attribute".)

2.3 Extended Example (Object and Manifest)

 The Manifest element is provided to meet additional requirements not
 directly addressed by the mandatory parts of this specification.  Two
 requirements and the way the Manifest satisfies them 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

Eastlake, et al. Standards Track [Page 11] RFC 3075 XML-Signature Syntax and Processing March 2001

 approach because it requires every Reference within SignedInfo to
 undergo reference validation -- the DigestValue elements are checked.
 These applications may wish to reserve reference validation decision
 logic to themselves.  For example, an application might receive a
 signature valid SignedInfo element that includes three Reference
 elements.  If a single Reference fails (the identified data object
 when digested does not yield the specified DigestValue) the signature
 would fail core validation.  However, the application may wish to
 treat the signature over the two valid Reference elements as valid or
 take different actions depending on which fails.  To accomplish this,
 SignedInfo would reference a Manifest element that contains one or
 more Reference elements (with the same structure as those in
 SignedInfo).  Then, reference validation of the Manifest is under
 application control.
 Second, consider an application where many signatures (using
 different keys) are applied to a large number of documents.  An
 inefficient solution is to have a separate signature (per key)
 repeatedly applied to a large SignedInfo element (with many
 References); this is wasteful and redundant.  A more efficient
 solution is to include many references in a single Manifest that is
 then referenced from multiple Signature elements.
 The example below includes a Reference that signs a Manifest found
 within the Object element.

[ ] … [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/09/

         xmldsig#sha1"/>

[m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] … [m06] <Object> [m07] <Manifest Id="MyFirstManifest"> [m08] <Reference> [m09] … [m10] </Reference> [m11] <Reference> [m12] … [m13] </Reference> [m14] </Manifest> [m15] </Object>

Eastlake, et al. Standards Track [Page 12] RFC 3075 XML-Signature Syntax and Processing March 2001

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

Eastlake, et al. Standards Track [Page 13] RFC 3075 XML-Signature Syntax and Processing March 2001

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:

Eastlake, et al. Standards Track [Page 14] RFC 3075 XML-Signature Syntax and Processing March 2001

 Schema Definition:

<!DOCTYPE schema

 PUBLIC "-//W3C//DTD XMLSCHEMA 200010//EN"
        "http://www.w3.org/2000/10/XMLSchema.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"/>

Eastlake, et al. Standards Track [Page 15] RFC 3075 XML-Signature Syntax and Processing March 2001

    <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> 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>

Eastlake, et al. Standards Track [Page 16] RFC 3075 XML-Signature Syntax and Processing March 2001

      <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

 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

Eastlake, et al. Standards Track [Page 17] RFC 3075 XML-Signature Syntax and Processing March 2001

       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) be 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>
 </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>

Eastlake, et al. Standards Track [Page 18] RFC 3075 XML-Signature Syntax and Processing March 2001

 </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"/>
     </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

Eastlake, et al. Standards Track [Page 19] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 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.

Eastlake, et al. Standards Track [Page 20] RFC 3075 XML-Signature Syntax and Processing March 2001

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

transformrequires a node-set, the signature application MUST

       attempt to parse the octets.
  • If the data object is a node-set and the next transformrequires

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.
 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 transformthat requires XML
 parsing is applied (See Transforms (section 4.3.3.1).)

Eastlake, et al. Standards Track [Page 21] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 transformrather 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

Eastlake, et al. Standards Track [Page 22] RFC 3075 XML-Signature Syntax and Processing March 2001

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

Eastlake, et al. Standards Track [Page 23] RFC 3075 XML-Signature Syntax and Processing March 2001

