XML Signature Syntax and Processing Version 2.0

2.1.1 Simple Example in "Compatibility Mode"

This example uses "Compatibility Mode".

[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> 
[s02]   <SignedInfo>  
[s03]   <CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> 
[s04]   <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> 
[s05]   <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> 
[s06]     <Transforms> 
[s07]       <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> 
[s08]     </Transforms> 
[s09]     <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> 
[s10]     <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> 
[s11]   </Reference> 
[s12] </SignedInfo> 
[s13]   <SignatureValue>...</SignatureValue> 
[s14]   <KeyInfo> 
[s15a]    <KeyValue>
[s15b]      <DSAKeyValue> 
[s15c]        <P>...</P><Q>...</Q><G>...</G><Y>...</Y> 
[s15d]      </DSAKeyValue> 
[s15e]    </KeyValue> 
[s16]   </KeyInfo> 
[s17] </Signature>

[s02-12] The required SignedInfo element is the information that is actually signed. Core validation of SignedInfo consists of two mandatory processes: validation of the signature over SignedInfo and validation of each Reference digest within SignedInfo. Note that the algorithms used in calculating the SignatureValue are also included in the signed information while the SignatureValue element is outside SignedInfo.

[s03] The CanonicalizationMethod is the algorithm that is used to canonicalize the SignedInfo element before it is digested as part of the signature operation. Note that this example is not in canonical form. (None of the examples in this specification are in canonical form.)

[s04] The SignatureMethod is the algorithm that is used to convert the canonicalized SignedInfo into the SignatureValue. It is a combination of a digest algorithm and a key dependent algorithm and possibly other algorithms such as padding, for example RSA-SHA1. The algorithm names are signed to resist attacks based on substituting a weaker algorithm. To promote application interoperability we specify a set of signature algorithms that must be implemented, though their use is at the discretion of the signature creator. We specify additional algorithms as recommended or optional for implementation; the design also permits arbitrary user specified algorithms.

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

[s14-16] KeyInfo indicates the key to be used to validate the signature. Possible forms for identification include certificates, key names, and key agreement algorithms and information -- we define only a few. KeyInfo is optional for two reasons. First, the signer may not wish to reveal key information to all document processing parties. Second, the information may be known within the application's context and need not be represented explicitly. Since KeyInfo is outside of SignedInfo, if the signer wishes to bind the keying information to the signature, a Reference can easily identify and include the KeyInfo as part of the signature. Use of KeyInfo is optional, however note that senders and receivers must agree on how it will be used through a mechanism out of scope for this specification.

2.1.2 More on Reference

These section explaining the lines [s05] to [s11] of the previous example. This signature is in "compatibility mode".

[s05]   <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> 
[s06]     <Transforms> 
[s07]       <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> 
[s08]     </Transforms> 
[s09]     <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> 
[s10]     <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> 
[s11]   </Reference>

[s05] The optional URI attribute of Reference identifies the data object to be signed. This attribute may be omitted on at most one Reference in a Signature. (This limitation is imposed in order to ensure that references and objects may be matched unambiguously.)

[s05-08] This identification, along with the transforms, is a description provided by the signer on how they obtained the signed data object in the form it was digested (i.e. the digested content). The verifier may obtain the digested content in another method so long as the digest verifies. In particular, the verifier may obtain the content from a different location such as a local store than that specified in the URI.

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

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

2.1.3 Simple Example in "2.0 Mode"

This is the same example using "2.0 Mode". The only differences are in the CanonicalizationMethod and Reference portions. The line numbers in this example match up with the line numbers in the "Compatibility Mode" example.

[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> 
[s02]   <SignedInfo>  
[s03]   <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/> 
[s04]   <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/> 
[s05]   <Reference> 
[s06]     <Transforms> 
[s07]       <Transform Algorithm="http://www.w3.org/2010/xmldsig2#transform">
[s07a]        <dsig2:Selection type="http://www.w3.org/2010/xmldsig2#xml" xmlns:dsig2="http://www.w3.org/2010/xmldsig2#"
URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126">
>
[s07b]        </dsig2:Selection>
[s07c]        <CanonicalizationMethod Algorithm="http://www.w3.org/2010/xml-c14n2"/>
[s07d]      </Transform> 
[s08]     </Transforms> 
[s09]     <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> 
[s10]     <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> 
[s11]   </Reference> 
[s12] </SignedInfo> 
[s13]   <SignatureValue>...</SignatureValue> 
[s14]   <KeyInfo> 
[s15a]    <KeyValue>
[s15b]      <DSAKeyValue> 
[s15c]        <P>...</P><Q>...</Q><G>...</G><Y>...</Y> 
[s15d]      </DSAKeyValue> 
[s15e]    </KeyValue> 
[s16]   </KeyInfo> 
[s17] </Signature>

[s03] In "2.0 Mode", the Canonicalization Method URI should be Canonical XML 2.0 (or a later version) and all the parameters for Canonical XML 2.0 should be present as subelements of this element.

