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RFC 3275

(Extensible Markup Language) XML-Signature Syntax and Processing

Pages: 73
Draft Standard
Errata
Obsoletes:  3075
Part 2 of 3 – Pages 15 to 44
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Top   ToC   RFC3275 - Page 15   prevText

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, and internal entity.
Top   ToC   RFC3275 - Page 16
      Schema Definition:

      <?xml version="1.0" encoding="utf-8"?>
      <!DOCTYPE schema
        PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
   "http://www.w3.org/2001/XMLSchema.dtd"
       [
         <!ATTLIST schema
           xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
         <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
         <!ENTITY % p ''>
         <!ENTITY % s ''>
        ]>

      <schema xmlns="http://www.w3.org/2001/XMLSchema"
              xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
              targetNamespace="http://www.w3.org/2000/09/xmldsig#"
              version="0.1" elementFormDefault="qualified">

      DTD:

      <!--

      The following entity declarations enable external/flexible content
      in the Signature content model.

      #PCDATA emulates schema:string; when combined with element types
      it emulates schema mixed="true".

      %foo.ANY permits the user to include their own element types from
      other namespaces, for example:
        <!ENTITY % KeyValue.ANY '| ecds:ECDSAKeyValue'>
        ...
        <!ELEMENT ecds:ECDSAKeyValue (#PCDATA)  >

      -->

      <!ENTITY % Object.ANY ''>
      <!ENTITY % Method.ANY ''>
      <!ENTITY % Transform.ANY ''>
      <!ENTITY % SignatureProperty.ANY ''>
      <!ENTITY % KeyInfo.ANY ''>
      <!ENTITY % KeyValue.ANY ''>
      <!ENTITY % PGPData.ANY ''>
      <!ENTITY % X509Data.ANY ''>
      <!ENTITY % SPKIData.ANY ''>
Top   ToC   RFC3275 - Page 17

4.0.1 The ds:CryptoBinary Simple Type

This specification defines the ds:CryptoBinary simple type for representing arbitrary-length integers (e.g., "bignums") in XML as octet strings. The integer value is first converted to a "big endian" bitstring. The bitstring is then padded with leading zero bits so that the total number of bits == 0 mod 8 (so that there are an integral number of octets). If the bitstring contains entire leading octets that are zero, these are removed (so the high-order octet 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). This type is used by "bignum" values such as RSAKeyValue and DSAKeyValue. If a value can be of type base64Binary or ds:CryptoBinary they are defined as base64Binary. For example, if the signature algorithm is RSA or DSA then SignatureValue represents a bignum and could be ds:CryptoBinary. However, if HMAC-SHA1 is the signature algorithm then SignatureValue could have leading zero octets that must be preserved. Thus SignatureValue is generically defined as of type base64Binary. Schema Definition: <simpleType name="CryptoBinary"> <restriction base="base64Binary"> </restriction> </simpleType>

4.1 The Signature element

The Signature element is the root element of an XML Signature. Implementation MUST generate laxly schema valid [XML-schema] Signature elements as specified by the following schema: Schema Definition: <element name="Signature" type="ds:SignatureType"/> <complexType name="SignatureType"> <sequence> <element ref="ds:SignedInfo"/> <element ref="ds:SignatureValue"/> <element ref="ds:KeyInfo" minOccurs="0"/> <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
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      DTD:

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

4.2 The SignatureValue Element

The SignatureValue element contains the actual value of the digital signature; it is always encoded using base64 [MIME]. While we identify two SignatureMethod algorithms, one mandatory and one optional to implement, user specified algorithms may be used as well. Schema Definition: <element name="SignatureValue" type="ds:SignatureValueType"/> <complexType name="SignatureValueType"> <simpleContent> <extension base="base64Binary"> <attribute name="Id" type="ID" use="optional"/> </extension> </simpleContent> </complexType> DTD: <!ELEMENT SignatureValue (#PCDATA) > <!ATTLIST SignatureValue Id ID #IMPLIED>

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.
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      Schema Definition:

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

      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 canonicalization algorithms. Alternatives to the REQUIRED canonicalization algorithms (section 6.5), such as Canonical XML with Comments (section 6.5.1) or a minimal canonicalization (such as CRLF and charset normalization), may be explicitly specified but are NOT REQUIRED. Consequently, their use may not interoperate with other applications that do not 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 non-XML aware canonicalization algorithms are not properly constrained (see section 8.2: Only What is "Seen" Should be Signed). The way in which the SignedInfo element is presented to the canonicalization method is dependent on that method. The following applies to algorithms which process XML as nodes or characters: * XML based canonicalization implementations MUST be provided with a [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.
Top   ToC   RFC3275 - Page 20
      *  Text based canonicalization algorithms (such as CRLF and
         charset normalization) should be provided with the UTF-8 octets
         that represent the well-formed SignedInfo element, from the
         first character to the last character of the XML
         representation, inclusive.  This includes the entire text of
         the start and end tags of the SignedInfo element as well as all
         descendant markup and character data (i.e., the text) between
         those tags.  Use of text based canonicalization of SignedInfo
         is NOT RECOMMENDED.

   We recommend applications that implement a text-based instead of
   XML-based canonicalization -- such as resource constrained apps --
   generate canonicalized XML as their output serialization so as to
   mitigate interoperability and security concerns.  For instance, such
   an implementation SHOULD (at least) generate standalone XML instances
   [XML].

   NOTE: The signature application must exercise great care in accepting
   and executing an arbitrary CanonicalizationMethod.  For example, the
   canonicalization method could rewrite the URIs of the References
   being validated.  Or, the method could massively transform SignedInfo
   so that validation would always succeed (i.e., converting it to a
   trivial signature with a known key over trivial data).  Since
   CanonicalizationMethod is inside SignedInfo, in the resulting
   canonical form it could erase itself from SignedInfo or modify the
   SignedInfo element so that it appears that a different
   canonicalization function was used! Thus a Signature which appears to
   authenticate the desired data with the desired key, DigestMethod, and
   SignatureMethod, can be meaningless if a capricious
   CanonicalizationMethod is used.

      Schema Definition:

      <element name="CanonicalizationMethod"
               type="ds:CanonicalizationMethodType"/>
      <complexType name="CanonicalizationMethodType" mixed="true">
        <sequence>
          <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
          <!-- (0,unbounded) elements from (1,1) namespace -->
        </sequence>
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>

      DTD:

      <!ELEMENT CanonicalizationMethod (#PCDATA %Method.ANY;)* >
      <!ATTLIST CanonicalizationMethod
       Algorithm CDATA #REQUIRED >
Top   ToC   RFC3275 - Page 21

4.3.2 The SignatureMethod Element

SignatureMethod is a required element that specifies the algorithm used for signature generation and validation. This algorithm identifies all cryptographic functions involved in the signature operation (e.g., hashing, public key algorithms, MACs, padding, etc.). This element uses the general structure here for algorithms described in section 6.1: Algorithm Identifiers and Implementation Requirements. While there is a single identifier, that identifier may specify a format containing multiple distinct signature values. Schema Definition: <element name="SignatureMethod" type="ds:SignatureMethodType"/> <complexType name="SignatureMethodType" mixed="true"> <sequence> <element name="HMACOutputLength" minOccurs="0" type="ds:HMACOutputLengthType"/> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) external namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType> DTD: <!ELEMENT SignatureMethod (#PCDATA|HMACOutputLength %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.
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      Schema Definition:

      <element name="Reference" type="ds:ReferenceType"/>
      <complexType name="ReferenceType">
        <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="anyURI" use="optional"/>
        <attribute name="Type" type="anyURI" use="optional"/>
      </complexType>

      DTD:

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

4.3.3.1 The URI Attribute
The URI attribute identifies a data object using a URI-Reference, as specified by RFC2396 [URI]. The set of allowed characters for URI attributes is the same as for XML, namely [Unicode]. However, some Unicode characters are disallowed from URI references including all non-ASCII characters and the excluded characters listed in RFC2396 [URI, section 2.4]. However, the number sign (#), percent sign (%), and square bracket characters re-allowed in RFC 2732 [URI-Literal] are permitted. Disallowed characters must be escaped as follows: 1. Each disallowed character is converted to [UTF-8] as one or more octets. 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 octet 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
Top   ToC   RFC3275 - Page 23
   parameter and state information, (such as HTTP cookies, HTML device
   profiles or content negotiation), may affect the content yielded by
   dereferencing a URI.