4.3.3.4 The Transforms Element

 The optional Transforms element contains an ordered list of Transform
 elements; these describe how the signer obtained the data object that
 was digested.  The output of each Transform serves as input to the
 next Transform.  The input to the first Transform is the result of
 dereferencing the URI attribute of the Reference element.  The output
 from the last Transform is the input for the DigestMethod algorithm.
 When transforms are applied the signer is not signing the native
 (original) document but the resulting (transformed) document.  (See
 Only What is Signed is Secure (section 8.1).)
 Each Transform consists of an Algorithm attribute and content
 parameters, if any, appropriate for the given algorithm.  The
 Algorithm attribute value specifies the name of the algorithm to be
 performed, and the Transform content provides additional data to
 govern the algorithm's processing of the transform input.  (See
 Algorithm Identifiers and Implementation Requirements (section 6).)
 As described in The Reference Processing Model (section  4.3.3.2),
 some transforms take an XPath node-set as input, while others require
 an octet stream.  If the actual input matches the input needs of the
 transform, then the transform operates on the unaltered input.  If
 the transform input requirement differs from the format of the actual
 input, then the input must be converted.
 Some Transform may require explicit MIME type, charset (IANA
 registered "character set"), or other such information concerning the
 data they are receiving from an earlier Transform or the source data,
 although no Transform algorithm specified in this document needs such
 explicit information.  Such data characteristics are provided as
 parameters to the Transform algorithm and should be described in the
 specification for the algorithm.
 Examples of transforms include but are not limited to base64 decoding
 [MIME], canonicalization [XML-C14N], XPath filtering [XPath], and
 XSLT [XSLT].  The generic definition of the Transform element also
 allows application-specific transform algorithms.  For example, the
 transform could be a decompression routine given by a Java class
 appearing as a base64 encoded parameter to a Java Transform
 algorithm.  However, applications should refrain from using
 application-specific transforms if they wish their signatures to be
 verifiable outside of their application domain.  Transform Algorithms
 (section 6.6) defines the list of standard transformations.
 Schema Definition:

Eastlake, et al. Standards Track [Page 24] RFC 3075 XML-Signature Syntax and Processing March 2001

<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"
       maxOccurs="unbounded"/>
      <element name="XSLT" type="string"/>
      <!-- should be an xsl:stylesheet element -->
      <element name="XPath" type="string"/>
    </choice>
    <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:

Eastlake, et al. Standards Track [Page 25] RFC 3075 XML-Signature Syntax and Processing March 2001

 <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:
 <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).

Eastlake, et al. Standards Track [Page 26] RFC 3075 XML-Signature Syntax and Processing March 2001

 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.
 In addition to the types above for which we define structures, we
 specify one additional type to indicate a binary X.509 Certificate
 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>
  <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.)

Eastlake, et al. Standards Track [Page 27] RFC 3075 XML-Signature Syntax and Processing March 2001

 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="KeyValue">
   <complexType mixed="true">
     <choice>
       <any namespace="##other" processContents="lax" minOccurs="0"
        maxOccurs="unbounded"/>
       <element ref="ds:DSAKeyValue"/>
       <element ref="ds:RSAKeyValue"/>
     </choice>
   </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 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,

Eastlake, et al. Standards Track [Page 28] RFC 3075 XML-Signature Syntax and Processing March 2001

 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
    [X509v3] certificate, and
 5. The X509CRL element, which contains a base64-encoded certificate
    revocation list (CRL) [X509v3].

Eastlake, et al. Standards Track [Page 29] RFC 3075 XML-Signature Syntax and Processing March 2001

 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"/>

Eastlake, et al. Standards Track [Page 30] RFC 3075 XML-Signature Syntax and Processing March 2001

            <element name="X509Certificate" type="ds:CryptoBinary"/>
          </choice>
        </sequence>
        <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>

Eastlake, et al. Standards Track [Page 31] RFC 3075 XML-Signature Syntax and Processing March 2001

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

Eastlake, et al. Standards Track [Page 32] RFC 3075 XML-Signature Syntax and Processing March 2001

 <!-- 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
 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"/>

Eastlake, et al. Standards Track [Page 33] RFC 3075 XML-Signature Syntax and Processing March 2001

