XML Signature Syntax and Processing Version 2.0

1 Introduction

This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element. More specifically, this specification defines an XML signature element type and an XML signature application; conformance requirements for each are specified by way of schema definitions and prose respectively. This specification also includes other useful types that identify methods for referencing collections of resources, algorithms, and keying and management information.

The XML Signature is a method of associating a key with referenced data (octets); it does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed. Consequently, while this specification is an important component of secure XML applications, it itself is not sufficient to address all application security/trust concerns, particularly with respect to using signed XML (or other data formats) as a basis of human-to-human communication and agreement. Such an application must specify additional key, algorithm, processing and rendering requirements. For further information, please see 9 Security Considerations.

XML Signature 2.0 includes a new transform model designed to address requirements including performance, simplicity and streamability. This model is significantly different than in XML Signature 1.x, see 10 Differences from 1.x version. XML Signature 2.0 is designed to be backward compatible, however, enabling the XML Signature 1.x model to be used where necessary. Details of this model are documented in XML Signature, Second Edition.

1.1 Editorial and Conformance Conventions

For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types. Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See 9.1 Check the Security Model

This specification provides a normative XML Schema [XML-schema].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]:

"they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"

Consequently, we use these capitalized key words to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the Namespaces in XML specification [XML-ns] is described as "REQUIRED."

1.2 Design Philosophy

The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature-RD].

1.3 Versions, Namespaces and Identifiers

This specification makes use of XML namespaces, and uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics.

Implementations of this specification MUST use the following XML namespace URIs:

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#">`
`http://www.w3.org/2008/xmlsec/experimental#`	`dsig2:`	`<!ENTITY dsig2 "http://www.w3.org/2008/xmlsec/experimental#">`

Editorial note
The Namespace URI for dsig2, listed as `http://www.w3.org/2008/xmlsec/experimental#`, is subject to change.

While implementations MUST support XML and XML namespaces, and while use of the above namespace URIs is REQUIRED, the namespace prefixes and entity declarations given are merely editorial conventions used in this document. Their use by implementations is OPTIONAL.

These namespace URIs are also used as the prefix for algorithm identifiers that are under control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN] or Uniform Resource Identifiers [URI] defined by the relevant normative external specification.

For instance:

SignatureProperties is identified and defined by the disg: namespace http://www.w3.org/2000/09/xmldsig#SignatureProperties
ECKeyValue is identified and defined by the dsig11: namespace http://www.w3.org/2009/xmldsig11#ECKeyValue
XSLT is identified and defined by an external URI http://www.w3.org/TR/1999/REC-xslt-19991116
SHA1 is identified via this specification's namespace and defined via a normative reference http://www.w3.org/2001/04/xmlenc#sha256 FIPS PUB 180-3. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.
Selection is identified and defined by the dsig2: namespace http://www.w3.org/2008/xmlsec/experimental#Selection

The http://www.w3.org/2000/09/xmldsig# (dsig:) namespace was introduced in the first edition of this specification, and http://www.w3.org/2009/xmldsig11# (dsig11:) namespace was introduced in 1.1. This version does not coin any new elements or algorithm identifiers in that namespace; instead, the dsig2: namespace is used.

No provision is made for an explicit version number in this syntax. If a future version of this specification requires explicit versioning of the document format, a different namespace will be used.

1.4 Acknowledgements

The contributions of the following Working Group members to this specification are gratefully acknowledged:

Mark Bartel, Adobe, was Accelio (Author)
John Boyer, IBM (Author)
Mariano P. Consens, University of Waterloo
John Cowan, Reuters Health
Donald Eastlake 3rd, Motorola (Chair, Author/Editor)
Barb Fox, Microsoft (Author)
Christian Geuer-Pollmann, University Siegen
Tom Gindin, IBM
Phillip Hallam-Baker, VeriSign Inc
Richard Himes, US Courts
Merlin Hughes, Baltimore
Gregor Karlinger, IAIK TU Graz
Brian LaMacchia, Microsoft (Author)
Peter Lipp, IAIK TU Graz
Joseph Reagle, NYU, was W3C (Chair, Author/Editor)
Ed Simon, XMLsec (Author)
David Solo, Citigroup (Author/Editor)
Petteri Stenius, Capslock
Raghavan Srinivas, Sun
Kent Tamura, IBM
Winchel Todd Vincent III, GSU
Carl Wallace, Corsec Security, Inc.
Greg Whitehead, Signio Inc.