   If a resource is identified by more than one URI, the most specific
   should be used (e.g., http://www.w3.org/2000/06/interop-
   pressrelease.html.en instead of http://www.w3.org/2000/06/interop-
   pressrelease).  (See the Reference Validation (section 3.2.1) for a
   further information on reference processing.)

   If the URI attribute is omitted altogether, the receiving application
   is expected to know the identity of the object.  For example, a
   lightweight data protocol might omit this attribute given the
   identity of the object is part of the application context.  This
   attribute may be omitted from at most one Reference in any particular
   SignedInfo, or Manifest.

   The optional Type attribute contains information about the type of
   object being signed.  This is represented as a URI.  For example:

   Type="http://www.w3.org/2000/09/xmldsig#Object"
   Type="http://www.w3.org/2000/09/xmldsig#Manifest"

   The Type attribute applies to the item being pointed at, not its
   contents.  For example, a reference that identifies an Object element
   containing a SignatureProperties element is still of type #Object.
   The type attribute is advisory.  No validation of the type
   information is required by this specification.

4.3.3.2 The Reference Processing Model
Note: XPath is RECOMMENDED. Signature applications need not conform to [XPath] specification in order to conform to this specification. However, the XPath data model, definitions (e.g., node-sets) and syntax is used within this document in order to describe functionality for those that want to process XML-as-XML (instead of octets) as part of signature generation. For those that want to use these features, a conformant [XPath] implementation is one way to implement these features, but it is not required. Such applications could use a sufficiently functional replacement to a node-set and implement only those XPath expression behaviors REQUIRED by this specification. However, for simplicity we generally will use XPath terminology without including this qualification on every point. Requirements over "XPath node-sets" can include a node-set functional equivalent. Requirements over XPath processing can include application behaviors that are equivalent to the corresponding XPath behavior.
Top   ToC   RFC3275 - Page 24
   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 an octet stream and the next transform
         requires a node-set, the signature application MUST attempt to
         parse the octets yielding the required node-set via [XML]
         well-formed processing.
      *  If the data object is a node-set and the next transform
         requires octets, the signature application MUST attempt to
         convert the node-set to an octet stream using Canonical XML
         [XML-C14N].

   Users may specify alternative transforms that override 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 transform that requires XML
   parsing is applied.  (See Transforms (section 4.3.3.1).)

   When a fragment is preceded by an absolute or relative URI in the
   URI-Reference, the meaning of the fragment is defined by the
   resource's MIME type.  Even for XML documents, URI dereferencing
   (including the fragment processing) might be done for the signature
   application by a proxy.  Therefore, reference validation might fail
   if fragment processing is not performed in a standard way (as defined
   in the following section for same-document references).
   Consequently, we RECOMMEND that the URI attribute not include
   fragment identifiers and that such processing be specified as an
   additional XPath Transform.

   When a fragment is not preceded by a URI in the URI-Reference, XML
   signature applications MUST support the null URI and barename
   XPointer.  We RECOMMEND support for the same-document XPointers
   '#xpointer(/)' and '#xpointer(id('ID'))' if the application also
   intends to support any canonicalization that preserves comments.
   (Otherwise URI="#foo" will automatically remove comments before the
   canonicalization can even be invoked.)  All other support for
   XPointers is OPTIONAL, especially all support for barename and other
Top   ToC   RFC3275 - Page 25
   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 an XML document
       given its file extension.
   URI="http://example.com/bar.xml#chapter1"
       Identifies the element with ID attribute value 'chapter1' of the
       external XML resource 'http://example.com/bar.xml', provided as
       an octet stream.  Again, for the sake of interoperability, the
       element identified as 'chapter1' should be obtained using an
       XPath transform rather than a URI fragment (barename XPointer
       resolution in external resources is not REQUIRED in this
       specification).
   URI=""
       Identifies the node-set (minus any comment nodes) of the XML
       resource containing the signature
   URI="#chapter1"
       Identifies a node-set containing the element with ID attribute
       value 'chapter1' of the XML resource containing the signature.
       XML Signature (and its applications) modify this node-set 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 [XML-C14N]. 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)
Top   ToC   RFC3275 - Page 26
   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.  It's necessary because when [XML-C14N] is
   passed a node-set, it processes the node-set as is: with or without
   comments.  Only when it's called with an octet stream does it invoke
   its own XPath expressions (default or without comments).  Therefore
   to retain the default behavior of stripping comments when passed a
   node-set, they are removed in the last step if the URI is not a full
   XPointer.  To retain comments while selecting an element by an
   identifier ID, use the following full XPointer:
   URI='#xpointer(id('ID'))'.  To retain comments while selecting the
   entire document, use the following full XPointer: URI='#xpointer(/)'.
   This XPointer contains a simple XPath expression that includes the
   root node, which the second to last step above replaces with all
   nodes of the parse tree (all descendants, plus all attributes, plus
   all namespaces nodes).

