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RFC 3275

(Extensible Markup Language) XML-Signature Syntax and Processing

Pages: 73
Draft Standard
Errata
Obsoletes:  3075
Part 1 of 3 – Pages 1 to 15
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Top   ToC   RFC3275 - Page 1
Network Working Group                                    D. Eastlake 3rd
Request for Comments: 3275                                      Motorola
Obsoletes: 3075                                                J. Reagle
Category: Standards Track                                            W3C
                                                                 D. Solo
                                                               Citigroup
                                                              March 2002


    (Extensible Markup Language) XML-Signature Syntax and Processing

Status of this Memo

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

   Copyright Notice

   Copyright (c) 2002 The Internet Society & W3C (MIT, INRIA, Keio), All
   Rights Reserved.

Abstract

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

Table of Contents

1. Introduction................................................... 3 1.1 Editorial and Conformance Conventions......................... 4 1.2 Design Philosophy............................................. 4 1.3 Versions, Namespaces and Identifiers.......................... 4 1.4 Acknowledgements.............................................. 6 1.5 W3C Status.................................................... 6 2. Signature Overview and Examples................................ 7 2.1 Simple Example (Signature, SignedInfo, Methods, and References) 8 2.1.1 More on Reference........................................... 9 2.2 Extended Example (Object and SignatureProperty)............... 10 2.3 Extended Example (Object and Manifest)........................ 12 3.0 Processing Rules.............................................. 13 3.1 Core Generation............................................... 13 3.1.1 Reference Generation........................................ 13
Top   ToC   RFC3275 - Page 2
   3.1.2 Signature Generation........................................ 13
   3.2 Core Validation............................................... 14
   3.2.1 Reference Validation........................................ 14
   3.2.2 Signature Validation........................................ 15
   4.0 Core Signature Syntax......................................... 15
   4.0.1 The ds:CryptoBinary Simple Type............................. 17
   4.1 The Signature element......................................... 17
   4.2 The SignatureValue Element.................................... 18
   4.3 The SignedInfo Element........................................ 18
   4.3.1 The CanonicalizationMethod Element.......................... 19
   4.3.2 The SignatureMethod Element................................. 21
   4.3.3 The Reference Element....................................... 21
   4.3.3.1 The URI Attribute......................................... 22
   4.3.3.2 The Reference Processing Model............................ 23
   4.3.3.3 Same-Document URI-References.............................. 25
   4.3.3.4 The Transforms Element.................................... 26
   4.3.3.5 The DigestMethod Element.................................. 28
   4.3.3.6 The DigestValue Element................................... 28
   4.4 The KeyInfo Element........................................... 29
   4.4.1 The KeyName Element......................................... 31
   4.4.2 The KeyValue Element........................................ 31
   4.4.2.1 The DSAKeyValue Element................................... 32
   4.4.2.2 The RSAKeyValue Element................................... 33
   4.4.3 The RetrievalMethod Element................................. 34
   4.4.4 The X509Data Element........................................ 35
   4.4.5 The PGPData Element......................................... 38
   4.4.6 The SPKIData Element........................................ 39
   4.4.7 The MgmtData Element........................................ 40
   4.5 The Object Element............................................ 40
   5.0 Additional Signature Syntax................................... 42
   5.1 The Manifest Element.......................................... 42
   5.2 The SignatureProperties Element............................... 43
   5.3 Processing Instructions in Signature Elements................. 44
   5.4 Comments in Signature Elements................................ 44
   6.0 Algorithms.................................................... 44
   6.1 Algorithm Identifiers and Implementation Requirements......... 44
   6.2 Message Digests............................................... 46
   6.2.1 SHA-1....................................................... 46
   6.3 Message Authentication Codes.................................. 46
   6.3.1 HMAC........................................................ 46
   6.4 Signature Algorithms.......................................... 47
   6.4.1 DSA......................................................... 47
   6.4.2 PKCS1 (RSA-SHA1)............................................ 48
   6.5 Canonicalization Algorithms................................... 49
   6.5.1 Canonical XML............................................... 49
   6.6 Transform Algorithms.......................................... 50
   6.6.1 Canonicalization............................................ 50
   6.6.2 Base64...................................................... 50
Top   ToC   RFC3275 - Page 3
   6.6.3 XPath Filtering............................................. 51
   6.6.4 Enveloped Signature Transform............................... 54
   6.6.5 XSLT Transform.............................................. 54
   7. XML Canonicalization and Syntax Constraint Considerations...... 55
   7.1 XML 1.0, Syntax Constraints, and Canonicalization............. 56
   7.2 DOM/SAX Processing and Canonicalization....................... 57
   7.3 Namespace Context and Portable Signatures..................... 58
   8.0 Security Considerations....................................... 59
   8.1 Transforms.................................................... 59
   8.1.1 Only What is Signed is Secure............................... 60
   8.1.2 Only What is 'Seen' Should be Signed........................ 60
   8.1.3 'See' What is Signed........................................ 61
   8.2 Check the Security Model...................................... 62
   8.3 Algorithms, Key Lengths, Certificates, Etc.................... 62
   9. Schema, DTD, Data Model, and Valid Examples.................... 63
   10. Definitions................................................... 63
   Appendix: Changes from RFC 3075................................... 67
   References........................................................ 67
   Authors' Addresses................................................ 72
   Full Copyright Statement.......................................... 73

