XML Signature Syntax and Processing Version 2.0

3.1 Common Conformance Requirements

The following conformance requirements must be met by all implementations, including those in compatibility mode.

3.2 XML Signature 2.0 Conformance

An implementation that conforms to this specification must support XML Signature 2.0 operation and conform to the following features when not operating in compatibility mode:

• must support the required steps of Signature generation, including the generation of Reference elements and the SignatureValue over SignedInfo as outlined in section 4.1 Signature Generation .
• must support the required steps of core validation as outlined in section 4.3 Core Validation .
• must support required XML Signature 2.0 Reference generation as outlined in section 4.2 Reference Generation .
• must conform to the syntax as outlined in text of this specification
• must not have a URI attribute in a Reference element
• Every Reference element must have a single Transforms element and that element must contain exactly one Transform element with an Algorithm of "http://www.w3.org/2010/xmldsig2#transform"
• The result of processing each Reference must be an octet stream with the digest algorithm applied to the resulting data octets
• Transforms must RetrievalMethod should not be used in RetrievalMethod . used; dsig11:KeyInfoReference should be used for key referencing in such cases. instead.

3.3 Compatibility Mode Conformance

An implementation that conforms to this specification may be conformant to Compatibility Mode. To conform to compatibility mode conformance with the following is required as well as conformance to common conformance requirements described in section 3.1 Common Conformance Requirements .

4.1 Signature Generation

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

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 . For XML Signature 2.0 signatures (i.e. not XML Signature 1.x or "Compatibility Mode" signatures), canonicalization in this step must use a canonicalization algorithm designated as compatible with XML Signature 2.0. This canonicalization algorithm should be the same as that used for Reference canonicalization.
3. Construct the Signature element that includes SignedInfo , Object (s) (if desired, encoding may be different than that used for signing), KeyInfo (if required), and SignatureValue .

   Note, if the Signature includes same-document references, [ XML10 ] or [ XMLSCHEMA-1 ] ,[ XMLSCHEMA-2 ] validation of the document might introduce changes that break the signature. Consequently, applications should be careful to consistently process the document or refrain from using external contributions (e.g., defaults and entities).

4.2 Reference Generation

For each Reference:

1. Decide how to represent the data object as a dsig2:Selection .
2. Use Canonicalization to convert the data object into an octet stream. This is not required for binary data.
3. Calculate the digest value over the resulting data object.
4. Create a Reference element, including the dsig2:Selection element, Canonicalization element, the digest algorithm and the DigestValue . (Note, it is the canonical form of these references that are signed in section 4.1 Signature Generation and validated in section 4.4 Reference Check .)

XML data objects must be canonicalized using Canonical XML 2.0 [ XML-C14N20 ] or an alternative algorithm that is compliant with its interface.

4.3 Core Validation

The required steps of core validation include

1. establishing trust in the signing key mentioned in the KeyInfo . (Note in some environments, the signing key is implicitly known, and KeyInfo is not used at all).
2. Checking each Reference to to see if the data object matches with the expected data object.
3. the cryptographic signature validation of the signature calculated over SignedInfo .
4. reference validation , the verification of the digest contained in each Reference in SignedInfo .

Note, there may be valid signatures that some signature applications are unable to validate. Reasons for this include failure to implement optional parts of this specification, inability or unwillingness to execute specified algorithms, or inability or unwillingness to dereference specified URIs (some URI schemes may cause undesirable side effects), etc.

Comparison of each value in reference and signature validation is over the numeric (e.g., integer) or decoded octet sequence of the value. Different implementations may produce different encoded digest and signature values when processing the same resources because of variances in their encoding, such as accidental white space. But if one uses numeric or octet comparison (choose one) on both the stated and computed values these problems are eliminated.

4.4 Reference Check

The absence of arbitrary transforms makes reference checking simpler in XML Signature 2.0. Implementations process the dsig2:Selection in each Reference to return a list of data objects that are included in the signature. For example each reference in a signature may point to a different part of the same document. The signature implementation should return all these parts (possibly as DOM elements) to the calling application, which can then compare them against its policy to make sure what was expected to be signed is actually signed.