4.3.3.4 The Transforms Element
The optional Transforms element contains an ordered list of Transform elements; these describe how the signer obtained the data object that was digested. The output of each Transform serves as input to the next Transform. The input to the first Transform is the result of dereferencing the URI attribute of the Reference element. The output from the last Transform is the input for the DigestMethod algorithm. When transforms are applied the signer is not signing the native (original) document but the resulting (transformed) document. (See 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).)
Top   ToC   RFC3275 - Page 27
   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 Transforms 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) define the list of standard transformations.

      Schema Definition:

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

      <element name="Transform" type="ds:TransformType"/>
      <complexType name="TransformType" mixed="true">
        <choice minOccurs="0" maxOccurs="unbounded">
          <any namespace="##other" processContents="lax"/>
          <!-- (1,1) elements from (0,unbounded) namespaces -->
          <element name="XPath" type="string"/>
        </choice>
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </complexType>
Top   ToC   RFC3275 - Page 28
      DTD:

      <!ELEMENT Transforms (Transform+)>

      <!ELEMENT Transform (#PCDATA|XPath %Transform.ANY;)* >
      <!ATTLIST Transform
       Algorithm    CDATA    #REQUIRED >

      <!ELEMENT XPath (#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 Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream. Schema Definition: <element name="DigestMethod" type="ds:DigestMethodType"/> <complexType name="DigestMethodType" mixed="true"> <sequence> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType> DTD: <!ELEMENT DigestMethod (#PCDATA %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].
Top   ToC   RFC3275 - Page 29
      Schema Definition:

      <element name="DigestValue" type="ds:DigestValueType"/>
      <simpleType name="DigestValueType">
        <restriction base="base64Binary"/>
      </simpleType>

      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 extend those types or all together replace them with their own key identification and exchange semantics using the XML namespace facility. [XML-ns] However, questions of trust of such key information (e.g., its authenticity or strength) are out of scope of this specification and left to the application. If KeyInfo is omitted, the recipient is expected to be able to identify the key based on application context. Multiple declarations within KeyInfo refer to the same key. While applications may define and use any mechanism they choose through inclusion of elements from a different namespace, compliant versions MUST implement KeyValue (section 4.4.2) and SHOULD implement RetrievalMethod (section 4.4.3). The schema/DTD specifications of many of KeyInfo's children (e.g., PGPData, SPKIData, X509Data) permit their content to be extended/complemented with elements from another namespace. This may be done only if it is safe to ignore these extension elements while claiming support for the types defined in this specification. Otherwise, external elements, including alternative structures to those defined by this specification, MUST be a child of KeyInfo. For example, should a complete XML-PGP standard be defined, its root element MUST be a child of KeyInfo. (Of course, new structures from external namespaces can incorporate elements from the &dsig; namespace via features of the type definition language. For instance, they can create a DTD that mixes their own and dsig qualified elements, or a schema that permits, includes, imports, or derives new types based on &dsig; elements.)
Top   ToC   RFC3275 - Page 30
   The following list summarizes the KeyInfo types that are allocated to
   an identifier in the &dsig; namespace; these can be used within the
   RetrievalMethod Type attribute to describe a remote KeyInfo
   structure.

      * http://www.w3.org/2000/09/xmldsig#DSAKeyValue
      * http://www.w3.org/2000/09/xmldsig#RSAKeyValue
      * 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 an XML structure,
   we specify one additional type to indicate a binary (ASN.1 DER) X.509
   Certificate.

