XML Signature Syntax and Processing Version 1.1

Abstract

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

URI	namespace prefix	XML internal entity
`http://www.w3.org/2000/09/xmldsig#`	default namespace, `ds:`, `dsig:`	`<!ENTITY dsig "http://www.w3.org/2000/09/xmldsig#">`
`http://www.w3.org/2009/xmldsig11#`	`dsig11:`	`<!ENTITY dsig11 "http://www.w3.org/2009/xmldsig11#">`

2. Signature Overview and Examples

This section provides an overview and examples of XML digital signature syntax. The specific processing is given in section 3. Processing Rules. The formal syntax is found in section 4. Core Signature Syntax and section 5. Additional Signature Syntax.

In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.

XML Signatures are applied to arbitrary digital content (data objects) via an indirection. Data objects are digested, the resulting value is placed in an element (with other information) and that element is then digested and cryptographically signed. XML digital signatures are represented by the Signature element which has the following structure (where "?" denotes zero or one occurrence; "+" denotes one or more occurrences; and "*" denotes zero or more occurrences):

Example 1

<Signature ID?> 
  <SignedInfo>
    <CanonicalizationMethod />
    <SignatureMethod />
   (<Reference URI? >
     (<Transforms>)?
      <DigestMethod>
      <DigestValue>
    </Reference>)+
  </SignedInfo>
  <SignatureValue> 
 (<KeyInfo>)?
 (<Object ID?>)*
</Signature>

Signatures are related to data objects via URIs [URI]. Within an XML document, signatures are related to local data objects via fragment identifiers. Such local data can be included within an enveloping signature or can enclose an enveloped signature. Detached signatures are over external network resources or local data objects that reside within the same XML document as sibling elements; in this case, the signature is neither enveloping (signature is parent) nor enveloped (signature is child). Since a Signature element (and its Id attribute value/name) may co-exist or be combined with other elements (and their IDs) within a single XML document, care should be taken in choosing names such that there are no subsequent collisions that violate the ID uniqueness validity constraint [XML10].

2.1 Simple Example (`Signature`, `SignedInfo`, `Methods`, and `Reference`s)

The following example is a detached signature of the content of the HTML4 in XML specification.

Example 2

[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.

2.1.1 More on `Reference`

Example 3

[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.

2.2 Extended Example (`Object` and `SignatureProperty`)

This specification does not address mechanisms for making statements or assertions. Instead, this document defines what it means for something to be signed by an XML Signature (integrity, message authentication, and/or signer authentication). Applications that wish to represent other semantics must rely upon other technologies, such as [XML10], [RDF-PRIMER]. For instance, an application might use a foo:assuredby attribute within its own markup to reference a Signature element. Consequently, it's the application that must understand and know how to make trust decisions given the validity of the signature and the meaning of assuredby syntax. We also define a SignatureProperties element type for the inclusion of assertions about the signature itself (e.g., signature semantics, the time of signing or the serial number of hardware used in cryptographic processes). Such assertions may be signed by including a Reference for the SignatureProperties in SignedInfo. While the signing application should be very careful about what it signs (it should understand what is in the SignatureProperty) a receiving application has no obligation to understand that semantic (though its parent trust engine may wish to). Any content about the signature generation may be located within the SignatureProperty element. The mandatory Target attribute references the Signature element to which the property applies.

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

Example 4

[   ]  <Signature Id="MySecondSignature" ...>
[p01]  <SignedInfo>  
[   ]   ...  
[p02]   <Reference URI="http://www.w3.org/TR/xml-stylesheet/">   
[   ]   ... 
[p03]   <Reference URI="#AMadeUpTimeStamp"  
[p04]         Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties">
[p05]    <Transforms> 
[p06]      <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> 
[p07]    </Transforms> 
[p08]    <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>    
[p09]    <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue>
[p10]   </Reference>    
[p11]  </SignedInfo>  
[p12]  ...  
[p13]  <Object> 
[p14]   <SignatureProperties> 
[p15]     <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> 
[p16]        <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt">  
[p17]          <date>19990914</date>  
[p18]          <time>14:34:34:34</time>  
[p19]        </timestamp>  
[p20]     </SignatureProperty> 
[p21]   </SignatureProperties> 
[p22]  </Object>  
[p23]</Signature>

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

[p13] Object is an optional element for including data objects within the signature element or elsewhere. The Object can be optionally typed and/or encoded.