4.5 Reference Validation

Reference Validation is very similar to that in XML Signature 1.x, except that SignedInfo need not be canonicalized, there are no arbitrary transforms to execute, and there is an optional dsig2:Verifications step.

1. Obtain the data object to be digested using the dsig2:Selection .
   1. Optional : If the selection relies on an ID-based reference, and there is a dsig2:Verification element with Type="http://www.w3.org/2010/xmldsig2#IDAttributes" , then its content may assist in obtaining the intended data object by identifying an ID attribute that the verifier may not otherwise recognize.
   2. Optional : If the selection relies on an ID-based reference, and there is a dsig2:Verification element with Type="http://www.w3.org/2010/xmldsig2#PositionAssertion" , then the verifier may confirm that the data object obtained is the same as that which would be obtained by resolving the XPath expression in the PositionAssertion attribute.
2. Perform the Canonicalization to compute an octet stream.
   1. Optional : If there is a dsig2:Verification element with Type="http://www.w3.org/2010/xmldsig2#DigestDataLength" , then verify that the length of the octet stream computed above is the same as the length specified in the DigestDataLength attribute.
3. Digest the resulting data object using the DigestMethod specified in its Reference specification. The canonicalization and digesting can be combined in one step for efficiency.
4. Compare the generated digest value against DigestValue in the SignedInfo Reference ; if there is any mismatch, validation fails.

4.6 Signature Validation

Signature Validation in XML Signature 2.0 is very similar to XML Signature 1.x, except that KeyInfo cannot contain any transforms, and the canonicalization of SignatureMethod is not required. These are the steps.

1. Obtain the keying information from KeyInfo or from an external source.
2. Using the CanonicalizationMethod (which must be Canonical XML 2.0 or an alternative algorithm that is compliant with its interface) and use the result (and previously obtained KeyInfo ) to confirm the SignatureValue over the SignedInfo element.

5.1 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 [ RFC2045 ] encoded. (The conversion from integer to octet string is equivalent to IEEE 1363's I2OSP [ IEEE1363 ] 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 would 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>

5.2 The Signature element

The Signature element is the root element of an XML Signature. Implementation must generate laxly schema valid [ XMLSCHEMA-1 ][ XMLSCHEMA-2 ] 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>

5.3 The SignatureValue Element

The SignatureValue element contains the actual value of the digital signature; it is always encoded using base64 [ RFC2045 ].

   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>

5.4 The SignedInfo Element

The structure of SignedInfo includes a canonicalization algorithm, a signature algorithm, and one or more references. Given the importance of reference processing, this is described separately in section 6. Referencing Content .

The SignedInfo element may contain an optional ID attribute allowing 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" 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>

   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>

   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>

   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>

   Schema Definition:

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

6.1 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" 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>

6.2 The Transforms Element

Each Reference must contain the Transforms element, and this must contain one and only one Transform element with an Algorithm of "http://www.w3.org/2010/xmldsig2#transform" . This signals the 2.0 syntax and processing (Compatibility mode transforms are described in section B.5 "Compatibility Mode" Transforms and Processing Model ).

   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>

The semantics of the Transform element in XML Signature 2.0 is that its input is determined solely from within the Transform itself rather than via the surrounding Reference . The output is guaranteed to be an octet stream.

The detailed definition of the XML Signature 2.0 Transform algorithm definitions can be found in section 10.5 The Transform Algorithm .

A difference from XML Signature 1.x (and the corresponding "Compatibility Mode") is that the use of extensible Transform algorithms is replaced with an extensible syntax for reference and selection processing. This construct is modeled as a fixed Transform, for compatibility with the original schema, and to ensure predictable failure modes for older implementations.

Legacy implementations should react to this as an undefined Transform and report failure in the fashion that is normal for them in such a case.

6.3 The dsig2:Selection Element

The dsig2:Selection element describes the data being signed for a "2.0 Mode" signature Reference . The content and processing model for this element depends on the value of the required Algorithm attribute, which identifies the selection algorithm/syntax in use. The required URI attribute and any child elements are passed to that algorithm as parameters to selection processing.