      * http://www.w3.org/2000/09/xmldsig#rawX509Certificate

      Schema Definition:

      <element name="KeyInfo" type="ds:KeyInfoType"/>
      <complexType name="KeyInfoType" mixed="true">
        <choice maxOccurs="unbounded">
          <element ref="ds:KeyName"/>
          <element ref="ds:KeyValue"/>
          <element ref="ds:RetrievalMethod"/>
          <element ref="ds:X509Data"/>
          <element ref="ds:PGPData"/>
          <element ref="ds:SPKIData"/>
          <element ref="ds:MgmtData"/>
          <any processContents="lax" namespace="##other"/>
          <!-- (1,1) elements from (0,unbounded) namespaces -->
        </choice>
        <attribute name="Id" type="ID" use="optional"/>
      </complexType>

      DTD:

      <!ELEMENT KeyInfo (#PCDATA|KeyName|KeyValue|RetrievalMethod|
                  X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* >
      <!ATTLIST KeyInfo
       Id  ID   #IMPLIED >
Top   ToC   RFC3275 - Page 31

4.4.1 The KeyName Element

The KeyName element contains a string value (in which white space is significant) which may be used by the signer to communicate a key identifier to the recipient. Typically, KeyName contains an identifier related to the key pair used to sign the message, but it may contain other protocol-related information that indirectly identifies a key pair. (Common uses of KeyName include simple string names for keys, a key index, a distinguished name (DN), an email address, etc.) Schema Definition: <element name="KeyName" type="string"/> 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). The KeyValue element may include externally defined public key values represented as PCDATA or element types from an external namespace. Schema Definition: <element name="KeyValue" type="ds:KeyValueType"/> <complexType name="KeyValueType" mixed="true"> <choice> <element ref="ds:DSAKeyValue"/> <element ref="ds:RSAKeyValue"/> <any namespace="##other" processContents="lax"/> </choice> </complexType> DTD: <!ELEMENT KeyValue (#PCDATA|DSAKeyValue|RSAKeyValue %KeyValue.ANY;)* >
Top   ToC   RFC3275 - Page 32
4.4.2.1 The DSAKeyValue Element
Identifier Type="http://www.w3.org/2000/09/xmldsig#DSAKeyValue" (this can be used within a RetrievalMethod or Reference element to identify the referent's type) DSA keys and the DSA signature algorithm are specified in [DSS]. DSA public key values can have the following fields: P a prime modulus meeting the [DSS] requirements Q an integer in the range 2**159 < Q < 2**160 which is a prime divisor of P-1 G an integer with certain properties with respect to P and Q Y G**X mod P (where X is part of the private key and not made public) J (P - 1) / Q seed a DSA prime generation seed pgenCounter a DSA prime generation counter Parameter J is available for inclusion solely for efficiency as it is calculatable from P and Q. Parameters seed and pgenCounter are used in the DSA prime number generation algorithm specified in [DSS]. As such, they are optional, but must either both be present or both be absent. This prime generation algorithm is designed to provide assurance that a weak prime is not being used and it yields a P and Q value. Parameters P, Q, and G can be public and common to a group of users. They might be known from application context. As such, they are optional but P and Q must either both appear or both be absent. If all of P, Q, seed, and pgenCounter are present, implementations are not required to check if they are consistent and are free to use either P and Q or seed and pgenCounter. All parameters are encoded as base64 [MIME] values. Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are represented in XML as octet strings as defined by the ds:CryptoBinary type.
Top   ToC   RFC3275 - Page 33
      Schema Definition:

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

      DTD Definition:

      <!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) >

4.4.2.2 The RSAKeyValue Element
Identifier Type="http://www.w3.org/2000/09/xmldsig#RSAKeyValue" (this can be used within a RetrievalMethod or Reference element to identify the referent's type) RSA key values have two fields: Modulus and Exponent. <RSAKeyValue> <Modulus> xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRg BUwUlV5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U= </Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue>
Top   ToC   RFC3275 - Page 34
   Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
   represented in XML as octet strings as defined by the ds:CryptoBinary
   type.