[p14-21] Signature properties, such as time of signing, can be optionally signed by identifying them from within a Reference. (These properties are traditionally called signature "attributes" although that term has no relationship to the XML term "attribute".)

2.3 Extended Example (`Object` and `Manifest`)

The Manifest element is provided to meet additional requirements not directly addressed by the mandatory parts of this specification. Two requirements and the way the Manifest satisfies them follow.

First, applications frequently need to efficiently sign multiple data objects even where the signature operation itself is an expensive public key signature. This requirement can be met by including multiple Reference elements within SignedInfo since the inclusion of each digest secures the data digested. However, some applications may not want the core validation behavior associated with this approach because it requires every Reference within SignedInfo to undergo reference validation -- the DigestValue elements are checked. These applications may wish to reserve reference validation decision logic to themselves. For example, an application might receive a signature valid SignedInfo element that includes three Reference elements. If a single Reference fails (the identified data object when digested does not yield the specified DigestValue) the signature would fail core validation. However, the application may wish to treat the signature over the two valid Reference elements as valid or take different actions depending on which fails. To accomplish this, SignedInfo would reference a Manifest element that contains one or more Reference elements (with the same structure as those in SignedInfo). Then, reference validation of the Manifest is under application control.

Second, consider an application where many signatures (using different keys) are applied to a large number of documents. An inefficient solution is to have a separate signature (per key) repeatedly applied to a large SignedInfo element (with many References); this is wasteful and redundant. A more efficient solution is to include many references in a single Manifest that is then referenced from multiple Signature elements.

The example below includes a Reference that signs a Manifest found within the Object element.

Example 5

[   ] ...
[m01]   <Reference URI="#MyFirstManifest"
[m02]     Type="http://www.w3.org/2000/09/xmldsig#Manifest">
[m03]     <Transforms> 
[m04]       <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> 
[m05]     </Transforms> 
[m06]     <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> 
[m07]     <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...=</DigestValue> 
[m08]   </Reference>  
[   ] ...
[m09] <Object>
[m10]   <Manifest Id="MyFirstManifest">
[m11]     <Reference>
[m12]     ...
[m13]     </Reference>   
[m14]     <Reference>
[m15]     ...
[m16]     </Reference>
[m17]   </Manifest>
[m18] </Object>

4. Core Signature Syntax

The general structure of an XML signature is described in section 2. Signature Overview and Examples. This section provides detailed syntax of the core signature features. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via an [XMLSCHEMA-1][XMLSCHEMA-2] with the following XML preamble, declaration, and internal entity.

Schema Definition:

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

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

Additional markup defined in version 1.1 of this specification uses the dsig11: namespace. The syntax is defined in an XML schema with the following preamble:

Schema Definition:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
[
<!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
<!ENTITY dsig11 'http://www.w3.org/2009/xmldsig11#'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>

<schema xmlns="http://www.w3.org/2001/XMLSchema"
        xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
        xmlns:dsig11="http://www.w3.org/2009/xmldsig11#"
        targetNamespace="http://www.w3.org/2009/xmldsig11#"
        version="0.1" elementFormDefault="qualified">

4.1 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 could be ds:CryptoBinary. However, if HMAC-SHA1 is the signature algorithm then SignatureValue could have leading zero octets that must be preserved. Thus SignatureValue is generically defined as of type base64Binary.

Schema Definition:

<simpleType name="CryptoBinary">
  <restriction base="base64Binary" />
</simpleType>

4.2 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>

4.3 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>

4.4 The `SignedInfo` Element

The structure of SignedInfo includes the canonicalization algorithm, a signature algorithm, and one or more references. The SignedInfo element may contain an optional ID attribute that will allow it to be referenced by other signatures and objects.

SignedInfo does not include explicit signature or digest properties (such as calculation time, cryptographic device serial number, etc.). If an application needs to associate properties with the signature or digest, it may include such information in a SignatureProperties element within an Object element.