1. Introduction

This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element. More specifically, this specification defines an XML signature element type and an XML signature application; conformance requirements for each are specified by way of schema definitions and prose respectively. This specification also includes other useful types that identify methods for referencing collections of resources, algorithms, and keying and management information. The XML Signature is a method of associating a key with referenced data (octets); it does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed. Consequently, while this specification is an important component of secure XML applications, it itself is not sufficient to address all application security/trust concerns, particularly with respect to using signed XML (or other data formats) as a basis of human-to-human communication and agreement. Such an application must specify additional key, algorithm, processing and rendering requirements. For further information, please see Security Considerations (section 8).
Top   ToC   RFC3275 - Page 4

1.1 Editorial and Conformance Conventions

For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types. Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See Check the Security Model, section 8.3.) This specification provides an XML Schema [XML-schema] and DTD [XML]. The schema definition is normative. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]: "they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)" Consequently, we use these capitalized key words to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the Namespaces in XML specification [XML-ns] is described as "REQUIRED."

1.2 Design Philosophy

The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature- RD].

1.3 Versions, Namespaces and Identifiers

No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace. The XML namespace [XML-ns] URI that MUST be used by implementations of this (dated) specification is: xmlns="http://www.w3.org/2000/09/xmldsig#"
Top   ToC   RFC3275 - Page 5
   This namespace is also used as the prefix for algorithm identifiers
   used by this specification.  While applications MUST support XML and
   XML namespaces, the use of internal entities [XML] or our "dsig" XML
   namespace prefix and defaulting/scoping conventions are OPTIONAL; we
   use these facilities to provide compact and readable examples.

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

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

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

   SHA1 is identified via this specification's namespace and defined via
   a normative reference
      http://www.w3.org/2000/09/xmldsig#sha1
      FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
      Commerce/National Institute of Standards and Technology.

   Finally, in order to provide for terse namespace declarations we
   sometimes use XML internal entities [XML] within URIs.  For instance:

      <?xml version='1.0'?>
      <!DOCTYPE Signature SYSTEM
        "xmldsig-core-schema.dtd" [ <!ENTITY dsig
        "http://www.w3.org/2000/09/xmldsig#"> ]>
      <Signature xmlns="&dsig;" Id="MyFirstSignature">
        <SignedInfo>
        ...
Top   ToC   RFC3275 - Page 6

1.4 Acknowledgements

The contributions of the following Working Group members to this specification are gratefully acknowledged: * Mark Bartel, Accelio (Author) * John Boyer, PureEdge (Author) * Mariano P. Consens, University of Waterloo * John Cowan, Reuters Health * Donald Eastlake 3rd, Motorola (Chair, Author/Editor) * Barb Fox, Microsoft (Author) * Christian Geuer-Pollmann, University Siegen * Tom Gindin, IBM * Phillip Hallam-Baker, VeriSign Inc * Richard Himes, US Courts * Merlin Hughes, Baltimore * Gregor Karlinger, IAIK TU Graz * Brian LaMacchia, Microsoft (Author) * Peter Lipp, IAIK TU Graz * Joseph Reagle, W3C (Chair, Author/Editor) * Ed Simon, XMLsec (Author) * David Solo, Citigroup (Author/Editor) * Petteri Stenius, DONE Information, Ltd * Raghavan Srinivas, Sun * Kent Tamura, IBM * Winchel Todd Vincent III, GSU * Carl Wallace, Corsec Security, Inc. * Greg Whitehead, Signio Inc. As are the Last Call comments from the following: * Dan Connolly, W3C * Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG. * Martin J. Duerst, W3C; and Masahiro Sekiguchi, Fujitsu; on behalf of the Internationalization WG/IG. * Jonathan Marsh, Microsoft, on behalf of the Extensible Stylesheet Language WG.