[s05-s08] Note "2.0 Mode" does not use the concept of transforms, instead each reference object has two parts - a dsig2:Selection element to choose the data object to be signed, and a Canonicalization element to convert the data object to a canonicalized octet stream. To fit in these two elements, without breaking backwards compatibility with the 1.0 schema, these elements have been put inside a special Transform with URI http://www.w3.org/2010/xmldsig2#transform. In "2.0 Mode" the Transforms element will contain only this particular Transform.

[s05] In "2.0 Mode", the URI attribute is omitted from the Reference. Instead it can be found in the dsig2:Selection.

[s07a-s07b] The dsig2:Selection element identifies the data object to be signed. This specification identifies only two types, "xml" and "binary", but user specified types are also possible. For example a new type "database-rows" could be defined to select rows from a database for signing. Usually a URI and a few other bits of information are used to identify the data object, but the URI is not required; for example, the "xml" type can identify a local document subset by using an XPath.

[s07c] The CanonicalizationMethod element provides the mechanism to convert the data object into a canonicalized octet stream. This specification addresses only canonicalization for xml data. Other forms of canonicalization can be defined - e.g. a scheme for signing mime attachments could define a canonicalization for mime headers and data. The output of the canonicalization is digested.

3.3.1 Reference Generation in "2.0 Mode"

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 3.1.3 and validated in 3.2.2.)

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

3.3.2 Reference check in "2.0 Mode"

The absence of arbitrary transforms makes reference checking simpler in "2.0 Mode". In this mode, 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.

3.3.3 Signature Validation in "2.0 Mode"

Signature Validation in "2.0 Mode" is very similar, except that in this mode 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.

3.3.4 Reference Validation in "2.0 Mode"

Reference Validation in "2.0 Mode" is very similar, except that SignedInfo need not be canonicalized, there are no arbitrary transforms to execute, and there is an optional dsig2:Verification 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 containing an dsig2:IDAssertion, its content may assist in obtaining the intended data object by identifying ID attributes 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 containing a dsig2: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 dsig2:PositionAssertion.
2. Perform the Canonicalization to compute an octet stream.
   1. Optional: If there is a dsig2:Verification element containing a dsig2:DigestDataLength, then verify that the length of the octet stream computed above is the same as the length specified.
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.

3.4.1 Reference Generation in "Compatibility Mode"

For each data object being signed:

1. Apply the Transforms, as determined by the application, to the data object.
2. Calculate the digest value over the resulting data object.
3. Create a Reference element, including the (optional) identification of the data object, any (optional) transform elements, the digest algorithm and the DigestValue. (Note, it is the canonical form of these references that are signed in 3.1.3 and validated in 3.2.1 .)

3.4.2 Reference check in "Compatibility Mode"

It is very important to check that the Reference actually includes the data that is expected to be signed. The [XMLDSIG-BESTPRACTICES] document describes a number of attacks, where what is apparently being signed is not actually signed.

One way to check the reference is to allow only certain combinations of transforms. For example [SAML2-CORE] and [EBXML-MSG] follow this approach.

Another option is for XML Signature libraries to return the pre-digest data to the application, so that application can inspect it to verify what is actually signed. This too may not be enough, for example in a Web Services scenario, if the reference is pointing to a soap:Body, it is not sufficient to just check the name of the "soap:Body" element, as it can lead to wrapping attacks [MCINTOSH-WRAP];Instead the application should check if this soap:Body is in the correct position, i.e. as a child of the top level soap:Envelope.

3.4.3 Signature Validation in "Compatibility Mode"

1. Obtain the keying information from KeyInfo or from an external source.
2. Obtain the canonical form of the SignatureMethod using the CanonicalizationMethod and use the result (and previously obtained KeyInfo) to confirm the SignatureValue over the SignedInfo element.

Note, KeyInfo (or some transformed version thereof) may be signed via a Reference element. Transformation and validation of this reference (3.2.1) is orthogonal to Signature Validation which uses the KeyInfo as parsed.

Additionally, the SignatureMethod URI may have been altered by the canonicalization of SignedInfo (e.g., absolutization of relative URIs) and it is the canonical form that must be used. However, the required canonicalization [XML-C14N] of this specification does not change URIs.

3.4.4 Reference Validation in "Compatibility Mode"

1. Canonicalize the SignedInfo element based on the CanonicalizationMethod in SignedInfo.
2. For each Reference in SignedInfo:
   1. Obtain the data object to be digested. (For example, the signature application may dereference the URI and execute Transforms provided by the signer in the Reference element, or it may obtain the content through other means such as a local cache.)
   2. Digest the resulting data object using the DigestMethod specified in its Reference specification.
   3. Compare the generated digest value against DigestValue in the SignedInfo Reference; if there is any mismatch, validation fails.

Note, SignedInfo is canonicalized in step 1. The application must ensure that the CanonicalizationMethod has no dangerous side effects, such as rewriting URIs, (see CanonicalizationMethod Note (section 4.4.1)) and that it Sees What is Signed, which is the canonical form.