As are the Last Call comments from the following:

Dan Connolly, W3C
Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG.
Martin J. Duerst, W3C; and Masahiro Sekiguchi, Fujitsu; on behalf of the Internationalization WG/IG.
Jonathan Marsh, Microsoft, on behalf of the Extensible Stylesheet Language WG.

The following members of the XML Security Specification Maintenance Working Group contributed to the second edition:

Juan Carlos Cruellas, Universitat Politècnica de Catalunya
Pratik Datta, Oracle Corporation
Phillip Hallam-Baker, VeriSign, Inc.
Frederick Hirsch, Nokia, (Chair, Editor)
Konrad Lanz, Applied Information processing and Kommunications (IAIK)
Hal Lockhart, BEA Systems, Inc.
Robert Miller, MITRE Corporation
Sean Mullan, Sun Microsystems, Inc.
Bruce Rich, IBM Corporation
Thomas Roessler, W3C/ERCIM, (Staff contact, Editor)
Ed Simon, W3C Invited Expert
Greg Whitehead, HP

Contributions for version 1.1 were received from the members of the XML Security Working Group:

Editorial note
TBD. See public list of participants for now.

2 Signature Overview and Examples

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

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

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

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

Signatures are related to data objects via "data selectors" which usually consist of a URI [URI] and some attributes.. 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 [XML].

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.

   [s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> 
   [s02]   <SignedInfo> 
   [s03]   <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2009/WD-xml-c14n2-20091022/"/> 
   [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/2008/xmlsec/experimental#newTransformModel">
   [s07a]        <Selection type="http://www.w3.org/2008/xmlsec/experimental#xml" xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s07b]        </Selection>
   [s07c]        <Canonicalization xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s07d]          <Inclusive>false</Inclusive>
   [s07e]        </Canonicalization>
   [s07f]      </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>

[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, and all examples in this specification, are not in canonical form. The URI should be Canonical XML 2.0 [XML-C14N20] (or a later version) and all the parameters for Canonical XML 2.0 should be present as subelements of this element.

[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. Each reference object has two parts - a Selection element to choose the data object to be signed, and a Canonicalization element to convert the data object to a canonicalized octet stream. Signature 2.0 does not use the concept of transforms, but the syntax still uses a Transforms element so that an Signature 1.x implementation can gracefully ignore 2.0 signature, by giving an "unrecognized transform" error message. A data object is signed by computing its digest value of that octet stream and then 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.

2.1.1 More on `Reference`

   [s05]   <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> 
   [s06]     <Transforms> 
   [s07]       <Transform Algorithm="http://www.w3.org/2008/xmlsec/experimental#newTransformModel">
   [s07a]        <Selection type="http://www.w3.org/2008/xmlsec/experimental#xml" xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s07b]        </Selection>
   [s07c]        <Canonicalization xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s07d]          <Inclusive>false</Inclusive>
   [s07e]        </Canonicalization>
   [s07f]      </Transform> 
   [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.)

[s07a-s07c] The Selection element identifies the data object to be signed. This specification only defines two types "xml" and "binary", but user specified types are also allowed. For example a new type "database-rows" can be defined to select rows from the database for signing. Usually a URI and a few other bits of information is 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.

Although the URI attribute is part of the Reference, it can be considered as logically part of the Selection and the URI is dereferenced in the context of the other Selection elements.

[s07d-s07f] The Canonicalization element provides the mechanism to convert the data object into a canonicalized octet stream. This specification only addresses canonicalization for xml data. Other forms of canonicalization can be defined - e.g. a scheme for signing mime attachments, can define a canonicalization for mime headers and data. The output of the canonicalization is digested.

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

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

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

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

   [   ]  <Signature Id="MySecondSignature" ...>
   [p01]  <SignedInfo>  
   [   ]   ...  
   [p02]   <Reference>   
   [   ]   ... 
   [   ]   </Reference>   
   [p03]   <Reference URI="#AMadeUpTimeStamp">  
   [p04]
   [p05]    <Transforms> 
   [s06]      <Transform Algorithm="http://www.w3.org/2008/xmlsec/experimental#newTransformModel">
   [s06a]       <Selection type="http://www.w3.org/2008/xmlsec/experimental#xml" xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s06b]       </Selection>
   [s06c]       <Canonicalization xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s06d]         <Inclusive>false</Inclusive>
   [s06e]       </Canonicalization>
   [p06f]     </Transform> 
   [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>

[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.