     </sequence>
     <attribute name="Id" type="ID" use="optional"/>
     <attribute name="MimeType" type="string" use="optional"/>
        <!-- add a grep facet -->
     <attribute name="Encoding" type="uriReference" use="optional"/>
   </complexType>
 </element>
 DTD:
 <!ELEMENT Object %Object.ANY; >
 <!ATTLIST Object
           Id ID  #IMPLIED
           MimeType   CDATA   #IMPLIED
           Encoding   CDATA   #IMPLIED >

5.0 Additional Signature Syntax

 This section describes the optional to implement Manifest and
 SignatureProperties elements and describes the handling of XML
 processing instructions and comments.  With respect to the elements
 Manifest and SignatureProperties this section specifies syntax and
 little behavior -- it is left to the application.  These elements can
 appear anywhere the parent's content model permits; the Signature
 content model only permits them within Object.

5.1 The Manifest Element

 Identifier
       Type="http://www.w3.org/2000/09/xmldsig#Manifest"
       (this can be used within a Reference element to identify the
       referent's type)
 The Manifest element provides a list of References.  The difference
 from the list in SignedInfo is that it is application defined which,
 if any, of the digests are actually checked against the objects
 referenced and what to do if the object is inaccessible or the digest
 compare fails.  If a Manifest is pointed to from SignedInfo, the
 digest over the Manifest itself will be checked by the core signature
 validation behavior.  The digests within such a Manifest are checked
 at the application's discretion.  If a Manifest is referenced from
 another Manifest, even the overall digest of this two level deep
 Manifest might not be checked.
 Schema Definition:
 <element name="Manifest">
   <complexType>
     <sequence>
       <element ref="ds:Reference" maxOccurs="unbounded"/>

Eastlake, et al. Standards Track [Page 34] RFC 3075 XML-Signature Syntax and Processing March 2001

     </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:
 <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"
          maxOccurs="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  >

Eastlake, et al. Standards Track [Page 35] RFC 3075 XML-Signature Syntax and Processing March 2001

 <!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.

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

Eastlake, et al. Standards Track [Page 36] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 - &dsig;sha1
 Encoding
    base64  - REQUIRED - &dsig;base64
 MAC
    HMAC-SHA1 - REQUIRED - &dsig;hmac-sha1
 Signature
    DSAwithSHA1(DSS) - REQUIRED - &dsig;dsa-sha1
    RSAwithSHA1 - RECOMMENDED - &dsig;rsa-sha1
 Canonicalization
    minimal - RECOMMENDED - &dsig;minimal
    Canonical XML with Comments - RECOMMENDED -
       http://www.w3.org/TR/2000/CR-xml-c14n-20001026#WithComments
    Canonical XML (omits comments) - REQUIRED -
       http://www.w3.org/TR/2000/CR-xml-c14n-20001026
 Transform
    XSLT - OPTIONAL - http://www.w3.org/TR/1999/REC-xslt-19991116
    XPath - RECOMMENDED -
       http://www.w3.org/TR/1999/REC-xpath-19991116
    Enveloped Signature* - REQUIRED - &dsig;enveloped-signature
  • The Enveloped Signature transform removes the Signature element

from the calculation of the signature when the signature is within

 the content that it is being signed.  This MAY be implemented via the
 RECOMMENDED XPath specification specified in 6.6.4: Enveloped
 Signature Transform; it MUST have the same effect as that specified
 by the XPath Transform.