Note, After a Signature element has been created during Signature Generation for a signature with a same document reference, an implementation can serialize the XML content with variations in that serialization. This means that Reference Validation needs to canonicalize the XML document before digesting in step 1 to avoid issues related to variations in serialization.

4.4.1 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.

   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>

4.4.2 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>

The ds:HMACOutputLength parameter is used for HMAC [HMAC] algorithms. The parameter specifies a truncation length in bits. If this parameter is trusted without further verification, then this can lead to a security bypass [CVE-2009-0217]. Signatures must be deemed invalid if the truncation length is below the larger of (a) half the underlying hash algorithm's output length, and (b) 80 bits. Note that some implementations are known to not accept truncation lengths that are lower than the underlying hash algorithm's output length.

4.4.3 The Reference Element

Reference is an element that may occur one or more times. It specifies a digest algorithm and digest value, and optionally an identifier of the object being signed, the type of the object, and/or a list of transforms to be applied prior to digesting. The identification (URI) and transforms describe how the digested content (i.e., the input to the digest method) was created. The Type attribute facilitates the processing of referenced data. For example, while this specification makes no requirements over external data, an application may wish to signal that the referent is a Manifest. An optional ID attribute permits a Reference to be referenced from elsewhere.

   Schema Definition:

   <element name="Reference" 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>

   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>

   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>

4.5.1 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"/>

4.5.2 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 Signature Algorithms (section 6.4). 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>

4.5.3 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 4.4.3.1) and The Reference Processing Model (section 4.4.3.2) 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 4.5) 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 SignatureValidation, 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 "2.0 Mode", Transforms are not allowed in RetrievalMethod. Use of dsig11:KeyInfoReference is encouraged instead, see section 4.5.10.

   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.

4.5.4 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 KeyInfo Element (section 4.5) 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>

4.5.5 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>

4.5.6 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>

4.5.7 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"/>
  

4.5.8 XML Encryption EncryptedKey and DerivedKey Elements

4.5.9 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 4.5.2 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).

4.5.10 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 4.4.3.2) and the Reference Processing Model (section 4.4.3.3) 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.

Note: The dsig11:KeyInfoReference element is a desirable alternative to the use of RetrievalMethod when the data being referred to is a KeyInfo element and the use of RetrievalMethod would require one or more Transform child elements, which introduce security risk and implementation challenges, and are precluded when using "2.0 Mode" signatures.

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>

6.1.1 SHA-1

Identifier:

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

The SHA-1 algorithm [FIPS-186-3] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:

<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>

A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. For example, the DigestValue element for the message digest:

A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D

from Appendix A of the SHA-1 standard would be:

<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>

6.1.2 SHA-256

Identifier:

http://www.w3.org/2001/04/xmlenc#sha256

The SHA-256 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-256 digest is a 256-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 32-octet octet stream.

6.1.3 SHA-384

Identifier:

http://www.w3.org/2000/09/xmldsig#sha384

The SHA-384 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-384 digest is a 384-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 48-octet octet stream.

6.1.4 SHA-512

Identifier:

http://www.w3.org/2001/04/xmlenc#sha512

The SHA-512 algorithm [FIPS-180-3] takes no explicit parameters. A SHA-512 digest is a 512-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 64-octet octet stream.

6.2.1 HMAC

Identifier:

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

http://www.w3.org/2001/04/xmldsig-more#hmac-sha256

http://www.w3.org/2001/04/xmldsig-more#hmac-sha384

http://www.w3.org/2001/04/xmldsig-more#hmac-sha512

The HMAC algorithm (RFC2104 [HMAC]) takes the output (truncation) length in bits as a parameter; this specification REQUIRES that the truncation length be a multiple of 8 (i.e. fall on a byte boundary) because Base64 encoding operates on full bytes.  If the truncation parameter is not specified then all the bits of the hash are output. Any signature with a truncation length that is less than half the output length of the underlying hash algorithm must be deemed invalid. An example of an HMAC SignatureMethod element:

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

The output of the HMAC algorithm is ultimately the output (possibly truncated) of the chosen digest algorithm. This value shall be base64 encoded in the same straightforward fashion as the output of the digest algorithms. Example: the SignatureValue element for the HMAC-SHA1 digest

9294727A 3638BB1C 13F48EF8 158BFC9D

from the test vectors in [HMAC] would be

<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>

   Schema Definition:

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

6.3.1 DSA

Identifier:

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

http://www.w3.org/2009/xmldsig11#dsa-sha256

The DSA family of algorithms is defined in FIPS 186-3 [FIPS-186-3].  FIPS 186-3 defines DSA in terms of two security parameters L and N where L = |p|, N = |q|, p is the prime modulus, q is a prime divisor of (p-1).  FIPS 186-3 defines four valid pairs of (L, N); they are: (1024, 160), (2048, 224), (2048, 256) and (3072, 256).  The pair (1024, 160) corresponds to the algorithm DSAwithSHA1, which is identified in this specification by the URI http://www.w3.org/2000/09/xmldsig#dsa-sha1.  The pairs (2048, 256) and (3072, 256) correspond to the algorithm DSAwithSHA256, which is identified in this specification by the URI http://www.w3.org/2009/xmldsig11#dsa-sha256.  This specification does not use the (2048, 224) instance of DSA (which corresponds to DSAwithSHA224).