  Schema Definition:

  <xs:element name="Selection" type="dsig2:SelectionType"/>
  <xs:complexType name="dsig2:SelectionType">
    <xs:sequence>
      <xs:any namespace="##any" processContents="lax" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="URI" type="xs:anyURI" use="required"/>
    <xs:attribute name="Algorithm" type="xs:anyURI" use="required"/>
  </xs:complexType>

The Algorithm attribute is an extensibility point enabling application-specific content selection approaches. Each Algorithm must define the parameters expected, how they are expressed within the dsig2:Selection element, how to process the selection, what user-defined object the selection produces, and what canonicalization algorithm(s) to allow for unambiguous conversation of the data into an octet stream.

The result of processing the dsig2:Selection element must be one of the following:

• one or more subtrees with optional exclusions (see Subtrees with Exclusions )
• an octet stream
• any user-defined object

In the first case, the current Signature node is implicitly added as an exclusion, and then a "2.0 Mode" canonicalization algorithm (one compatible with these inputs) must be applied to produce an octet stream for the digest algorithm. The contents of the sibling CanonicalizationMethod element, if present, will specify the algorithm to use, and supply any non-default parameters to that algorithm. If no sibling CanonicalizationMethod element is present, then the XML Canonicalization 2.0 Algorithm [ XML-C14N20 ] must be applied with no non-default parameters.

For an octet stream, no further processing is applied, and the octets are supplied directly to the digest algorithm.

For a user-defined object (the result of a user-defined selection process), processing is subject to the definition of that process.

6.4 The dsig2:Verifications Element

dsig2:Verifications is an optional element containing information that aids in signature verification. It contains one or more dsig2:Verification elements identifying the type(s) of verification information available.

Use of the dsig2:Verifications element by validators is optional, even if the element is present. For example, validators may ignore a dsig2:Verification element of Type "http://www.w3.org/2010/xmldsig2#PositionAssertion" , and rely on ID-based referencing (with the risk of being vulnerable to signature wrapping attacks unless other steps are taken) for simplicity.

  Schema Definition:

  <xs:element name="Verifications" type="dsig2:VerificationsType"/>
  <xs:complexType name="VerificationsType">
    <xs:sequence>
      <xs:element ref="dsig2:Verification" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  
  <xs:element name="Verification" type="dsig2:VerificationType"/>
  <xs:complexType name="VerificationType">
    <xs:choice minOccurs="0" maxOccurs="unbounded">
      <xs:any namespace="##other" processContents="lax"/>
      <xs:element ref="dsig2:QualifiedAttr"/>
      <xs:element ref="dsig2:UnqualifiedAttr"/>
    <xs:/choice>
    <xs:attribute name="Type" type="xs:anyURI" use="required"/>
    <xs:attribute name="DigestDataLength" type="xs:nonNegativeInteger"/>
    <xs:attribute name="PositionAssertion" type="xs:string"/>
    <xs:anyAttribute namespace="##other" processContents="lax"/>
  <xs:/complexType>

7.1 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"/>

7.2 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 ), RSA ( required ) and ECDSA ( required ) public keys are defined in section 10.3 Signature Algorithms . The KeyValue element may include externally defined public keys 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"/>
      <!-- <element ref="dsig11:ECKeyValue"/> -->
      <!-- ECC keys (XMLDsig 1.1) will use the any element -->
      <any namespace="##other" processContents="lax"/>
    </choice>
   </complexType>

   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>

<RSAKeyValue>
  <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
   jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
   5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
  </Modulus>
  <Exponent>AQAB</Exponent>

</RSAKeyValue>

   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>

<ECKeyValue xmlns="http://www.w3.org/2009/xmldsig11#">
  <NamedCurve URI="urn:oid:1.2.840.10045.3.1.7" />
  <PublicKey>
     vWccUP6Jp3pcaMCGIcAh3YOev4gaa2ukOANC7Ufg
Cf8KDO7AtTOsGJK7/TA8IC3vZoCy9I5oPjRhyTBulBnj7Y
  </PublicKey>

</ECKeyValue>

    Schema Definition:
    