Eastlake, et al. Standards Track [Page 37] RFC 3075 XML-Signature Syntax and Processing March 2001

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">
    <HMACOutputLength>128</HMACOutputLength>
 </SignatureMethod>

Eastlake, et al. Standards Track [Page 38] RFC 3075 XML-Signature Syntax and Processing March 2001

 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

Eastlake, et al. Standards Track [Page 39] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 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

Eastlake, et al. Standards Track [Page 40] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 1363's I2OSP [1363] with minimal length).
 The expression "RSA algorithm" as used in this document refers to the
 RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1].  The RSA
 algorithm takes no explicit parameters.  An example of an RSA
 SignatureMethod element is:  <SignatureMethod Algorithm="&dsig;rsa-
 sha1"/>
 The SignatureValue content for an RSA signature is the base64 [MIME]
 encoding of the octet string computed as per RFC 2437 [PKCS1, section
 8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature
 scheme].  As specified in the EMSA-PKCS1-V1_5-ENCODE function RFC
 2437 [PKCS1, section 9.2.1], the value input to the signature
 function MUST contain a pre-pended algorithm object identifier for
 the hash function, but the availability of an ASN.1 parser and
 recognition of OIDs 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>

Eastlake, et al. Standards Track [Page 41] RFC 3075 XML-Signature Syntax and Processing March 2001

 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"/>
     </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 RECOMMEND that externally

Eastlake, et al. Standards Track [Page 42] RFC 3075 XML-Signature Syntax and Processing March 2001

 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).
 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/CR-xml-c14n-20001026
 Identifier for Canonical XML with Comments:
       http://www.w3.org/TR/2000/CR-xml-c14n-20001026#WithComments
 An example of an XML canonicalization element is:

Eastlake, et al. Standards Track [Page 43] RFC 3075 XML-Signature Syntax and Processing March 2001

 <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/CR-xml-
 c14n-20001026"/>
 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].  The base64 Transform element has no content.  The input is
 decoded by the algorithms.  This transform is useful if an
 application needs to sign the raw data associated with the encoded
 content of an element.
 This transform requires an octet stream for input.  If an XPath
 node-set (or sufficiently functional alternative) is given as input,
 then it is converted to an octet stream by performing operations
 logically equivalent to 1) applying an XPath transform with
 expression self::text(), then 2) taking the string-value of the
 node-set.  Thus, if an XML element is identified by a barename
 XPointer in the Reference URI, and its content consists solely of
 base64 encoded character data, then this transform automatically

Eastlake, et al. Standards Track [Page 44] RFC 3075 XML-Signature Syntax and Processing March 2001

 strips away the start and end tags of the identified element and any
 of its descendant elements as well as any descendant comments and
 processing instructions.  The output of this transform is an octet
 stream.

6.6.3 XPath Filtering

 Identifier:
       http://www.w3.org/TR/1999/REC-xpath-19991116
 The normative specification for XPath expression evaluation is
 [XPath].  The XPath expression to be evaluated appears as the
 character content of a transform parameter child element named XPath.
 The input required by this transform is an XPath node-set.  Note that
 if the actual input is an XPath node-set resulting from a null URI or
 barename XPointer dereference, then comment nodes will have been
 omitted.  If the actual input is an octet stream, then the
 application MUST convert the octet stream to an XPath node-set
 suitable for use by Canonical XML with Comments (a subsequent
 application of the REQUIRED Canonical XML algorithm would strip away
 these comments).  In other words, the input node-set should be
 equivalent to the one that would be created by the following process:
 1. Initialize an XPath evaluation context by setting the initial node
    equal to the input XML document's root node, and set the context
    position and size to 1.
 2. Evaluate the XPath expression (//. | //@* | //namespace::*)
 The evaluation of this expression includes all of the document's
 nodes (including comments) in the node-set representing the octet
 stream.
 The transform output is also an XPath node-set.  The XPath expression
 appearing in the XPath parameter is evaluated once for each node in
 the input node-set.  The result is converted to a boolean.  If the
 boolean is true, then the node is included in the output node-set.
 If the boolean is false, then the node is omitted from the output
 node-set.
 Note: Even if the input node-set has had comments removed, the
 comment nodes still exist in the underlying parse tree and can
 separate text nodes.  For example, the markup <e>Hello, <!-- comment
 --> world!</e> contains two text nodes.  Therefore, the expression
 self::text()[string()="Hello, world!"] would fail.  Should this
 problem arise in the application, it can be solved by either
 canonicalizing the document before the XPath transform to physically

Eastlake, et al. Standards Track [Page 45] RFC 3075 XML-Signature Syntax and Processing March 2001

 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

Eastlake, et al. Standards Track [Page 46] RFC 3075 XML-Signature Syntax and Processing March 2001

 that directly bears the XPath expression.  This expression results in
 an error if the containing XPath expression does not appear in the
 same XML document against which the XPath expression is being
 evaluated.
 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;.