 DSA takes no explicit parameters; an example of a DSA SignatureMethod element is:

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

The output of the DSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value consists of the base64 encoding of the concatenation of two octet-streams that respectively result from the octet-encoding of the values r and s in that order. Integer to octet-stream conversion must be done according to the I2OSP operation defined in the RFC 3447 [PKCS1] specification with a l parameter equal to 20. For example, the SignatureValue element for a DSA signature (r, s) with values specified in hexadecimal:

r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8

from the example in Appendix 5 of the DSS standard would be

<SignatureValue>
i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>

Security considerations regarding DSA key sizes

Per FIPS 186-3 [FIPS-186-3], the DSA security parameter L is defined to be 1024, 2048 or 3072 bits and the corresponding DSA q value is defined to be 160, 224/256 and 256 bits respectively. Special Publication SP 800-57 Part 1 [SP800-57], NIST recommends using at least at 2048-bit public keys for securing information beyond 2010 (and 3072-bit keys for securing information beyond 2030).

Since XML Signature 1.0 requires implementations to support DSA-based digital signatures, this XML Signature 1.1 revision REQUIRES signature verifiers to implement DSA only for keys of 1024 bits in order to guarantee interoperability with XML Signature 1.0 generators. XML Signature 1.1 implementations may but are not required to support DSA-based signature generation, and given the short key size and the SP800-57 guidelines, DSA with 1024-bit prime moduli should not be used for signatures that will be verified beyond 2010.

6.3.2 RSA (PKCS#1 v1.5)

Identifier:

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

http://www.w3.org/2001/04/xmldsig-more#rsa-sha256

http://www.w3.org/2001/04/xmldsig-more#rsa-sha384

http://www.w3.org/2001/04/xmldsig-more#rsa-sha512

The expression "RSA algorithm" as used in this specification refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 3447 [PKCS1]. The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:

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

The SignatureValue content for an RSA signature is the base64 [RFC2045] encoding of the octet string computed as per RFC 3447 [PKCS1], section 8.2.1: Signature generation for the RSASSA-PKCS1-v1_5 signature scheme]. Computation of the signature will require concatenation of the hash value and a constant string determined by RFC 3447. Signature computation and verification does not require implementation of an ASN.1 parser.

The resulting base64 [RFC2045] string is the value of the child text node of the SignatureValue element, e.g.

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

Security considerations regarding RSA key sizes

6.3.3 ECDSA

Identifiers:

http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1

http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256

http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha384

http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha512

The ECDSA algorithm [FIPS-186-3] takes no explicit parameters. An example of a ECDSA SignatureMethod element is:

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

The output of the ECDSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value consists of the base64 encoding of the concatenation of two octet-streams that respectively result from the octet-encoding of the values r and s in that order. Integer to octet-stream conversion must be done according to the I2OSP operation defined in the RFC 3447 [PKCS1] specification with the l parameter equal to the size of the base point order of the curve in bytes (e.g. 32 for the P-256 curve and 66 for the P-521 curve).

This specification REQUIRES implementations to support the ECDSAwithSHA256 signature algorithm, which is ECDSA over the P-256 prime curve specified in Section D.2.3 of FIPS 186-3 [FIPS-186-3] (and using the SHA-256 hash algorithm). It is further recommended that implementations also support ECDSA over the P-384 and P-521 prime curves; these curves are defined in Sections D.2.4 and D.2.5 of FIPS 186-3, respectively.

6.4.1 Canonical XML 2.0

Identifier for Canonical XML 2.0:

http://www.w3.org/2010/xml-c14n2

An example of a Canonical XML 2.0 element is:

<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>

There is no Canonical XML 2.0 Transform. Instead the same CanonicalizationMethod element is reused within the dsig2:Selection element for specifying canonicalization of referenced data,

The normative specification of Canonical XML 2.0 is [XML-C14N20].

6.5.1 Canonical XML 1.0

Identifier for required Canonical XML 1.0 (omits comments):

http://www.w3.org/TR/2001/REC-xml-c14n-20010315

Identifier for Canonical XML 1.0 with Comments:

http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments

Input:

octet-stream, node-set

Output:

octet-stream

An example of an XML canonicalization element is:

<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>

The normative specification of Canonical XML1.0 is [XML-C14N]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML is easily parameterized (via an additional URI) to omit or retain comments.