    <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->

    
    <element name="ECKeyValue" type="dsig11:ECKeyValueType"/>
    <complexType name="ECKeyValueType">
      <sequence>
        <choice>
          <element name="ECParameters" type="dsig11:ECParametersType"/>
          <element name="NamedCurve" type="dsig11:NamedCurveType"/>
        </choice>
        <element name="PublicKey" type="dsig11:ECPointType"/>
      </sequence>
      <attribute name="Id" type="ID" use="optional"/>
    </complexType>

    
    <complexType name="NamedCurveType">
      <attribute name="URI" type="anyURI" use="required"/>
    </complexType>

      
    <simpleType name="ECPointType">
      <restriction base="ds:CryptoBinary"/>
    </simpleType>

Schema Definition:
  
    <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->


    <complexType name="ECParametersType">
      <sequence>
        <element name="FieldID" type="dsig11:FieldIDType"/>
        <element name="Curve" type="dsig11:CurveType"/>
        <element name="Base" type="dsig11:ECPointType"/>
        <element name="Order" type="ds:CryptoBinary"/>
        <element name="CoFactor" type="integer" minOccurs="0"/>
        <element name="ValidationData" type="dsig11:ECValidationDataType" minOccurs="0"/>
      </sequence>
    </complexType>

    
    <complexType name="FieldIDType">
      <choice>
        <element ref="dsig11:Prime"/>
        <element ref="dsig11:TnB"/>
        <element ref="dsig11:PnB"/>
        <element ref="dsig11:GnB"/>
        <any namespace="##other" processContents="lax"/>
      </choice>
    </complexType>


    <complexType name="CurveType">
      <sequence>
        <element name="A" type="ds:CryptoBinary"/>
        <element name="B" type="ds:CryptoBinary"/>
      </sequence>
    </complexType>


  <complexType name="ECValidationDataType">
    <sequence>
      <element name="seed" type="ds:CryptoBinary"/>
    </sequence>
    <attribute name="hashAlgorithm" type="anyURI" use="required"/>
  </complexType>

Schema Definition:
  
   <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->

  
    <element name="Prime" type="dsig11:PrimeFieldParamsType"/>
    <complexType name="PrimeFieldParamsType">
      <sequence>
        <element name="P" type="ds:CryptoBinary"/>
      </sequence>
    </complexType>

Schema Definition:
  
   <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->

  
    <element name="GnB" type="dsig11:CharTwoFieldParamsType"/>
    <complexType name="CharTwoFieldParamsType">
      <sequence>
        <element name="M" type="positiveInteger"/>
      </sequence>
    </complexType>

    
    <element name="TnB" type="dsig11:TnBFieldParamsType"/>
    <complexType name="TnBFieldParamsType">
      <complexContent>
        <extension base="dsig11:CharTwoFieldParamsType">
          <sequence>
            <element name="K" type="positiveInteger"/>
          </sequence>
        </extension>
      </complexContent>
    </complexType>


    <element name="PnB" type="dsig11:PnBFieldParamsType"/>
    <complexType name="PnBFieldParamsType">
      <complexContent>
        <extension base="dsig11:CharTwoFieldParamsType">
          <sequence>
            <element name="K1" type="positiveInteger"/>
            <element name="K2" type="positiveInteger"/>
            <element name="K3" type="positiveInteger"/>
          </sequence>
        </extension>
      </complexContent>
    </complexType>

<ECDSAKeyValue xmlns="http://www.w3.org/2001/04/xmldsig-more#">
  <DomainParameters>
    <NamedCurve URN="urn:oid:1.2.840.10045.3.1.7" />
  </DomainParameters>
  <PublicKey>
      <X Value="5851106065380174439324917904648283332
0204931884267326155134056258624064349885">
      <Y Value="1024033521368277752409102672177795083
59028642524881540878079119895764161434936">
  </PublicKey>

</ECDSAKeyValue>

7.3 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 section B.4 The URI Attribute in "Compatibility Mode" and section B.4.1 The "Compatibility Mode" Reference Processing Model except that there are no DigestMethod or DigestValue child elements and presence of the URI attribute is mandatory.