Eastlake, et al. Standards Track [Page 47] RFC 3075 XML-Signature Syntax and Processing March 2001

 A more elegant solution uses the here function to omit only the
 Signature containing the XPath Transform, thus allowing enveloped
 signatures to sign other signatures.  In the example above, use the
 XPath element:
    <XPath xmlns:dsig="&dsig;">
    count(ancestor-or-self::dsig:Signature |
    here()/ancestor::dsig:Signature[1]) >
    count(ancestor-or-self::dsig:Signature)</XPath>
 Since the XPath equality operator converts node sets to string values
 before comparison, we must instead use the XPath union operator (|).
 For each node of the document, the predicate expression is true if
 and only if the node-set containing the node and its Signature
 element ancestors does not include the enveloped Signature element
 containing the XPath expression (the union does not produce a larger
 set if the enveloped Signature element is in the node-set given by
 ancestor-or-self::Signature).

6.6.4 Enveloped Signature Transform

 Identifier:
       http://www.w3.org/2000/09/xmldsig#enveloped-signature
 An enveloped signature transform T removes the whole Signature
 element containing T from the digest calculation of the Reference
 element containing T.  The entire string of characters used by an XML
 processor to match the Signature with the XML production element is
 removed.  The output of the transform is equivalent to the output
 that would result from replacing T with an XPath transform containing
 the following XPath parameter element:
    <XPath xmlns:dsig="&dsig;">
    count(ancestor-or-self::dsig:Signature |
    here()/ancestor::dsig:Signature[1]) >
    count(ancestor-or-self::dsig:Signature)</XPath>
 The input and output requirements of this transform are identical to
 those of the XPath transform.  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

Eastlake, et al. Standards Track [Page 48] RFC 3075 XML-Signature Syntax and Processing March 2001

 The normative specification for XSL Transformations is [XSLT].  The
 XSL style sheet or transform to be evaluated appears as the character
 content of a transform parameter child element named XSLT.  The root
 element of a XSLT style sheet 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 for the XSL style sheet or transform element are stated in the
 XSLT specification [XSLT].  We RECOMMEND that XSLT transformauthors
 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 transformafter the XSLT
 transformto 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

Eastlake, et al. Standards Track [Page 49] RFC 3075 XML-Signature Syntax and Processing March 2001

 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
 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 Normalization 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,

Eastlake, et al. Standards Track [Page 50] RFC 3075 XML-Signature Syntax and Processing March 2001

 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 constraints are observed when generating any
 signed material including the SignedInfo element:
 1. attributes having default values be explicitly present,
 2. all entity references (except "amp", "lt", "gt", "apos", "quot",
    and other character entities not representable in the encoding
    chosen) be expanded,
 3. attribute value white space be normalized

7.2 DOM/SAX Processing and Canonicalization

 In addition to the canonicalization and syntax constraints discussed
 above, many XML applications use the Document Object Model [DOM] or
 The Simple API for XML [SAX].  DOM maps XML into a tree structure of
 nodes and typically assumes it will be used on an entire document
 with subsequent processing being done on this tree.  SAX converts XML
 into a series of events such as a start tag, content, etc.  In either
 case, many surface characteristics such as the ordering of attributes
 and insignificant white space within start/end tags is lost.  In
 addition, namespace declarations are mapped over the nodes to which
 they apply, losing the namespace prefixes in the source text and, in
 most cases, losing where namespace declarations appeared in the
 original instance.
 If an XML Signature is to be produced or verified on a system using
 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