6.5.2 Canonical XML 1.1

Identifier for required Canonical XML 1.1 (omits comments):

http://www.w3.org/2006/12/xml-c14n11

Identifier for Canonical XML 1.1 with Comments:

http://www.w3.org/2006/12/xml-c14n11#WithComments

Input:

octet-stream, node-set

Output:

octet-stream

The normative specification of Canonical XML 1.1 is [XML-C14N11]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML 1.1 is easily parameterized (via an additional URI) to omit or retain comments.

6.5.3 Exclusive XML Canonicalization 1.0

Identifier for Exclusive XML Canonicalization 1.0 (omits comments):

http://www.w3.org/2001/10/xml-exc-c14n#

Identifier for Exclusive XML Canonicalization 1.0 with Comments:

http://www.w3.org/2001/10/xml-exc-c14n#WithComments

Input:

octet-stream, node-set

Output:

octet-stream

The normative specification of Exclusive XML Canonicalization 1.0 is [XML-EXC-C14N]].

6.6.1 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 Type and optional Subtype attributes, which identify 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="Type" type="xs:anyURI" use="required"/>
    <xs:attribute name="Subtype" type="xs:string" use="optional"/>
  </xs:complexType>

The Type and optional Subtype attributes are an extensibility point enabling application-specific content selection approaches. Each Type/Subtype pair 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 following values for Type and Subtype are defined by this specification and are required:

Type="http://www.w3.org/2010/xmldsig2#xml"

Selection of complete XML documents or XML fragments.

Type="http://www.w3.org/2010/xmldsig2#binary" Subtype="http://www.w3.org/2010/xmldsig2#fromURI"

Selection of binary data from an external URI.

Type="http://www.w3.org/2010/xmldsig2#binary" Subtype="http://www.w3.org/2010/xmldsig2#fromBase64Node"

Selection of binary data from a base64 encoded text node inside an XML document.

These algorithms are defined in detail below.

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.

  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"/>

6.7.1 The dsig2:DigestDataLength Element

dsig2:DigestDataLength contains an integer that specifies the number of bytes that were digested for the containing Reference. This can be used for multiple purposes:

• to debug digest verification failures
• to indicate intentional signing of 0 bytes, such as if an XPath expression selects nothing
• to bypass the expensive digest calculation if during verification the length of the byte array containing the canonicalized bytes doesn't match the value found in the message

  Schema Definition:

  <xs:element name="DigestDataLength" type="xs:nonNegativeInteger"/>

6.7.2 The dsig2:PositionAssertion Element

dsig2:PositionAssertion is used to increase the resistance of ID-based referencing to signature wrapping attacks. It contains an XPath expression that must match the referenced content's position in the document. In this way, instead of "selecting" the referenced element via an XPath, its position is verified by one (which enables flexibility in the actual use of XPath by the verifier). The actual selection process remains ID-based, which is simpler for many implementers.

  Schema Definition:

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

6.7.3 The dsig2:IDAttributes Element

dsig2:IDAttributes is used in conjunction with ID-based references, to specify the ID attribute node(s) that the signer used. Ordinarily, ID attribute knowledge is imparted through a variety of normative and informal means, including DTDs, XML Schemas, use of xml:id, and application-specific content knowledge. A signer is not required to use this mechanism to identify ID attributes, but may do so to transfer its own ID knowledge to the verifier through the signature itself. Verifiers may incorporate this knowledge, or use more traditional means of recognizing ID attributes.

The IDAttributes element may specify more than one ID attribute node. A particular selection may involve more than one, or a complete set of ID attribute nodes within a document may be specified without regard for which one is actually involved in the selection.

The IDAttributes element contains any number of the following two child elements in any order:

dsig2:QualifiedAttr

Specifies a namespace-qualified ID attribute node, by means of Name and NS attributes.

dsig2:UnqualifiedAttr

Specifies an unqualified ID attribute node, by means of a required Name attribute, and required ParentName and optional ParentNS attributes to identify the owning element.

  Schema Definition:

  <xs:element name="IDAttributes" type="dsig2:IDAttributesType"/>
  <xs:complexType name="IDAttributesType">
    <xs:choice maxOccurs="unbounded">
      <xs:element name="QualifiedAttr">
        <xs:complexType>
          <xs:attribute name="Name" type="xs:NCName" use="required"/>
          <xs:attribute name="NS" type="xs:anyURI" use="required"/>
        </xs:complexType>
	  </xs:element>
	  <xs:element name="UnqualifiedAttr">
        <xs:complexType>
          <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>
      </xs:element>
    </xs:choice>
  </xs:complexType>

Without a DTD, there is technically no way to define IDness in an XML document. In practice, this typing was extended to documents validated by an XML Schema, and then to the creation of xml:id. Unfortunately, DTDs have mostly fallen out of practice, and schemas are expensive, rarely used in many runtime scenarios, and can't be relied on to be completely known by the verifier in the presence of extensible XML scenarios.
xml:id has not yet seen wide adoption, mainly because a lot of the standards that needed it (SAML, WS-Security) were completed prior to its invention.
The result is that applications that rely on ID-based references for signing have typically made insecure assumptions about the IDness of attributes based on their name (ID, id, Id, etc.), or have to provide APIs for applications to call before verification (which is also a problem in the face of extensibility). DOM level 3, which is now fairly widely implemented, also provides the ability to identify attributes as an ID at runtime, although often without guaranteeing the uniqueness property.
The dsig2:IDAttributes element provides a deterministic way of defining an ID attribute used during signing, that is independent of DTD, XML Schema, DOM 3 or other application-specific mechanisms.