Type is an optional identifier for the type of data retrieved after all transforms have been applied. The result of dereferencing a RetrievalMethod Reference for all KeyInfo types defined by this specification ( section 7. The KeyInfo Element ) 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.

Note that when referencing one of the defined KeyInfo types within the same document, or some remote documents, at least one Transform is required to turn an ID-based reference to a KeyInfo element into a child element located inside it. This is due to the lack of an XML ID attribute on the defined KeyInfo types.

Transforms in RetrievalMethod are more attack prone, since they need to be evaluated in the first step of the signature validation, where the trust in the key has not yet been established, and the SignedInfo has not yet been verified. As noted in the [ XMLDSIG-BESTPRACTICES ] an attacker can easily causes a Denial of service, by adding a specially crafted transform in the RetrievalMethod without even bothering to have the key validate or the signature match.

In XML Signature 2.0, Note: The Transforms KeyInfoReference are not allowed in element is preferred over use of RetrievalMethod . Use as it avoids use of dsig11:KeyInfoReference Transform is encouraged instead, see section 7.10 The dsig11:KeyInfoReference Element . child elements that introduce security risk and implementation challenges.

   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>

Note: The schema for the URI attribute of RetrievalMethod erroneously omitted the attribute: use="required" . However, this error only results in a more lax schema which permits all valid RetrievalMethod elements. Because the existing schema is embedded in many applications, which may include the schema in their signatures, the schema has not been corrected to be more restrictive.

7.4 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 at least one element, from the following set of element types; any of these may appear together or more than once iff (if and only if) each instance describes or is related to the same certificate:

• The deprecated X509IssuerSerial element, which contains an X.509 issuer distinguished name/serial number pair. The distinguished name should be represented as a string that complies with section 3 of RFC4514 [ LDAP-DN ], to be generated according to the Distinguished Name Encoding Rules section below,
• The X509SubjectName element, which contains an X.509 subject distinguished name that should be represented as a string that complies with section 3 of RFC4514 [ LDAP-DN ], to be generated according to the Distinguished Name Encoding Rules section below,
• The X509SKI element, which contains the base64 encoded plain (i.e. non-DER-encoded) value of a X509 V.3 SubjectKeyIdentifier extension,
• The X509Certificate element, which contains a base64-encoded [ X509V3 ] certificate, and
• The X509CRL element, which contains a base64-encoded certificate revocation list (CRL) [ X509V3 ].
• The dsig11:X509Digest element contains a base64-encoded digest of a certificate. The digest algorithm URI is identified with a required Algorithm attribute. The input to the digest must be the raw octets that would be base64-encoded were the same certificate to appear in the X509Certificate element.
• The dsig11:OCSPResponse element contains a base64-encoded OCSP response in DER encoding. [ OCSP ].
• Elements from an external namespace which accompanies/complements any of the elements above.

Any X509IssuerSerial , X509SKI , X509SubjectName , and dsig11:X509Digest 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 , X509SubjectName , and dsig11:X509Digest elements that relate to the same key but different certificates must be grouped within a single KeyInfo but may occur in multiple X509Data elements.

Note that if X509Data child elements are used to identify a trusted certificate (rather than solely as an untrusted hint supplemented by validation by policy), the complete set of such elements that are intended to identify a certificate should be integrity protected, typically by signing an entire X509Data or KeyInfo element.

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.

While in principle many certificate encodings are possible, it is recommended that certificates appearing in an X509Certificate element be limited to an encoding of BER or its DER subset, allowing that within the certificate other content may be present. The use of other encodings may lead to interoperability issues. In any case, XML Signature implementations should not alter or re-encode certificates, as doing so could invalidate their signatures.

Deployments that expect to make use of the X509IssuerSerial element should be aware that many Certificate Authorities issue certificates with large, random serial numbers. XML Schema validators may not support integer types with decimal data exceeding 18 decimal digits [XML-schema]. Therefore such deployments should avoid schema-validating the X509IssuerSerial element, or make use of a local copy of the schema that adjusts the data type of the X509SerialNumber child element from "integer" to "string" .