Schema Definition:

<element name="SignedInfo" 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>

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 section 6.1 Algorithm Identifiers and Implementation Requirements. Implementations must support the required canonicalization algorithms.

Alternatives to the required canonicalization algorithms (section 6.5), such as Canonical XML with Comments (section 6.5.1) or a minimal canonicalization (such as CRLF and charset normalization) , may be explicitly specified but are not required. Consequently, their use may not interoperate with other applications that do not support the specified algorithm (see XML Canonicalization and Syntax Constraint Considerations, section 7). Security issues may also arise in the treatment of entity processing and comments if non-XML aware canonicalization algorithms are not properly constrained (see section 8.1.2: Only What is "Seen" Should be Signed).

The way in which the SignedInfo element is presented to the canonicalization method is dependent on that method. The following applies to algorithms which process XML as nodes or characters:

XML based canonicalization implementations must be provided with an [XPATH] node-set originally formed from the document containing the SignedInfo and currently indicating the SignedInfo, its descendants, and the attribute and namespace nodes of SignedInfo and its descendant elements.
Text based canonicalization algorithms (such as CRLF and charset normalization) should be provided with the UTF-8 octets that represent the well-formed SignedInfo element, from the first character to the last character of the XML representation, inclusive. This includes the entire text of the start and end tags of the SignedInfo element as well as all descendant markup and character data (i.e., the text) between those tags. Use of text based canonicalization of SignedInfo is not recommended.

We recommend applications that implement a text-based instead of XML-based canonicalization -- such as resource constrained apps -- generate canonicalized XML as their output serialization so as to mitigate interoperability and security concerns. For instance, such an implementation should (at least) generate standalone XML instances [XML10].

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

Schema Definition:

<element name="CanonicalizationMethod" type="ds:CanonicalizationMethodType"/> 

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

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>

4.4.3.1 The `URI` Attribute

The URI attribute identifies a data object using a URI-Reference [URI].

The mapping from this attribute's value to a URI reference must be performed as specified in section 3.2.17 of [XMLSCHEMA-2]. Additionally: Some existing implementations are known to verify the value of the URI attribute against the grammar in [URI]. It is therefore safest to perform any necessary escaping while generating the URI attribute.

We RECOMMEND XML Signature applications be able to dereference URIs in the HTTP scheme. Dereferencing a URI in the HTTP scheme must comply with the Status Code Definitions of [HTTP11] (e.g., 302, 305 and 307 redirects are followed to obtain the entity-body of a 200 status code response). Applications should also be cognizant of the fact that protocol parameter and state information, (such as HTTP cookies, HTML device profiles or content negotiation), may affect the content yielded by dereferencing a URI.

If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease). (See section 3.2 Core Validation for further information on reference processing.)

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

The optional Type attribute contains information about the type of object being signed after all ds:Reference transforms have been applied. This is represented as a URI. For example:

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

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

4.4.3.2 The Reference Processing Model

Note: XPath is recommended. Signature applications need not conform to [XPATH] specification in order to conform to this specification. However, the XPath data model, definitions (e.g., node-sets) and syntax is used within this document in order to describe functionality for those that want to process XML-as-XML (instead of octets) as part of signature generation. For those that want to use these features, a conformant [XPATH] implementation is one way to implement these features, but it is not required. Such applications could use a sufficiently functional replacement to a node-set and implement only those XPath expression behaviors required by this specification. However, for simplicity we generally will use XPath terminology without including this qualification on every point. Requirements over "XPath node-sets" can include a node-set functional equivalent. Requirements over XPath processing can include application behaviors that are equivalent to the corresponding XPath behavior.

The data-type of the result of URI dereferencing or subsequent Transforms is either an octet stream or an XPath node-set.

The Transforms specified in this document are defined with respect to the input they require. The following is the default signature application behavior:

If the data object is an octet stream and the next transform requires a node-set, the signature application must attempt to parse the octets yielding the required node-set via [XML10] well-formed processing.
If the data object is a node-set and the next transform requires octets, the signature application must attempt to convert the node-set to an octet stream using Canonical XML [XML-C14N].