6.8.1 Canonicalization

Any canonicalization algorithm that can be used for CanonicalizationMethod (such as those in  Canonicalization Algorithms (section 6.5)) can be used as a Transform.

6.8.2 Base64

Identifiers:

http://www.w3.org/2000/09/xmldsig#base64

Input:

octet-stream, node-set

Output:

octet-stream

The normative specification for base64 decoding transforms is [RFC2045]. The base64 Transform element has no content. The input is decoded by the algorithms. This transform is useful if an application needs to sign the raw data associated with the encoded content of an element.

This transform accepts either an octet-stream or a node-set as input. If an octet-string is given as input, then this octet-stream is processed directly. If an XPath node-set (or sufficiently functional alternative) is given as input, then it is converted to an octet stream by performing operations logically equivalent to 1) applying an XPath transform with expression self::text(), then 2) taking the string-value of the node-set. Thus, if an XML element is identified by a shortname XPointer in the Reference URI, and its content consists solely of base64 encoded character data, then this transform automatically strips away the start and end tags of the identified element and any of its descendant elements as well as any descendant comments and processing instructions. The output of this transform is an octet stream.

6.8.3 XPath Filtering

Identifier:

http://www.w3.org/TR/1999/REC-xpath-19991116

Input:

octet-stream, node-set

Output:

node-set

The normative specification for XPath expression evaluation is [XPATH]. The XPath expression to be evaluated appears as the character content of a transform parameter child element named XPath.

The input required by this transform is an XPath node-set or an octet-stream. Note that if the actual input is an XPath node-set resulting from a null URI or shortname XPointer dereference, then comment nodes will have been omitted. If the actual input is an octet stream, then the application must convert the octet stream to an XPath node-set suitable for use by Canonical XML with Comments. (A subsequent application of the required Canonical XML algorithm would strip away these comments.) In other words, the input node-set should be equivalent to the one that would be created by the following process:

1. Initialize an XPath evaluation context by setting the initial node equal to the input XML document's root node, and set the context position and size to 1.
2. Evaluate the XPath expression (//. | //@* | //namespace::*)

The evaluation of this expression includes all of the document's nodes (including comments) in the node-set representing the octet stream.

The transform output is always an XPath node-set. The XPath expression appearing in the XPath parameter is evaluated once for each node in the input node-set. The result is converted to a boolean. If the boolean is true, then the node is included in the output node-set. If the boolean is false, then the node is omitted from the output node-set.

Note: Even if the input node-set has had comments removed, the comment nodes still exist in the underlying parse tree and can separate text nodes. For example, the markup <e>Hello, <!-- comment -->world!</e> contains two text nodes. Therefore, the expression self::text()[string()="Hello, world!"] would fail. Should this problem arise in the application, it can be solved by either canonicalizing the document before the XPath transform to physically remove the comments or by matching the node based on the parent element's string value (e.g. by using the expression self::text()[string(parent::e)="Hello, world!"]).

The primary purpose of this transform is to ensure that only specifically defined changes to the input XML document are permitted after the signature is affixed. This is done by omitting precisely those nodes that are allowed to change once the signature is affixed, and including all other input nodes in the output. It is the responsibility of the XPath expression author to include all nodes whose change could affect the interpretation of the transform output in the application context.

Note that the XML-Signature XPath Filter 2.0 Recommendation [XMLDSIG-XPATH-FILTER2] may be used for this purpose. That recommendation defines an XPath transform that permits the easy specification of subtree selection and omission that can be efficiently implemented.

An important scenario would be a document requiring two enveloped signatures. Each signature must omit itself from its own digest calculations, but it is also necessary to exclude the second signature element from the digest calculations of the first signature so that adding the second signature does not break the first signature.

The XPath transform establishes the following evaluation context for each node of the input node-set:

• A context node equal to a node of the input node-set.
• A context position, initialized to 1.
• A context size, initialized to 1.
• A library of functions equal to the function set defined in [XPATH] a function named here.
• A set of variable bindings. No means for initializing these is defined. Thus, the set of variable bindings used when evaluating the XPath expression is empty, and use of a variable reference in the XPath expression results in an error.
• The set of namespace declarations in scope for the XPath expression.

As a result of the context node setting, the XPath expressions appearing in this transform will be quite similar to those used in used in [XSLT], except that the size and position are always 1 to reflect the fact that the transform is automatically visiting every node (in XSLT, one recursively calls the command apply-templates to visit the nodes of the input tree).

The function here() is defined as follows:

Function: node-set here()

The here function returns a node-set containing the attribute or processing instruction node or the parent element of the text node that directly bears the XPath expression.  This expression results in an error if the containing XPath expression does not appear in the same XML document against which the XPath expression is being evaluated.