Eastlake, et al. Standards Track [Page 51] RFC 3075 XML-Signature Syntax and Processing March 2001

 Signature to be verifiable by an implementation using DOM or SAX, not
 only must the XML1.0 syntax constraints given in the previous section
 be followed but an appropriate XML canonicalization MUST be specified
 so that the verifier can re-serialize DOM/SAX mediated input into the
 same 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 transforms,
 canonicalization instructions or even XSLT transformations.  Three
 cautions are raised with respect to this feature in the following
 sections.
 Note, core validation behavior does not confirm that the signed data
 was obtained by applying each step of the indicated transforms.
 (Though it does check that the digest of the resulting content
 matches that specified in the signature.)  For example, some
 application may be satisfied with verifying an XML signature over a
 cached copy of already transformed data.  Other applications might
 require that content be freshly dereferenced and transformed.

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

Eastlake, et al. Standards Track [Page 52] RFC 3075 XML-Signature Syntax and Processing March 2001

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

8.1.3 "See" What is Signed

 Just as a user should only sign what 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 style sheet [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 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

Eastlake, et al. Standards Track [Page 53] RFC 3075 XML-Signature Syntax and Processing March 2001

 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,
 protection of cryptographic processing from hostile observation and
 tampering, etc.

Eastlake, et al. Standards Track [Page 54] RFC 3075 XML-Signature Syntax and Processing March 2001

 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
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
         core-schema.xsd   Valid XML schema instance based on the
       20000922 Schema/DTD [XML-Schema].
 XML Signature DTD
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
         core-schema.dtd
 RDF Data Model
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
         datamodel-20000112.gif
 XML Signature Object Example
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/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
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature-
         example-rsa.xml
       An XML Signature example with generated cryptographic values by
          Merlin Hughes and validated by Gregor Karlinger.
 DSA XML Signature Example
       http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature-
         example-dsa.xml   Similar to above but uses DSA.

Eastlake, et al. Standards Track [Page 55] RFC 3075 XML-Signature Syntax and Processing March 2001

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
       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

Eastlake, et al. Standards Track [Page 56] RFC 3075 XML-Signature Syntax and Processing March 2001

       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.
 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 identifier or transform).
 Signature, Enveloped
       The signature is over the XML content that contains the
       signature as an element.  The content provides the root XML

Eastlake, et al. Standards Track [Page 57] RFC 3075 XML-Signature Syntax and Processing March 2001

       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.
 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/xmldtd.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/

Eastlake, et al. Standards Track [Page 58] RFC 3075 XML-Signature Syntax and Processing March 2001

 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              Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                   Keyed-Hashing for Message Authentication", RFC
                   2104, February 1997.
                   http://www.ietf.org/rfc/rfc2104.txt
 HTTP              Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P. and T. Berners-Lee,
                   "Hypertext Transfer Protocol -- HTTP/1.1", RFC
                   2616, June 1999.
                   http://www.ietf.org/rfc/rfc2616.txt
 KEYWORDS          Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.
                   http://www.ietf.org/rfc/rfc2119.txt
 LDAP-DN           Wahl, M., Kille, S. and T. Howes, "Lightweight
                   Directory Access Protocol (v3): UTF-8 String
                   Representation of Distinguished Names", RFC 2253,
                   December 1997.  http://www.ietf.org/rfc/rfc2253.txt
 MD5               Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                   1321, April 1992.
                   http://www.ietf.org/rfc/rfc1321.txt
 MIME              Freed, N. and N. Borenstein, "Multipurpose Internet
                   Mail Extensions (MIME) Part One: Format of Internet
                   Message Bodies", RFC 2045, November 1996.
                   http://www.ietf.org/rfc/rfc2045.txt
 NFC               TR15. Unicode Normalization Forms. M. Davis, M.
                   Drst. Revision 18: November 1999.
 PGP               Callas, J., Donnerhacke, L., Finney, H. and R.
                   Thayer, "OpenPGP Message Format", November 1998.
                   http://www.ietf.org/rfc/rfc2440.txt
 RANDOM            Eastlake, D., Crocker, S. and J. Schiller,
                   "Randomness Recommendations for Security", RFC
                   1750, December 1994.
                   http://www.ietf.org/rfc/rfc1750.txt