Users may specify alternative transforms that override these defaults in transitions between transforms that expect different inputs. The final octet stream contains the data octets being secured. The digest algorithm specified by DigestMethod is then applied to these data octets, resulting in the DigestValue.

Note: The section 3.1.1 Reference Generation includes further restrictions on the reliance upon defined default transformations when applications generate signatures.

In this specification, a 'same-document' reference is defined as a URI-Reference that consists of a hash sign ('#') followed by a fragment or alternatively consists of an empty URI [URI].

Unless the URI-Reference is such a 'same-document' reference , the result of dereferencing the URI-Reference must be an octet stream. In particular, an XML document identified by URI is not parsed by the signature application unless the URI is a same-document reference or unless a transform that requires XML parsing is applied. (See Transforms (section 4.4.3.4).)

When a fragment is preceded by an absolute or relative URI in the URI-Reference, the meaning of the fragment is defined by the resource's MIME type [RFC2045]. Even for XML documents, URI dereferencing (including the fragment processing) might be done for the signature application by a proxy. Therefore, reference validation might fail if fragment processing is not performed in a standard way (as defined in the following section for same-document references). Consequently, we RECOMMEND in this case that the URI attribute not include fragment identifiers and that such processing be specified as an additional XPath Transform or XPath Filter 2 Transform [XMLDSIG-XPATH-FILTER2].

When a fragment is not preceded by a URI in the URI-Reference, XML Signature applications must support the null URI and shortname XPointer [XPTR-FRAMEWORK]. We RECOMMEND support for the same-document XPointers '#xpointer(/)' and '#xpointer(id('ID'))' if the application also intends to support any canonicalization that preserves comments. (Otherwise URI="#foo" will automatically remove comments before the canonicalization can even be invoked due to the processing defined in Same-Document URI-References (section 4.4.3.3).) All other support for XPointers is optional, especially all support for shortname and other XPointers in external resources since the application may not have control over how the fragment is generated (leading to interoperability problems and validation failures).

'#xpointer(/)' must be interpreted to identify the root node [XPATH] of the document that contains the URI attribute.

'#xpointer(id('ID'))' must be interpreted to identify the element node identified by '#element(ID)' [XPTR-ELEMENT] when evaluated with respect to the document that contains the URI attribute.

The original edition of this specification [XMLDSIG-CORE] referenced the XPointer Candidate Recommendation [XPTR-XPOINTER-CR2001] and some implementations support it optionally. That Candidate Recommendation has been superseded by the [XPTR-FRAMEWORK], [XPTR-XMLNS] and [XPTR-ELEMENT] Recommendations, and -- at the time of this edition -- the [XPTR-XPOINTER] Working Draft. Therefore, the use of the xpointer() scheme [XPTR-XPOINTER] beyond the usage discussed in this section is discouraged.

The following examples demonstrate what the URI attribute identifies and how it is dereferenced:

URI="http://example.com/bar.xml": Identifies the octets that represent the external resource 'http://example.com/bar.xml', that is probably an XML document given its file extension.
URI="http://example.com/bar.xml#chapter1": Identifies the element with ID attribute value 'chapter1' of the external XML resource 'http://example.com/bar.xml', provided as an octet stream. Again, for the sake of interoperability, the element identified as 'chapter1' should be obtained using an XPath transform rather than a URI fragment (shortname XPointer resolution in external resources is not required in this specification).
URI="": Identifies the node-set (minus any comment nodes) of the XML resource containing the signature
URI="#chapter1": Identifies a node-set containing the element with ID attribute value 'chapter1' of the XML resource containing the signature. XML Signature (and its applications) modify this node-set to include the element plus all descendants including namespaces and attributes -- but not comments.