1.5 W3C Status

The World Wide Web Consortium Recommendation corresponding to this RFC is at: http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
Top   ToC   RFC3275 - Page 7

2. Signature Overview and Examples

This section provides an overview and examples of XML digital signature syntax. The specific processing is given in Processing Rules (section 3). The formal syntax is found in Core Signature Syntax (section 4) and Additional Signature Syntax (section 5). In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later. XML Signatures are applied to arbitrary digital content (data objects) via an indirection. Data objects are digested, the resulting value is placed in an element (with other information) and that element is then digested and cryptographically signed. XML digital signatures are represented by the Signature element which has the following structure (where "?" denotes zero or one occurrence; "+" denotes one or more occurrences; and "*" denotes zero or more occurrences): <Signature ID?> <SignedInfo> <CanonicalizationMethod/> <SignatureMethod/> (<Reference URI? > (<Transforms>)? <DigestMethod> <DigestValue> </Reference>)+ </SignedInfo> <SignatureValue> (<KeyInfo>)? (<Object ID?>)* </Signature> Signatures are related to data objects via URIs [URI]. Within an XML document, signatures are related to local data objects via fragment identifiers. Such local data can be included within an enveloping signature or can enclose an enveloped signature. Detached signatures are over external network resources or local data objects that reside within the same XML document as sibling elements; in this case, the signature is neither enveloping (signature is parent) nor enveloped attribute (signature is child). Since a Signature element (and its Id value/name) may co-exist or be combined with other elements (and their IDs) within a single XML document, care should be taken in choosing names such that there are no subsequent collisions that violate the ID uniqueness validity constraint [XML].
Top   ToC   RFC3275 - Page 8

2.1 Simple Example (Signature, SignedInfo, Methods, and References)

The following example is a detached signature of the content of the HTML4 in XML specification. [s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature> [s02-12] The required SignedInfo element is the information that is actually signed. Core validation of SignedInfo consists of two mandatory processes: validation of the signature over SignedInfo and validation of each Reference digest within SignedInfo. Note that the algorithms used in calculating the SignatureValue are also included in the signed information while the SignatureValue element is outside SignedInfo. [s03] The CanonicalizationMethod is the algorithm that is used to canonicalize the SignedInfo element before it is digested as part of the signature operation. Note that this example, and all examples in this specification, are not in canonical form.
Top   ToC   RFC3275 - Page 9
   [s04] The SignatureMethod is the algorithm that is used to convert
   the canonicalized SignedInfo into the SignatureValue.  It is a
   combination of a digest algorithm and a key dependent algorithm and
   possibly other algorithms such as padding, for example RSA-SHA1.  The
   algorithm names are signed to resist attacks based on substituting a
   weaker algorithm.  To promote application interoperability we specify
   a set of signature algorithms that MUST be implemented, though their
   use is at the discretion of the signature creator.  We specify
   additional algorithms as RECOMMENDED or OPTIONAL for implementation;
   the design also permits arbitrary user specified algorithms.

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

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

2.1.1 More on Reference

[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s05] The optional URI attribute of Reference identifies the data object to be signed. This attribute may be omitted on at most one Reference in a Signature. (This limitation is imposed in order to ensure that references and objects may be matched unambiguously.)
Top   ToC   RFC3275 - Page 10
   [s05-08] This identification, along with the transforms, is a
   description provided by the signer on how they obtained the signed
   data object in the form it was digested (i.e., the digested content).
   The verifier may obtain the digested content in another method so
   long as the digest verifies.  In particular, the verifier may obtain
   the content from a different location such as a local store, as
   opposed to that specified in the URI.