Eastlake, et al. Standards Track [Page 59] RFC 3075 XML-Signature Syntax and Processing March 2001

 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/
 1363              IEEE 1363: Standard Specifications for Public Key
                   Cryptography.  August 2000.
 PKCS1             Kaliski, B. and J. Staddon, "PKCS #1: RSA
                   Cryptography Specifications Version 2.0", RFC 2437,
                   October 1998.  http://www.ietf.org/rfc/rfc2437.txt
 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            Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                   ISO 10646", RFC 2781, February 2000.
                   http://www.ietf.org/rfc/rfc2781.txt
 UTF-8             Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", RFC 2279, January 1998.
                   http://www.ietf.org/rfc/rfc2279.txt
 URI               Berners-Lee, T., Fielding, R. and L. Masinter,
                   "Uniform Resource Identifiers (URI): Generic
                   Syntax", RFC 2396, August 1998.
                   http://www.ietf.org/rfc/rfc2396.txt
 URI-Literal       Hinden, R., Carpenter, B. and L. Masinter, "Format
                   for Literal IPv6 Addresses in URL's", RFC 2732,
                   December 1999.  http://www.ietf.org/rfc/rfc2732.txt
 URL               Berners-Lee, T., Masinter, L. and M. McCahill,
                   "Uniform Resource Locators (URL)", RFC 1738,
                   December 1994.  http://www.ietf.org/rfc/rfc1738.txt

Eastlake, et al. Standards Track [Page 60] RFC 3075 XML-Signature Syntax and Processing March 2001

 URN               Moats, R., "URN Syntax" RFC 2141, May 1997.
                   http://www.ietf.org/rfc/rfc2141.txt
                   Daigle, L., van Gulik, D., Iannella, R. and P.
                   Faltstrom, "URN Namespace Definition Mechanisms",
                   RFC 2611, June 1999.
                   http://www.ietf.org/rfc/rfc2611.txt
 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          J. Boyer, "Canonical XML Version 1.0", RFC 3076,
                   September 2000.  http://www.w3.org/TR/2000/CR-xml-
                   c14n-20001026
                   http://www.ietf.org/rfc/rfc3076.txt
 XML-Japanese      XML Japanese Profile. W3C NOTE. M. MURATA April
                   2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-
                   20000414/
 XML-MT            Whitehead, E. and M. Murata, "XML Media Types",
                   July 1998.  http://www.ietf.org/rfc/rfc2376.txt
 XML-ns            Namespaces in XML Recommendation. T. Bray, D.
                   Hollander, A. Layman. Janury 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/

Eastlake, et al. Standards Track [Page 61] RFC 3075 XML-Signature Syntax and Processing March 2001

                   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  Reagle, J., "XML Signature Requirements", RFC 2907,
                   April 2000.  http://www.w3.org/TR/1999/WD-xmldsig-
                   requirements-19991014
                   http://www.ietf.org/rfc/rfc2807.txt
 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

Eastlake, et al. Standards Track [Page 62] RFC 3075 XML-Signature Syntax and Processing March 2001

12. Authors' Addresses

 Donald E. Eastlake 3rd
 Motorola, Mail Stop: M2-450
 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
 Cambridge, MA 02139
 Phone: 1.617.258.7621
 EMail: reagle@w3.org
 David Solo
 Citigroup
 909 Third Ave, 16th Floor
 NY, NY 10043 USA
 Phone: +1-212-559-2900
 EMail: dsolo@alum.mit.edu

Eastlake, et al. Standards Track [Page 63] RFC 3075 XML-Signature Syntax and Processing March 2001

13. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Eastlake, et al. Standards Track [Page 64]

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