4.4.3.3 Same-Document URI-References

Dereferencing a same-document reference must result in an XPath node-set suitable for use by Canonical XML [XML-C14N]. Specifically, dereferencing a null URI (URI="") must result in an XPath node-set that includes every non-comment node of the XML document containing the URI attribute. In a fragment URI, the characters after the number sign ('#') character conform to the XPointer syntax [XPTR-FRAMEWORK]. When processing an XPointer, the application must behave as if the XPointer was evaluated with respect to the XML document containing the URI attribute . The application must behave as if the result of XPointer processing [XPTR-FRAMEWORK] were a node-set derived from the resultant subresource as follows:

include XPath nodes having full or partial content within the subresource
replace the root node with its children (if it is in the node-set)
replace any element node E with E plus all descendants of E (text, comment, PI, element) and all namespace and attribute nodes of E and its descendant elements.
if the URI has no fragment identifier or the fragment identifier is a shortname XPointer, then delete all comment nodes

The second to last replacement is necessary because XPointer typically indicates a subtree of an XML document's parse tree using just the element node at the root of the subtree, whereas Canonical XML treats a node-set as a set of nodes in which absence of descendant nodes results in absence of their representative text from the canonical form.

The last step is performed for null URIs and shortname XPointers . It is necessary because when [XML-C14N] or [XML-C14N11] is passed a node-set, it processes the node-set as is: with or without comments. Only when it is called with an octet stream does it invoke its own XPath expressions (default or without comments). Therefore to retain the default behavior of stripping comments when passed a node-set, they are removed in the last step if the URI is not a scheme-based XPointer. To retain comments while selecting an element by an identifier ID, use the following scheme-based XPointer: URI='#xpointer(id('ID'))'. To retain comments while selecting the entire document, use the following scheme-based XPointer: URI='#xpointer(/)'.

The interpretation of these XPointers is defined in The Reference Processing Model (section 4.4.3.2).

4.4.3.4 The `Transforms` Element

The optional Transforms element contains an ordered list of Transform elements; these describe how the signer obtained the data object that was digested. The output of each Transform serves as input to the next Transform. The input to the first Transform is the result of dereferencing the URI attribute of the Reference element. The output from the last Transform is the input for the DigestMethod algorithm. When transforms are applied the signer is not signing the native (original) document but the resulting (transformed) document. (See Only What is Signed is Secure (section 8.1.1).)

Each Transform consists of an Algorithm attribute and content parameters, if any, appropriate for the given algorithm. The Algorithm attribute value specifies the name of the algorithm to be performed, and the Transform content provides additional data to govern the algorithm's processing of the transform input. (See section 6.1 Algorithm Identifiers and Implementation Requirements)

As described in The Reference Processing Model (section 4.4.3.2), some transforms take an XPath node-set as input, while others require an octet stream. If the actual input matches the input needs of the transform, then the transform operates on the unaltered input. If the transform input requirement differs from the format of the actual input, then the input must be converted.

Some Transforms may require explicit MIME type, charset (IANA registered "character set"), or other such information concerning the data they are receiving from an earlier Transform or the source data, although no Transform algorithm specified in this document needs such explicit information. Such data characteristics are provided as parameters to the Transform algorithm and should be described in the specification for the algorithm.

Examples of transforms include but are not limited to base64 decoding [RFC2045], canonicalization [XML-C14N], XPath filtering [XPATH], and XSLT [XSLT]. The generic definition of the Transform element also allows application-specific transform algorithms. For example, the transform could be a decompression routine given by a Java class appearing as a base64 encoded parameter to a Java Transform algorithm. However, applications should refrain from using application-specific transforms if they wish their signatures to be verifiable outside of their application domain. Transform Algorithms (section 6.6) defines the list of standard transformations.

Schema Definition:

<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>

4.4.3.5 The `DigestMethod` Element

DigestMethod is a required element that identifies the digest algorithm to be applied to the signed object. This element uses the general structure here for algorithms specified in section 6.1 Algorithm Identifiers and Implementation Requirements.

If the result of the URI dereference and application of Transforms is an XPath node-set (or sufficiently functional replacement implemented by the application) then it must be converted as described in section 4.4.3.2 The Reference Processing Model. If the result of URI dereference and application of transforms is an octet stream, then no conversion occurs (comments might be present if the Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream.

Schema Definition:

<element name="DigestMethod" type="ds:DigestMethodType"/>

<complexType name="DigestMethodType" mixed="true"> 
  <sequence>
    <any namespace="##other" processContents="lax" 
         minOccurs="0" maxOccurs="unbounded"/>
  </sequence>    
  <attribute name="Algorithm" type="anyURI" use="required"/> 
</complexType>

4.4.3.6 The `DigestValue` Element

DigestValue is an element that contains the encoded value of the digest. The digest is always encoded using base64 [RFC2045].