   [s06-08] Transforms is an optional ordered list of processing steps
   that were applied to the resource's content before it was digested.
   Transforms can include operations such as canonicalization,
   encoding/decoding (including compression/inflation), XSLT, XPath, XML
   schema validation, or XInclude.  XPath transforms permit the signer
   to derive an XML document that omits portions of the source document.
   Consequently those excluded portions can change without affecting
   signature validity.  For example, if the resource being signed
   encloses the signature itself, such a transform must be used to
   exclude the signature value from its own computation.  If no
   Transforms element is present, the resource's content is digested
   directly.  While the Working Group has specified mandatory (and
   optional) canonicalization and decoding algorithms, user specified
   transforms are permitted.

   [s09-10] DigestMethod is the algorithm applied to the data after
   Transforms is applied (if specified) to yield the DigestValue.  The
   signing of the DigestValue is what binds a resources content to the
   signer's key.

2.2 Extended Example (Object and SignatureProperty)

This specification does not address mechanisms for making statements or assertions. Instead, this document defines what it means for something to be signed by an XML Signature (integrity, message authentication, and/or signer authentication). Applications that wish to represent other semantics must rely upon other technologies, such as [XML, RDF]. For instance, an application might use a foo:assuredby attribute within its own markup to reference a Signature element. Consequently, it's the application that must understand and know how to make trust decisions given the validity of the signature and the meaning of assuredby syntax. We also define a SignatureProperties element type for the inclusion of assertions about the signature itself (e.g., signature semantics, the time of signing or the serial number of hardware used in cryptographic processes). Such assertions may be signed by including a Reference for the SignatureProperties in SignedInfo. While the signing application should be very careful about what it signs (it should understand what is in the SignatureProperty) a receiving application has no obligation to understand that semantic (though its parent
Top   ToC   RFC3275 - Page 11
   trust engine may wish to).  Any content about the signature
   generation may be located within the SignatureProperty element.  The
   mandatory Target attribute references the Signature element to which
   the property applies.

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

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

   [p04] The optional Type attribute of Reference provides information
   about the resource identified by the URI.  In particular, it can
   indicate that it is an Object, SignatureProperty, or Manifest
   element.  This can be used by applications to initiate special
   processing of some Reference elements.  References to an XML data
   element within an Object element SHOULD identify the actual element
   pointed to.  Where the element content is not XML (perhaps it is
   binary or encoded data) the reference should identify the Object and
   the Reference Type, if given, SHOULD indicate Object.  Note that Type
   is advisory and no action based on it or checking of its correctness
   is required by core behavior.
Top   ToC   RFC3275 - Page 12
   [p10] Object is an optional element for including data objects within
   the signature element or elsewhere.  The Object can be optionally
   typed and/or encoded.

   [p11-18] Signature properties, such as time of signing, can be
   optionally signed by identifying them from within a Reference.
   (These properties are traditionally called signature "attributes"
   although that term has no relationship to the XML term "attribute".)

2.3 Extended Example (Object and Manifest)

The Manifest element is provided to meet additional requirements not directly addressed by the mandatory parts of this specification. Two requirements and the way the Manifest satisfies them follow. First, applications frequently need to efficiently sign multiple data objects even where the signature operation itself is an expensive public key signature. This requirement can be met by including multiple Reference elements within SignedInfo since the inclusion of each digest secures the data digested. However, some applications may not want the core validation behavior associated with this approach because it requires every Reference within SignedInfo to undergo reference validation -- the DigestValue elements are checked. These applications may wish to reserve reference validation decision logic to themselves. For example, an application might receive a signature valid SignedInfo element that includes three Reference elements. If a single Reference fails (the identified data object when digested does not yield the specified DigestValue) the signature would fail core validation. However, the application may wish to treat the signature over the two valid Reference elements as valid or take different actions depending on which fails. To accomplish this, SignedInfo would reference a Manifest element that contains one or more Reference elements (with the same structure as those in SignedInfo). Then, reference validation of the Manifest is under application control. Second, consider an application where many signatures (using different keys) are applied to a large number of documents. An inefficient solution is to have a separate signature (per key) repeatedly applied to a large SignedInfo element (with many References); this is wasteful and redundant. A more efficient solution is to include many references in a single Manifest that is then referenced from multiple Signature elements. The example below includes a Reference that signs a Manifest found within the Object element.
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    [   ] ...
    [m01]   <Reference URI="#MyFirstManifest"
    [m02]     Type="http://www.w3.org/2000/09/xmldsig#Manifest">
    [m03]     <DigestMethod
   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    [m04]     <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
    [m05]   </Reference>
    [   ] ...
    [m06] <Object>
    [m07]   <Manifest Id="MyFirstManifest">
    [m08]     <Reference>
    [m09]     ...
    [m10]     </Reference>
    [m11]     <Reference>
    [m12]     ...
    [m13]     </Reference>
    [m14]   </Manifest>
    [m15] </Object>