   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"/>
         <!-- <element ref="dsig11:OCSPResponse"/> -->
         <!-- <element ref="dsig11:X509Digest"/> -->
         <!-- OCSPResponse and X509Digest elements (XMLDsig 1.1) will use the any element -->
         <any namespace="##other" processContents="lax"/>
       </choice>
     </sequence>
   </complexType>


   <complexType name="X509IssuerSerialType"> 
     <sequence> 
       <element name="X509IssuerName" type="string"/> 
       <element name="X509SerialNumber" type="integer"/> 
     </sequence>
   </complexType>


   <!-- Note, this schema permits X509Data to be empty; this is 
   precluded by the text in section 7. The KeyInfo Element 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. -->

  <!-- targetNameSpace="http://www.w3.org/2009/xmldsig11#" -->
  
  <element name="OCSPResponse" type="base64Binary" />

  
  <element name="X509Digest" type="dsig11:X509DigestType"/>
  <complexType name="X509DigestType">
    <simpleContent>
      <extension base="base64Binary">
        <attribute name="Algorithm" type="anyURI" use="required"/>
      </extension>
    </simpleContent>
  </complexType>

7.5 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.

   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>

7.6 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 .

   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>

7.7 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)

   Schema Definition:

   <element name="MgmtData" type="string"/>

7.8 XML Encryption EncryptedKey and DerivedKey Elements

7.9 The dsig11:DEREncodedKeyValue Element

Identifier

Type=" http://www.w3.org/2009/xmldsig11#DEREncodedKeyValue "
(this can be used within a RetrievalMethod or Reference element to identify the referent's type)

The public key algorithm and value are DER-encoded in accordance with the value that would be used in the Subject Public Key Info field of an X.509 certificate, per section 4.1.2.7 of [ RFC5280 ]. The DER-encoded value is then base64-encoded.

For the key value types supported in this specification, refer to the following for normative references on the format of Subject Public Key Info and the relevant OID values that identify the key/algorithm type:

RSA

See section 2.3.1 of [ RFC3279 ]

DSA

See section 2.3.2 of [ RFC3279 ]

EC

See section 2 of [ RFC5480 ]

Specifications that define additional key types should provide such a normative reference for their own key types where possible.

   Schema Definition:

   <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->
  <element name="DEREncodedKeyValue" type="dsig11:DEREncodedKeyValueType"/>
  <complexType name="DEREncodedKeyValueType">
    <simpleContent>
      <extension base="base64Binary">
        <attribute name="Id" type="ID" use="optional"/>
      </extension>
    </simpleContent>
  </complexType>

Historical note: The dsig11:DEREncodedKeyValue element was added to XML Signature 1.1 in order to support certain interoperability scenarios where at least one of signer and/or verifier are not able to serialize keys in the XML formats described in section 7.2 The KeyValue Element above. The KeyValue element is to be used for "bare" XML key representations (not XML wrappings around other binary encodings like ASN.1 DER); for this reason the dsig11:DEREncodedKeyValue element is not a child of KeyValue . The dsig11:DEREncodedKeyValue element is also not a child of the X509Data element, as the keys represented by dsig11:DEREncodedKeyValue may not have X.509 certificates associated with them (a requirement for X509Data ).

7.10 The dsig11:KeyInfoReference Element

A dsig11:KeyInfoReference element within KeyInfo is used to convey a reference to a KeyInfo element at another location in the same or different document. 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 dsig11:KeyInfoReference element instead of including the entire chain with a sequence of X509Certificate elements repeated in multiple places.

dsig11:KeyInfoReference uses the same syntax and dereferencing behavior as Reference 's URI ( section B.4 The URI Attribute in "Compatibility Mode" ) and the Reference Processing Model ( section B.4.1 The "Compatibility Mode" Reference Processing Model ) except that there are no child elements and the presence of the URI attribute is mandatory.

The result of dereferencing a dsig11:KeyInfoReference must be a KeyInfo element, or an XML document with a KeyInfo element as the root.