Schema Definition:

<element name="DigestValue" type="ds:DigestValueType"/>

<simpleType name="DigestValueType">
  <restriction base="base64Binary"/>
</simpleType>

4.5 The `KeyInfo` Element

KeyInfo is an optional element that enables the recipient(s) to obtain the key needed to validate the signature. KeyInfo may contain keys, names, certificates and other public key management information, such as in-band key distribution or key agreement data. This specification defines a few simple types but applications may extend those types or all together replace them with their own key identification and exchange semantics using the XML namespace facility [XML-NAMES]. However, questions of trust of such key information (e.g., its authenticity or strength) are out of scope of this specification and left to the application. Details of the structure and usage of element children of KeyInfo other than simple types described in this specification are out of scope. For example, the definition of PKI certificate contents, certificate ordering, certificate revocation and CRL management are out of scope.

If KeyInfo is omitted, the recipient is expected to be able to identify the key based on application context. Multiple declarations within KeyInfo refer to the same key. While applications may define and use any mechanism they choose through inclusion of elements from a different namespace, compliant versions must implement KeyValue (section 4.5.2 The KeyValue Element) and should implement KeyInfoReference (section 4.5.10 The KeyInfoReference Element). KeyInfoReference is preferred over use of RetrievalMethod as it avoids use of Transform child elements that introduce security risk and implementation challenges. Support for other children of KeyInfo is optional.

The schema specification of many of KeyInfo's children (e.g., PGPData, SPKIData, X509Data) permit their content to be extended/complemented with elements from another namespace. This may be done only if it is safe to ignore these extension elements while claiming support for the types defined in this specification. Otherwise, external elements, including alternative structures to those defined by this specification, must be a child of KeyInfo. For example, should a complete XML-PGP standard be defined, its root element must be a child of KeyInfo. (Of course, new structures from external namespaces can incorporate elements from the dsig: namespace via features of the type definition language. For instance, they can create a schema that permits, includes, imports, or derives new types based on dsig: elements.)

The following list summarizes the KeyInfo types that are allocated an identifier in the dsig: namespace; these can be used within the RetrievalMethod Type attribute to describe a remote KeyInfo structure.

The following list summarizes the additional KeyInfo types that are allocated an identifier in the dsig11: namespace.

In addition to the types above for which we define an XML structure, we specify one additional type to indicate a binary (ASN.1 DER) X.509 Certificate.

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

Schema Definition:

<element name="KeyInfo" type="ds:KeyInfoType"/> 

<complexType name="KeyInfoType" mixed="true">
  <choice maxOccurs="unbounded">     
    <element ref="ds:KeyName"/> 
    <element ref="ds:KeyValue"/> 
    <element ref="ds:RetrievalMethod"/> 
    <element ref="ds:X509Data"/> 
    <element ref="ds:PGPData"/> 
    <element ref="ds:SPKIData"/>
    <element ref="ds:MgmtData"/>
    <!-- <element ref="dsig11:DEREncodedKeyValue"/> -->
    <!-- DEREncodedKeyValue (XMLDsig 1.1) will use the any element -->
    <!-- <element ref="dsig11:KeyInfoReference"/> -->
    <!-- KeyInfoReference (XMLDsig 1.1) will use the any element -->
    <!-- <element ref="xenc:EncryptedKey"/> -->
    <!-- EncryptedKey (XMLEnc) will use the any element -->
    <!-- <element ref="xenc:Agreement"/> -->
    <!-- Agreement (XMLEnc) will use the any element -->
    <any processContents="lax" namespace="##other"/>
    <!-- (1,1) elements from (0,unbounded) namespaces -->
  </choice>
  <attribute name="Id" type="ID" use="optional"/>
</complexType>

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 section 6.4 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>

4.5.2.1 The `DSAKeyValue` Element

Identifier: Type="http://www.w3.org/2000/09/xmldsig#DSAKeyValue" (this can be used within a RetrievalMethod or Reference element to identify the referent's type)