      Schema Definition:

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

      DTD Definition:

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

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. Type is an optional identifier for the type of data to be retrieved. The result of dereferencing a RetrievalMethod Reference for all KeyInfo types defined by this specification (section 4.4) with a corresponding XML structure is an XML element or document with that element as the root. The rawX509Certificate KeyInfo (for which there is no XML structure) returns a binary X509 certificate.
Top   ToC   RFC3275 - Page 35
      Schema Definition:

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

      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 a revocation list). The content of X509Data is: 1. At least one element, from the following set of element types; any of these may appear together or more than once if (if and only if) each instance describes or is related to the same certificate: 2. o The X509IssuerSerial element, which contains an X.509 issuer distinguished name/serial number pair that SHOULD be compliant with RFC 2253 [LDAP-DN], o The X509SubjectName element, which contains an X.509 subject distinguished name that SHOULD be compliant with RFC 2253 [LDAP-DN], o The X509SKI element, which contains the base64 encoded plain (i.e., non-DER-encoded) value of a X509 V.3 SubjectKeyIdentifier extension. o The X509Certificate element, which contains a base64-encoded [X509v3] certificate, and o Elements from an external namespace which accompanies/complements any of the elements above. o The X509CRL element, which contains a base64-encoded certificate revocation list (CRL) [X509v3].
Top   ToC   RFC3275 - Page 36
   Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
   appear MUST refer to the certificate or certificates containing the
   validation key.  All such elements that refer to a particular
   individual certificate MUST be grouped inside a single X509Data
   element and if the certificate to which they refer appears, it MUST
   also be in that X509Data element.

   Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
   relate to the same key but different certificates MUST be grouped
   within a single KeyInfo but MAY occur in multiple X509Data elements.

   All certificates appearing in an X509Data element MUST relate to the
   validation key by either containing it or being part of a
   certification chain that terminates in a certificate containing the
   validation key.

   No ordering is implied by the above constraints.  The comments in the
   following instance demonstrate these constraints:

   <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>
     <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=FVT,O=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 and CRLs 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.
Top   ToC   RFC3275 - Page 37
   Also, strings in DNames (X509IssuerSerial,X509SubjectName, and
   KeyNameif appropriate) should be encoded as follows:

      *  Consider the string as consisting of Unicode characters.
      *  Escape occurrences of the following special characters by
         prefixing it with the "\" character: a "#" character occurring
         at the beginning of the string or one of the characters ",",
         "+", """, "\", "<", ">" or ";"
      *  Escape all occurrences of ASCII control characters (Unicode
         range \x00 - \x 1f) by replacing them with "\" followed by a
         two digit hex number showing its Unicode number.
      *  Escape any trailing white space by replacing "\ " with "\20".
      *  Since a XML document logically consists of characters, not
         octets, the resulting Unicode string is finally encoded
         according to the character encoding used for producing the
         physical representation of the XML document.

      Schema Definition:

      <element name="X509Data" type="ds:X509DataType"/>
      <complexType name="X509DataType">
        <sequence maxOccurs="unbounded">
          <choice>
            <element name="X509IssuerSerial"
                     type="ds:X509IssuerSerialType"/>
            <element name="X509SKI" type="base64Binary"/>
            <element name="X509SubjectName" type="string"/>
            <element name="X509Certificate" type="base64Binary"/>
            <element name="X509CRL" type="base64Binary"/>
            <any namespace="##other" processContents="lax"/>
          </choice>
        </sequence>
      </complexType>
      <complexType name="X509IssuerSerialType">
        <sequence>
          <element name="X509IssuerName" type="string"/>
          <element name="X509SerialNumber" type="integer"/>
        </sequence>
      </complexType>
Top   ToC   RFC3275 - Page 38
      DTD:

      <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName
                           | X509Certificate | X509CRL)+ %X509.ANY;)>
      <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
      <!ELEMENT X509IssuerName (#PCDATA) >
      <!ELEMENT X509SubjectName (#PCDATA) >
      <!ELEMENT X509SerialNumber (#PCDATA) >
      <!ELEMENT X509SKI (#PCDATA) >
      <!ELEMENT X509Certificate (#PCDATA) >
      <!ELEMENT X509CRL (#PCDATA) >

   <!-- Note, this DTD and schema permit X509Data to be empty; this is
   precluded by the text in KeyInfo Element (section 4.4) which states
   that at least one element from the dsig namespace should be present
   in the PGP, SPKI, and X509 structures.  This is easily expressed for
   the other key types, but not for X509Data because of its rich
   structure. -->