3.0 Processing Rules

The sections below describe the operations to be performed as part of signature generation and validation.

3.1 Core Generation

The REQUIRED steps include the generation of Reference elements and the SignatureValue over SignedInfo.

3.1.1 Reference Generation

For each data object being signed: 1. Apply the Transforms, as determined by the application, to the data object. 2. Calculate the digest value over the resulting data object. 3. Create a Reference element, including the (optional) identification of the data object, any (optional) transform elements, the digest algorithm and the DigestValue. (Note, it is the canonical form of these references that are signed in 3.1.2 and validated in 3.2.1.)

3.1.2 Signature Generation

1. Create SignedInfo element with SignatureMethod, CanonicalizationMethod and Reference(s). 2. Canonicalize and then calculate the SignatureValue over SignedInfo based on algorithms specified in SignedInfo.
Top   ToC   RFC3275 - Page 14
   3. Construct the Signature element that includes SignedInfo,
      Object(s) (if desired, encoding may be different than that used
      for signing), KeyInfo (if required), and SignatureValue.

   Note, if the Signature includes same-document references, [XML] or
   [XML-schema] validation of the document might introduce changes that
   break the signature.  Consequently, applications should be careful to
   consistently process the document or refrain from using external
   contributions (e.g., defaults and entities).

3.2 Core Validation

The REQUIRED steps of core validation include (1) reference validation, the verification of the digest contained in each Reference in SignedInfo, and (2) the cryptographic signature validation of the signature calculated over SignedInfo. Note, there may be valid signatures that some signature applications are unable to validate. Reasons for this include failure to implement optional parts of this specification, inability or unwillingness to execute specified algorithms, or inability or unwillingness to dereference specified URIs (some URI schemes may cause undesirable side effects), etc. Comparison of values in reference and signature validation are over the numeric (e.g., integer) or decoded octet sequence of the value. Different implementations may produce different encoded digest and signature values when processing the same resources because of variances in their encoding, such as accidental white space. But if one uses numeric or octet comparison (choose one) on both the stated and computed values these problems are eliminated.

3.2.1 Reference Validation

1. Canonicalize the SignedInfo element based on the CanonicalizationMethod in SignedInfo. 2. For each Reference in SignedInfo: 2.1 Obtain the data object to be digested. (For example, the signature application may dereference the URI and execute Transforms provided by the signer in the Reference element, or it may obtain the content through other means such as a local cache.) 2.2 Digest the resulting data object using the DigestMethod specified in its Reference specification. 2.3 Compare the generated digest value against DigestValue in the SignedInfo Reference; if there is any mismatch, validation fails.
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   Note, SignedInfo is canonicalized in step 1.  The application must
   ensure that the CanonicalizationMethod has no dangerous side affects,
   such as rewriting URIs, (see CanonicalizationMethod (section 4.3))
   and that it Sees What is Signed, which is the canonical form.

3.2.2 Signature Validation

1. Obtain the keying information from KeyInfo or from an external source. 2. Obtain the canonical form of the SignatureMethod using the CanonicalizationMethod and use the result (and previously obtained KeyInfo) to confirm the SignatureValue over the SignedInfo element. Note, KeyInfo (or some transformed version thereof) may be signed via a Reference element. Transformation and validation of this reference (3.2.1) is orthogonal to Signature Validation which uses the KeyInfo as parsed. Additionally, the SignatureMethod URI may have been altered by the canonicalization of SignedInfo (e.g., absolutization of relative URIs) and it is the canonical form that MUST be used. However, the required canonicalization [XML-C14N] of this specification does not change URIs.


(page 15 continued on part 2)

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