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RFC 3075

XML-Signature Syntax and Processing

Pages: 64
Obsoleted by:  3275
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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
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         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
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   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
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   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#"
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   This namespace is also used as the prefix for algorithm identifiers
   used by this specification.  While applications MUST support XML and
   XML-namespaces, the use of internal entities [XML] or our "dsig" XML
   namespace prefix and defaulting/scoping conventions are OPTIONAL; we
   use these facilities to provide compact and readable examples.

   This specification uses Uniform Resource Identifiers [URI] to
   identify resources, algorithms, and semantics.  The URI in the
   namespace declaration above is also used as a prefix for URIs under
   the control of this specification.  For resources not under the
   control of this specification, we use the designated Uniform Resource
   Names [URN] or Uniform Resource Locators [URL] defined by its
   normative external specification.  If an external specification has
   not allocated itself a Uniform Resource Identifier we allocate an
   identifier under our own namespace.  For instance:

   SignatureProperties is identified and defined by this specification's
         namespace
         http://www.w3.org/2000/09/xmldsig#SignatureProperties

   XSLT is identified and defined by an external URI
         http://www.w3.org/TR/1999/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
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      *  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
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   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"/>
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[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.
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   [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
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   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>
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[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
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   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>
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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.
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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:
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   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"/>
ToP   noToC   RFC3075 - Page 16
      <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>
ToP   noToC   RFC3075 - Page 17
        <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
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         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>
ToP   noToC   RFC3075 - Page 19
   </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
ToP   noToC   RFC3075 - Page 20
   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.
ToP   noToC   RFC3075 - Page 21
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).)
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   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
ToP   noToC   RFC3075 - Page 23
   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).
ToP   noToC   RFC3075 - Page 24
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:
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<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:
ToP   noToC   RFC3075 - Page 26
   <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 -->


(page 26 continued on part 2)

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