DSA keys and the DSA signature algorithm are specified in [FIPS-186-3]. DSA public key values can have the following fields:

P: a prime modulus meeting the [FIPS-186-3] requirements
Q: an integer in the range 2**159 < Q < 2**160 which is a prime divisor of P-1
G: an integer with certain properties with respect to P and Q
Y: G**X mod P (where X is part of the private key and not made public)
J: (P - 1) / Q
seed: a DSA prime generation seed
pgenCounter: a DSA prime generation counter

Parameter J is available for inclusion solely for efficiency as it is calculatable from P and Q. Parameters seed and pgenCounter are used in the DSA prime number generation algorithm specified in [FIPS-186-3]. As such, they are optional but must either both be present or both be absent. This prime generation algorithm is designed to provide assurance that a weak prime is not being used and it yields a P and Q value. Parameters P, Q, and G can be public and common to a group of users. They might be known from application context. As such, they are optional but P and Q must either both appear or both be absent. If all of P, Q, seed, and pgenCounter are present, implementations are not required to check if they are consistent and are free to use either P and Q or seed and pgenCounter. All parameters are encoded as base64 [RFC2045] values.

Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are represented in XML as octet strings as defined by the ds:CryptoBinary type.

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>

4.5.2.2 The `RSAKeyValue` Element

Identifier: Type="http://www.w3.org/2000/09/xmldsig#RSAKeyValue" (this can be used within a RetrievalMethod or Reference element to identify the referent's type)

RSA key values have two fields: Modulus and Exponent.

Example 6

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

Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are represented in XML as octet strings as defined by the ds:CryptoBinary type.

Schema Definition:

<element name="RSAKeyValue" type="ds:RSAKeyValueType" />

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

4.5.2.3 The `ECKeyValue` Element

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

The ECKeyValue element is defined in the http://www.w3.org/2009/xmldsig11# namespace.

EC public key values consists of two sub components: Domain parameters and PublicKey.

Example 7

<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>

Note - A line break has been added to the PublicKey content to preserve printed page width.

Domain parameters can be encoded explicitly using the dsig11:ECParameters element or by reference using the dsig11:NamedCurve element. A named curve is specified through the URI attribute. For named curves that are identified by OIDs, such as those defined in [RFC3279] and [RFC4055], the OID should be encoded according to [URN-OID]. Conformant applications must support the dsig11:NamedCurve element and the 256-bit prime field curve as identified by the OID 1.2.840.10045.3.1.7.

The PublicKey element contains a Base64 encoding of a binary representation of the x and y coordinates of the point. Its value is computed as follows:

Convert the elliptic curve point (x,y) to an octet string by first converting the field elements x and y to octet strings as specified in Section 6.2 of [ECC-ALGS] (note), and then prepend the concatenated result of the conversion with 0x04. Support for Elliptic-Curve-Point-to-Octet-String conversion without point compression is required.
Base64 encode the octet string resulting from the conversion in Step 1.

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>

4.5.2.3.1 Explicit Curve Parameters

The ECParameters element consists of the following subelements. Note these definitions are based on the those described in [RFC3279].

The FieldID element identifies the finite field over which the elliptic curve is defined. Additional details on the structures for defining prime and characteristic two fields is provided below.
The dsig11:Curve element specifies the coefficients a and b of the elliptic curve E. Each coefficient is first converted from a field element to an octet string as specified in section 6.2 of [ECC-ALGS], then the resultant octet string is encoded in base64.
The Base element specifies the base point P on the elliptic curve. The base point is represented as a value of type ECPointType.
The Order element specifies the order n of the base point and is encoded as a positiveInteger.
The Cofactor element is an optional element that specifies the integer h = #E(Fq)/n. The cofactor is not required to support ECDSA, except in parameter validation. The cofactor may be included to support parameter validation for ECDSA keys. Parameter validation is not required by this specification. The cofactor is required in ECDH public key parameters.
The dsig11:ValidationData element is an optional element that specifies the hash algorithm used to generate the elliptic curve E and the base point G verifiably at random. It also specifies the seed that was used to generate the curve and the base point.

