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Network Working Group                                        D. EastlakeRequest for Comments: 3075                                      MotorolaCategory: Standards Track                                      J. Reagle                                                                 W3C/MIT                                                                 D. Solo                                                               Citigroup                                                              March 2001XML-Signature Syntax and ProcessingStatus 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 Contents1.  Introduction ................................................31. Editorial Conventions ..................................32. Design Philosophy ......................................43. Versions, Namespaces and Identifiers ...................44. Acknowledgements .......................................52.  Signature Overview and Examples .............................6         1. Simple Example (Signature, SignedInfo, Methods, and            References) ............................................71. More on Reference .................................92. Extended Example (Object and SignatureProperty) ........103. Extended Example (Object and Manifest) .................113.  Processing Rules ............................................131. Core Generation .... ...................................131. Reference Generation ..............................132. Signature Generation ..............................13Eastlake, et al.            Standards Track                     [Page 1]

RFC 3075          XML-Signature Syntax and Processing         March 20012. Core Validation ........................................131. Reference Validation ..............................142. Signature Validation ..............................144.  Core Signature Syntax .......................................141. The Signature element ..................................152. The SignatureValue Element .............................163. The SignedInfo Element .................................161. The CanonicalizationMethod Element ................172. The SignatureMethod Element .......................183. The Reference Element .............................191. The URI Attribute ............................192. The Reference Processing Model ...............213. Same-Document URI-References .................234. The Transforms Element .......................245. The DigestMethod Element .....................256. The DigestValue Element ......................264. The KeyInfo Element ....................................261. The KeyName Element ...............................272. The KeyValue Element ..............................283. The RetrievalMethod Element .......................284. The X509Data Element ..............................295. The PGPData Element ...............................316. The SPKIData Element ..............................327. The MgmtData Element ..............................325. The Object Element .....................................335.  Additional Signature Syntax .................................341. The Manifest Element ...................................342. The SignatureProperties Element ........................353. Processing Instructions ................................364. Comments in dsig Elements ..............................366.  Algorithms ..................................................361. Algorithm Identifiers and Implementation Requirements ..362. Message Digests ........................................381. SHA-1 .............................................383. Message Authentication Codes ...........................381. HMAC ..............................................384. Signature Algorithms ...................................391. DSA ...............................................392. PKCS1 .............................................405. Canonicalization Algorithms ............................421. Minimal Canonicalization ..........................432. Canonical XML .....................................436. Transform Algorithms ...................................441. Canonicalization ..................................442. Base64 ............................................443. XPath Filtering ...................................454. Enveloped Signature Transform .....................485. XSLT Transform ....................................48Eastlake, et al.            Standards Track                     [Page 2]

RFC 3075          XML-Signature Syntax and Processing         March 20017.  XML Canonicalization and Syntax Constraint Considerations ...491. XML 1.0, Syntax Constraints, and Canonicalization  .....502. DOM/SAX Processing and Canonicalization ................518.  Security Considerations .....................................521. Transforms .............................................521. Only What is Signed is Secure .....................522. Only What is "Seen" Should be Signed ..............533. "See" What is Signed ..............................532. Check the Security Model ...............................543. Algorithms, Key Lengths, Etc. ..........................549.  Schema, DTD, Data Model,and Valid Examples ..................5510. Definitions .................................................5611. References ..................................................5812. Authors' Addresses ..........................................6313. Full Copyright Statement ....................................641.0 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 toEastlake, 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 inRFC2119   [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         namespacehttp://www.w3.org/2000/09/xmldsig#SignatureProperties   XSLT is identified and defined by an external URIhttp://www.w3.org/TR/1999/PR-xslt-19991008   SHA1 is identified via this specification's namespace and defined via         a normative referencehttp://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;">        <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 WaterlooEastlake, 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 hasEastlake, 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       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 (andEastlake, 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 ...>[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          >[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 thisEastlake, 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>[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 20013.0 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 20013.2.1 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 insection 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 (seesection 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 ofEastlake, 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 insection 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 byRFC2396 [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 allEastlake, et al.            Standards Track                    [Page 19]

RFC 3075          XML-Signature Syntax and Processing         March 2001   non-ASCII characters and the excluded characters listed inRFC2396   [URI,section 2.4].  However, the number sign (#), percent sign (%),   and square bracket characters re-allowed inRFC 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 ofhttp://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 20014.3.3.2 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 signatureEastlake, 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 20014.3.3.4 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.      *http://www.w3.org/2000/09/xmldsig#X509Data      *http://www.w3.org/2000/09/xmldsig#PGPData      *http://www.w3.org/2000/09/xmldsig#SPKIData      *http://www.w3.org/2000/09/xmldsig#MgmtData   In addition to the types above for which we define structures, we   specify one additional type to indicate a binary X.509 Certificate      *http://www.w3.org/2000/09/xmldsig#rawX509Certificate   Schema Definition:<element name="KeyInfo">  <complexType>    <choice maxOccurs="unbounded">      <any processContents="lax" namespace="##other" minOccurs="0"       maxOccurs="unbounded"/>      <element name="KeyName" type="string"/>      <element ref="ds:KeyValue"/>      <element ref="ds:RetrievalMethod"/>      <element ref="ds:X509Data"/>      <element ref="ds:PGPData"/>      <element ref="ds:SPKIData"/>      <element name="MgmtData" type="string"/>    </choice>    <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      withRFC2253 [LDAP-DN],   2. The X509SubjectName element, which contains an X.509 subject      distinguished name that SHOULD be compliant withRFC2253 [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 roleEastlake, 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 20016.2 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   fromAppendix 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 theRFC 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 B6DCD8C8Eastlake, 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 paddedEastlake, 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 inRFC 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 perRFC 2437 [PKCS1,section8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature   scheme].  As specified in the EMSA-PKCS1-V1_5-ENCODE functionRFC2437 [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 externallyEastlake, 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 automaticallyEastlake, 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 physicallyEastlake, 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 nodeEastlake, 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-19991116Eastlake, 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, XMLEastlake, 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 normalized7.2 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 XMLEastlake, 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) SHOULDEastlake, 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 butEastlake, 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 Instancehttp://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 DTDhttp://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-core-schema.dtd   RDF Data Modelhttp://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-datamodel-20000112.gif   XML Signature Object Examplehttp://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 Examplehttp://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 Examplehttp://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 200110.0 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 aEastlake, 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 XMLEastlake, 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",RFC2104, 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",RFC2616, 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",RFC1321, 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",RFC1750, December 1994.http://www.ietf.org/rfc/rfc1750.txtEastlake, 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.txtEastlake, 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-20001026http://www.ietf.org/rfc/rfc3076.txt   XML-Japanese      XML Japanese Profile. W3C NOTE. M. MURATA April                     2000http://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-19991014http://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.htmlEastlake, et al.            Standards Track                    [Page 62]

RFC 3075          XML-Signature Syntax and Processing         March 200112. 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.eduEastlake, et al.            Standards Track                    [Page 63]

RFC 3075          XML-Signature Syntax and Processing         March 200113. 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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