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 base64Binary sequence 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]. These children element types can be complemented/extended by siblings from an external namespace within PGPData, or PGPData can be replaced all together with an alternative PGP XML structure as a child of KeyInfo. PGPData must contain one PGPKeyID and/or one PGPKeyPacket and 0 or more elements from an external namespace.
Top   ToC   RFC3275 - Page 39
      Schema Definition:

      <element name="PGPData" type="ds:PGPDataType"/>
      <complexType name="PGPDataType">
        <choice>
          <sequence>
            <element name="PGPKeyID" type="base64Binary"/>
            <element name="PGPKeyPacket" type="base64Binary"
                     minOccurs="0"/>
            <any namespace="##other" processContents="lax" minOccurs="0"
             maxOccurs="unbounded"/>
          </sequence>
          <sequence>
            <element name="PGPKeyPacket" type="base64Binary"/>
            <any namespace="##other" processContents="lax" minOccurs="0"
             maxOccurs="unbounded"/>
          </sequence>
        </choice>
      </complexType>

      DTD:

      <!ELEMENT PGPData ((PGPKeyID, PGPKeyPacket?) | (PGPKeyPacket)
                        %PGPData.ANY;) >
      <!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. SPKISexp is the base64 encoding of a SPKI canonical S-expression. SPKIData must have at least one SPKISexp; SPKISexp can be complemented/extended by siblings from an external namespace within SPKIData, or SPKIData can be entirely replaced with an alternative SPKI XML structure as a child of KeyInfo.
Top   ToC   RFC3275 - Page 40
   Schema Definition:

   <element name="SPKIData" type="ds:SPKIDataType"/>
   <complexType name="SPKIDataType">
     <sequence maxOccurs="unbounded">
       <element name="SPKISexp" type="base64Binary"/>
       <any namespace="##other" processContents="lax" minOccurs="0"/>
     </sequence>
   </complexType>

   DTD:

   <!ELEMENT SPKIData (SPKISexp %SPKIData.ANY;)  >
   <!ELEMENT SPKISexp  (#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. Use of this element is NOT RECOMMENDED. It provides a syntactic hook where in-band key distribution or agreement data can be placed. However, superior interoperable child elements of KeyInfo for the transmission of encrypted keys and for key agreement are being specified by the W3C XML Encryption Working Group and they should be used instead of MgmtData. Schema Definition: <element name="MgmtData" type="string"/> 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)
Top   ToC   RFC3275 - Page 41
   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 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).

   The MimeType attribute is an optional attribute which describes the
   data within the Object (independent of its encoding).  This is a
   string with values defined by [MIME].  For example, if the Object
   contains base64 encoded PNG, the Encoding may be specified as
   'base64' and the MimeType as 'image/png'.  This attribute is purely
   advisory; no validation of the MimeType information is required by
   this specification.  Applications which require normative type and
   encoding information for signature validation should specify
   Transforms with well defined resulting types and/or encodings.

   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.

   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" type="ds:ObjectType"/>
      <complexType name="ObjectType" mixed="true">
        <sequence minOccurs="0" maxOccurs="unbounded">
          <any namespace="##any" processContents="lax"/>
        </sequence>
        <attribute name="Id" type="ID" use="optional"/>
        <attribute name="MimeType" type="string" use="optional"/>
        <attribute name="Encoding" type="anyURI" use="optional"/>
      </complexType>
Top   ToC   RFC3275 - Page 42
      DTD:

      <!ELEMENT Object (#PCDATA|Signature|SignatureProperties|Manifest
                        %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" type="ds:ManifestType"/> <complexType name="ManifestType"> <sequence> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
Top   ToC   RFC3275 - Page 43
      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" type="ds:SignaturePropertiesType"/> <complexType name="SignaturePropertiesType"> <sequence> <element ref="ds:SignatureProperty" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType> <element name="SignatureProperty" type="ds:SignaturePropertyType"/> <complexType name="SignaturePropertyType" mixed="true"> <choice maxOccurs="unbounded"> <any namespace="##other" processContents="lax"/> <!-- (1,1) elements from (1,unbounded) namespaces --> </choice> <attribute name="Target" type="anyURI" use="required"/> <attribute name="Id" type="ID" use="optional"/> </complexType>
Top   ToC   RFC3275 - Page 44
      DTD:

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

      <!ELEMENT SignatureProperty (#PCDATA %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 identified 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.


(page 44 continued on part 3)

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