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>

Prime fields are described by a single subelement P, which represents the field size in bits. It is encoded as a positiveInteger.

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>

Structures are defined for three types of characteristic two fields: gaussian normal basis, pentanomial basis and trinomial basis.

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>

4.5.2.3.2 Compatibility with RFC 4050

Implementations that need to support the [RFC4050] format for ECDSA keys can avoid known interoperability problems with that specification by adhering to the following profile:

Avoid validating the ECDSAKeyValue element against the [RFC4050] schema. XML schema validators may not support integer types with decimal data exceeding 18 decimal digits. [XMLSCHEMA-1][XMLSCHEMA-2].
Support only the NamedCurve element.
Support the 256-bit prime field curve, as identified by the URN urn:oid:1.2.840.10045.3.1.7.

The following is an example of a ECDSAKeyValue element that meets the profile described in this section.

Example 8

<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>

Note - A line break has been added to the X and Y Value attribute values to preserve printed page width.

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 the Reference URI attribute (section 4.4.3.1 The URI Attribute) and the 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 4.5 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. In such cases, use of KeyInfoReference is encouraged instead, see section 4.5.10 The KeyInfoReference Element.

Note: The KeyInfoReference element is preferred over use of RetrievalMethod as it avoids use of Transform 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.

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.
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:

Example 9

<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.

The X509IssuerSerial element has been deprecated in favor of the newly-introduced dsig11:X509Digest element. The XML Schema type of the serial number was defined to be an integer, and XML Schema validators may not support integer types with decimal data exceeding 18 decimal digits [XMLSCHEMA-2]. This has proven insufficient, because many Certificate Authorities issue certificates with large, random serial numbers that exceed this limit. As a result, deployments that do make use of this element should take care if schema validation is involved. New deployments should avoid use of the element.

4.5.4.1 Distinguished Name Encoding Rules

To encode a distinguished name (X509IssuerSerial,X509SubjectName, and KeyName if appropriate), the encoding rules in section 2 of RFC 4514 [LDAP-DN] should be applied, except that the character escaping rules in section 2.4 of RFC 4514 [LDAP-DN] may be augmented as follows:

Escape all occurrences of ASCII control characters (Unicode range \x00 - \x1f) by replacing them with "\" followed by a two digit hex number showing its Unicode number.
Escape any trailing space characters (Unicode \x20) by replacing them with "\20", instead of using the escape sequence "\ ".

Since an XML document logically consists of characters, not octets, the resulting Unicode string is finally encoded according to the character encoding used for producing the physical representation of the XML document.

Schema Definition:

<element name="X509Data" type="ds:X509DataType"/> 

<complexType name="X509DataType">
  <sequence maxOccurs="unbounded">
    <choice>
      <element name="X509IssuerSerial" type="ds:X509IssuerSerialType"/>
      <element name="X509SKI" type="base64Binary"/>
      <element name="X509SubjectName" type="string"/>
      <element name="X509Certificate" type="base64Binary"/>
      <element name="X509CRL" type="base64Binary"/>
      <!-- <element ref="dsig11:X509Digest"/> -->
      <!-- The X509Digest element (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

			




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XML Signature Syntax and Processing Version 1.1

W3C Proposed Recommendation 24 January 2013

Abstract

Status of This Document

Table of Contents

1. Introduction

1.1 Conformance

1.2 Design Philosophy

1.3 Versions, Namespaces and Identifiers

1.4 Acknowledgements

2. Signature Overview and Examples

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

2.1.1 More on Reference

2.2 Extended Example (Object and SignatureProperty)

2.3 Extended Example (Object and Manifest)

3. Processing Rules

3.1 Signature Generation

3.1.1 Reference Generation

3.1.2 Signature Generation

3.2 Core Validation

3.2.1 Reference Validation

3.2.2 Signature Validation

4. Core Signature Syntax

4.1 The ds:CryptoBinary Simple Type

4.2 The Signature element

4.3 The SignatureValue Element

4.4 The SignedInfo Element

4.4.1 The CanonicalizationMethod Element

4.4.2 The SignatureMethod Element

4.4.3 The Reference Element

4.4.3.1 The URI Attribute