As an example, consider creating an enveloped signature (a Signature element that is a descendant of an element being signed). Although the signed content should not be changed after signing, the elements within the Signature element are changing (e.g. the digest value must be put inside the DigestValue and the SignatureValue must be subsequently calculated). One way to prevent these changes from invalidating the digest value in DigestValue is to add an XPath Transform that omits all Signature elements and their descendants. For example,

<Document>
...   
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
  <SignedInfo>
   ...
    <Reference URI="">
      <Transforms>
        <Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
          <XPath xmlns:dsig="&dsig;">
          not(ancestor-or-self::dsig:Signature)
          </XPath>
        </Transform>
      </Transforms>
      <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
      <DigestValue></DigestValue>
    </Reference>
  </SignedInfo>
  <SignatureValue></SignatureValue>
 </Signature>
 ...
</Document>

Due to the null Reference URI in this example, the XPath transform input node-set contains all nodes in the entire parse tree starting at the root node (except the comment nodes). For each node in this node-set, the node is included in the output node-set except if the node or one of its ancestors has a tag of Signature that is in the namespace given by the replacement text for the entity &dsig;.

A more elegant solution uses the here function to omit only the Signature containing the XPath Transform, thus allowing enveloped signatures to sign other signatures. In the example above, use the XPath element:

<XPath xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>

Since the XPath equality operator converts node sets to string values before comparison, we must instead use the XPath union operator (|). For each node of the document, the predicate expression is true if and only if the node-set containing the node and its Signature element ancestors does not include the enveloped Signature element containing the XPath expression (the union does not produce a larger set if the enveloped Signature element is in the node-set given by ancestor-or-self::Signature).

6.8.4 Signature Transform

Identifier:

http://www.w3.org/2000/09/xmldsig#enveloped-signature

Input:

node-set

Output:

node-set

An enveloped signature transform T removes the whole Signature element containing T from the digest calculation of the Reference element containing T. The entire string of characters used by an XML processor to match the Signature with the XML production element is removed. The output of the transform is equivalent to the output that would result from replacing T with an XPath transform containing the following XPath parameter element:

   <XPath xmlns:dsig="&dsig;">
   count(ancestor-or-self::dsig:Signature |
   here()/ancestor::dsig:Signature[1]) >
   count(ancestor-or-self::dsig:Signature)</XPath>

The input and output requirements of this transform are identical to those of the XPath transform, but may only be applied to a node-set from its parent XML document. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform must produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.

6.8.5 XSLT Transform

Identifier:

http://www.w3.org/TR/1999/REC-xslt-19991116

Input:

octet-stream

Output:

octet-stream

The normative specification for XSL Transformations is [XSLT]. Specification of a namespace-qualified stylesheet element, which must be the sole child of the Transform element, indicates that the specified style sheet should be used. Whether this instantiates in-line processing of local XSLT declarations within the resource is determined by the XSLT processing model; the ordered application of multiple stylesheet may require multiple Transforms. No special provision is made for the identification of a remote stylesheet at a given URI because it can be communicated via an xsl:include or xsl:import within the stylesheet child of the Transform.

This transform requires an octet stream as input.

The output of this transform is an octet stream. The processing rules for the XSL style sheet [XSL10] or transform element are stated in the XSLT specification [XSLT].

We RECOMMEND that XSLT transform authors use an output method of xml for XML and HTML. As XSLT implementations do not produce consistent serializations of their output, we further RECOMMEND inserting a transform after the XSLT transform to canonicalize the output. These steps will help to ensure interoperability of the resulting signatures among applications that support the XSLT transform. Note that if the output is actually HTML, then the result of these steps is logically equivalent [XHTML10].

8.1.1 Only What is Signed is Secure

First, obviously, signatures over a transformed document do not secure any information discarded by transforms: only what is signed is secure.

Note that the use of Canonical  XML [XML-C14N] ensures that all internal entities and XML namespaces are expanded within the content being signed. All entities are replaced with their definitions and the canonical form explicitly represents the namespace that an element would otherwise inherit. Applications that do not canonicalize XML content (especially the SignedInfo element) should not use internal entities and should represent the namespace explicitly within the content being signed since they can not rely upon canonicalization to do this for them. Also, users concerned with the integrity of the element type definitions associated with the XML instance being signed may wish to sign those definitions as well (i.e., the schema, DTD, or natural language description associated with the namespace/identifier).

Second, an envelope containing signed information is not secured by the signature. For instance, when an encrypted envelope contains a signature, the signature does not protect the authenticity or integrity of unsigned envelope headers nor its ciphertext form, it only secures the plaintext actually signed.