Schema Definition

   <!-- targetNamespace="http://www.w3.org/2009/xmldsig11#" -->


   <element name="KeyInfoReference" type="dsig11:KeyInfoReferenceType"/> 
   <complexType name="KeyInfoReferenceType">
     <attribute name="URI" type="anyURI" use="required"/>
     <attribute name="Id" type="ID" use="optional"/>
   </complexType>

9.1 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 Reference s. 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>

9.2 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>

9.3 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 canonicalization algorithms 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.

9.4 Comments in Signature Elements

XML comments are not used by this specification.

Note that unless the 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.

10.1 Message Digests

This specification defines several possible digest algorithms for the DigestMethod element, including required algorithm SHA-256. Use of SHA-256 is strongly recommended over SHA-1 because recent advances in cryptanalysis (see e.g. [ SHA-1-Analysis ]) have cast doubt on the long-term collision resistance of SHA-1. Therefore, SHA-1 support is required in this specification only for backwards-compatibility reasons.

Digest algorithms that are known not to be collision resistant should not be used in DigestMethod elements. For example, the MD5 message digest algorithm should not be used as specific collisions have been demonstrated for that algorithm.

<DigestMethod


Algorithm


=


"




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




sha1"


/>

A9993E36
4706816A
BA3E2571
7850C26C
9CD0D89D

<DigestValue>

qZk+NkcGgWq6PiVxeFDCbJzQ2J0=

</DigestValue>

10.2 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.

<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1">
   <HMACOutputLength>128</HMACOutputLength>

</SignatureMethod>

9294727A
3638BB1C
13F48EF8
158BFC9D

<SignatureValue>

kpRyejY4uxwT9I74FYv8nQ==

</SignatureValue>

   Schema Definition:

   <simpleType name="HMACOutputLengthType">
     <restriction base="integer"/>
   </simpleType>

10.3 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.

   <SignatureMethod Algorithm="http://www.w3.org/2009/xmldsig11#dsa-sha256"/>

r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0

s
=
41E2345F
1F56DF24
58F426D1
55B4BA2D
B6DCD8C8

<SignatureValue>

i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==

</SignatureValue>

<SignatureMethod


Algorithm


=


"http://www.w3.org/2000/09/xmldsig#rsa-sha1"


/>

<SignatureValue>
IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=


</SignatureValue>

    <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"/>

10.4 Canonicalization Algorithms

The input to any canonicalization algorithm compatible with XML Signature 2.0 signatures is a set of document subtrees and exclusions in the form of subtrees or XML attributes. The actual representation of these inputs depends on the processing model and may be in terms of DOM nodes or representations suitable for streaming-based processing. The output is an octet stream.

Note: The input passed to "2.0 Mode" canonicalization algorithms must always exclude the current Signature element node (i.e., the Signature must be passed as one of the exclusion elements. This is equivalent to an implicit Enveloped Signature Transform in "Compatibility Mode", and has no effect for non-enveloped signatures.

This specification REQUIRES implementation of Canonical XML 2.0 [ XML-C14N20 ]. Applications may support other canonicalization algorithms with the same input model (subtrees with exclusions). A Reference to non-XML data may not use canonicalization at all, or may use a custom canonicalization algorithm with this input model or a completely different one.

<CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"
  xmlns:c14n2="http://www.w3.org/2010/xml-c14n2">
  <c14n2:PrefixRewrite>sequential</c14n2:PrefixRewrite>
  <c14n2:TrimTextNodes>true</c14n2:TrimTextNodes>
  <c14n2:QNameAware>
    <c14n2:QualifiedAttr Name="type" NS="http://www.w3.org/2001/XMLSchema-instance"/>
  </c14n2:QNameAware>
</CanonicalizationMethod>

10.5 The Transform Algorithm

In XML Signature 2.0, the Transforms element contains exactly one Transform element with an Algorithm of "http://www.w3.org/2010/xmldsig2#transform" . This transform encapsulates the process of selecting the content to sign, canonicalizing it, and attaching optional material that may aid the verifier.

This fixed Transform element consists of a single required dsig2:Selection element, followed by an optional CanonicalizationMethod element, and an optional dsig2:Verifications element.

10.6 dsig2:Selection Algorithms

  Schema Definition:

  <xs:element name="IncludedXPath" type="xs:string"/>

  Schema Definition:

  <xs:element name="ExcludedXPath" type="xs:string"/>

  Schema Definition:

  <xs:element name="ByteRange" type="xs:string"/>

10.7 The dsig2:Verification Types

  Schema Definition:

   <xs:element name="QualifiedAttr" type="dsig2:QualifiedAttrType"/>
   <xs:complexType type="QualifiedAttrType">
      <xs:attribute name="Name" type="xs:NCName" use="required"/>
      <xs:attribute name="NS" type="xs:anyURI" use="required"/>
   <xs:/complexType>
   <xs:element name="UnqualifiedAttr" type="dsig2:UnqualifiedAttrType"/>
   <xs:complexType type="UnqualifiedAttrType">
      <xs:attribute name="Name" type="xs:NCName" use="required"/>
      <xs:attribute name="ParentName" type="xs:NCName" use="required"/>
      <xs:attribute name="ParentNS" type="xs:anyURI"/>
   <xs:/complexType>

11.1 XML 1.0 Syntax Constraints, and Canonicalization

XML 1.0 [ XML10 ]] 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,
4. entity references are replaced with the corresponding declared entity,
5. attribute values are normalized by
   1. replacing character and entity references as above,
   2. replacing occurrences of #x9, #xA, and #xD with #x20 (space) except that the sequence #xD#xA is replaced by a single space, and
   3. 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 (5.3) depend on the presence of a schema, DTD or similar declarations. The Signature element type is laxly schema valid [ XMLSCHEMA-1 ][ XMLSCHEMA-2 ], consequently external XML or even XML within the same document as the signature may be (only) well-formed or from another namespace (where permitted by the signature schema); the noted items may not be present. Thus, a signature with such content will only be verifiable by other signature applications if the following syntax constraints are observed when generating any signed material including the SignedInfo element:

1. attributes having default values be explicitly present,
2. all entity references (except "amp", "lt", "gt", "apos", "quot", and other character entities not representable in the encoding chosen) be expanded,
3. attribute value white space be normalized

11.2 DOM/SAX Processing and Canonicalization

In addition to the canonicalization and syntax constraints discussed above, many XML applications use the Document Object Model [ DOM-LEVEL-1 ] or the Simple API for XML [ SAX ]. DOM maps XML into a tree structure of nodes and typically assumes it will be used on an entire document with subsequent processing being done on this tree. SAX converts XML into a series of events such as a start tag, content, etc. In either case, many surface characteristics such as the ordering of attributes and insignificant white space within start/end tags is lost. In addition, namespace declarations are mapped over the nodes to which they apply, losing the namespace prefixes in the source text and, in most cases, losing where namespace declarations appeared in the original instance.

If an XML Signature is to be produced or verified on a system using the DOM or SAX processing, a canonical method is needed to serialize the relevant part of a DOM tree or sequence of SAX events. XML canonicalization specifications, such as [ XML-C14N ], are based only on information which is preserved by DOM and SAX. For an XML Signature to be verifiable by an implementation using DOM or SAX, not only must XML 1.0 syntax constraints given in the section 11.1 XML 1.0 Syntax Constraints, and Canonicalization be followed but an appropriate XML canonicalization must be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octet stream that was signed.

12.1 Transforms

A requirement of this specification is to permit signatures to "apply to a part or totality of a XML document." (See [ XMLDSIG-REQUIREMENTS ], 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 applications 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.

12.2 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.

12.3 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.

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.

13.1 XSD Schema

XML Signature Core Schema Instance

xmldsig-core-schema.xsd

Valid XML schema instance based on [ XMLSCHEMA-1 ][ XMLSCHEMA-2 ].

XML Signature 1.1 Schema Instance

xmldsig11-schema.xsd

This schema document defines the additional elements defined in this version of the XML Signature specification.

XML Signature 1.1 Schema Driver

xmldsig1-schema.xsd

This schema instance binds together the XML Signature Core Schema Instance and the XML Signature 1.1 Schema Instance