8.1.2 Only What is "Seen" Should be Signed

Additionally, the signature secures any information introduced by the transform: only what is "seen" (that which is represented to the user via visual, auditory or other media) should be signed. If signing is intended to convey the judgment or consent of a user (an automated mechanism or person), then it is normally necessary to secure as exactly as practical the information that was presented to that user. Note that this can be accomplished by literally signing what was presented, such as the screen images shown a user. However, this may result in data which is difficult for subsequent software to manipulate. Instead, one can sign the data along with whatever filters, style sheets, client profile or other information that affects its presentation.

8.1.3 "See" What is Signed

Just as a user should only sign what he or she "sees," persons and automated mechanism that trust the validity of a transformed document on the basis of a valid signature should operate over the data that was transformed (including canonicalization) and signed, not the original pre-transformed data. This recommendation applies to transforms specified within the signature as well as those included as part of the document itself. For instance, if an XML document includes an embedded style sheet [XSLT] it is the transformed document that should be represented to the user and signed. To meet this recommendation where a document references an external style sheet, the content of that external resource should also be signed as via a signature Reference otherwise the content of that external content might change which alters the resulting document without invalidating the signature.

Some applications might operate over the original or intermediary data but should be extremely careful about potential weaknesses introduced between the original and transformed data. This is a trust decision about the character and meaning of the transforms that an application needs to make with caution. Consider a canonicalization algorithm that normalizes character case (lower to upper) or character composition ('e and accent' to 'accented-e'). An adversary could introduce changes that are normalized and consequently inconsequential to signature validity but material to a DOM processor. For instance, by changing the case of a character one might influence the result of an XPath selection. A serious risk is introduced if that change is normalized for signature validation but the processor operates over the original data and returns a different result than intended.

As a result:

• All documents operated upon and generated by signature applications must be in [NFC] (otherwise intermediate processors might unintentionally break the signature)
• Encoding normalizations should not be done as part of a signature transform, or (to state it another way) if normalization does occur, the application should always "see" (operate over) the normalized form.

10.1.1 2.0 Mode Algorithm Identifiers and Implementation Requirements

This section identifies algorithms used with the XML digital signature specification. Entries contain the identifier to be used in Signature elements, a reference to the formal specification, and definitions, where applicable, for the representation of keys and the results of cryptographic operations.

Note that the algorithms required for 2.0 conformance are fewer than for compatibility mode, and that some algorithms required or optional are disallowed in 2.0.

10.2.1 Compatibility Mode Algorithm Identifiers and Implementation Requirements

The following algorithm support is required in addition to the algorithms listed for 2.0 mode.

Digest

Required

1. SHA1 (Use is DISCOURAGED; see SHA-1 Warning) http://www.w3.org/2000/09/xmldsig#sha1

Optional

Encoding

Required

MAC

Required

1. HMAC-SHA1 (Use is DISCOURAGED; see SHA-1 Warning) http://www.w3.org/2000/09/xmldsig#hmac-sha1

Recommended

Signature

Required

1. DSAwithSHA1 (signature verification) http://www.w3.org/2000/09/xmldsig#dsa-sha1 [RFC4051]

Recommended

1. RSAwithSHA1 (signature verification; use for signature generation is DISCOURAGED; see SHA-1 Warning) http://www.w3.org/2000/09/xmldsig#rsa-sha1

Optional

1. ECDSAwithSHA1 (Use is DISCOURAGED; see SHA-1 Warning) http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1 [RFC4051]
2. DSAwithSHA1 (signature generation) http://www.w3.org/2000/09/xmldsig#dsa-sha1

Canonicalization

Required

1. Canonical XML 1.0 (omits comments) http://www.w3.org/TR/2001/REC-xml-c14n-20010315
2. Canonical XML 1.1 (omits comments) http://www.w3.org/2006/12/xml-c14n11
3. Exclusive XML Canonicalization 1.0 (omits comments) http://www.w3.org/2001/10/xml-exc-c14n#

Recommended

1. Canonical XML 1.0 with Comments http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments
2. Canonical XML 1.1 with Comments http://www.w3.org/2006/12/xml-c14n11#WithComments
3. Exclusive XML Canonicalization 1.0 with Comments http://www.w3.org/2001/10/xml-exc-c14n#WithComments

Transform

Required

1. Enveloped Signature* http://www.w3.org/2000/09/xmldsig#enveloped-signature

Recommended

1. XPath http://www.w3.org/TR/1999/REC-xpath-19991116
2. XPath Filter 2.0 http://www.w3.org/2002/06/xmldsig-filter2

Optional

1. XSLT http://www.w3.org/TR/1999/REC-xslt-19991116

* The Enveloped Signature transform removes the Signature element from the calculation of the signature when the signature is within the content that it is being signed. This may be implemented via the XPath specification specified in 6.6.4: Enveloped Signature Transform; it must have the same effect as that specified by the XPath Transform.

When using transforms, we RECOMMEND selecting the least expressive choice that still accomplishes the needs of the use case at hand: Use of XPath filter 2.0 is recommended over use of XPath filter. Use of XPath filter is recommended over use of XSLT.

Note: Implementation requirements for the XPath transform may be downgraded to optional in a future version of this specification.