   [   ] ...
   [m01]   <Reference URI="#MyFirstManifest"
   [m02]     Type="http://www.w3.org/2000/09/xmldsig#Manifest">
   [m03]     <Transforms> 
   [m04]      <Transform Algorithm="http://www.w3.org/2008/xmlsec/experimental#newTransformModel">
   [s04a]       <Selection type="http://www.w3.org/2008/xmlsec/experimental#xml" xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s04b]       </Selection>
   [s04c]       <Canonicalization xmlns="http://www.w3.org/2008/xmlsec/experimental#">
   [s04d]         <Inclusive>false</Inclusive>
   [s04e]       </Canonicalization>
   [p04f]     </Transform> 
   [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>

3 Processing Rules

The sections below describe the operations to be performed as part of signature generation and validation.

3.1 Core Generation

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

3.1.1 Reference Generation

For each Reference:

Decide how to represent the data object as a Selection.
Use the Canonicalization to convert the data object into an octet stream. This is not required for binary data.
Calculate the digest value over the resulting data object.
Create a Reference element, including the Selection element, Canonicalization element, the digest algorithm and the DigestValue. (Note, it is the canonical form of these references that are signed in 3.1.2 and validated in 3.2.1 .)

XML data objects should be canonicalized using Canonical XML 2.0 [XML-C14N20] or later.

3.1.2 Signature Generation

Create SignedInfo element with SignatureMethod, CanonicalizationMethod and Reference(s).
Canonicalize and then calculate the SignatureValue over SignedInfo based on algorithms specified in SignedInfo.
Construct the Signature element that includes SignedInfo, Object(s) (if desired, encoding may be different than that used for signing), KeyInfo (if required), and SignatureValue.

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

3.2 Core Validation

The REQUIRED steps of core validation include

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

These steps are present in ascending order of complexity, which ensures that the verifier rejects invalid signatures as quickly as possible.

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

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

3.2.1 Selection Validation

Process the Selection of 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 should then compare against its policy to make sure what was expected to be signed is actually signed.

It is very important to check the selection accurately - 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];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.

3.2.2 Signature Validation

Obtain the keying information from 4.5 KeyInfo Element or from an external source.
Canonicalize the SignedInfo element based on the CanonicalizationMethod (which must be C14N 2.0 [XML-C14N20]) in SignedInfo.
Use the algorithm from the SignatureMethod and the key obtained from KeyInfo to verify the SignatureValue over the canonicalized octets.

Note, 4.5 KeyInfo Element (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.

3.2.3 Reference Validation

For each Reference in SignedInfo:
1. Obtain the data object to be digested by looking at the Selection
2. Perform the Canonicalization to compute an octet stream.
3. Digest the resulting octet stream 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 SignedInfoReference; if there is any mismatch, validation fails.

4 Core Signature Syntax

The general structure of an XML signature is described in 2 Signature Overview and Examples (section 2). This section provides detailed syntax of the core signature features. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via an [XML-schema] 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" "http://www.w3.org/2001/XMLSchema.dtd"
     [
      <!ATTLIST schema
        xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
      <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
      <!ENTITY % p ''>
      <!ENTITY % s ''>
     ]>

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

Additional markup defined in version 1.1 of this specification uses the dsig11: namespace, markup defined in version 2.0 used xmldsig2 namespace.

   <?xml version="1.0" encoding="utf-8"?>
   <!DOCTYPE schema
     PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd"
     [
      <!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 CryptoBinary Simple Type

This specification defines the ds:CryptoBinary simple type for representing arbitrary-length integers (e.g. "bignums") in XML as octet strings. The integer value is first converted to a "big endian" bitstring. The bitstring is then padded with leading zero bits so that the total number of bits == 0 mod 8 (so that there are an integral number of octets). If the bitstring contains entire leading octets that are zero, these are removed (so the high-order octet is always non-zero). This octet string is then base64 [MIME] encoded. (The conversion from integer to octet string is equivalent to IEEE 1363's I2OSP [1363] with minimal length).

This type is used by "bignum" values such as RSAKeyValue and DSAKeyValue. If a value can be of type base64Binary or ds:CryptoBinary they are defined as base64Binary. For example, if the signature algorithm is RSA or DSA then SignatureValue represents a bignum and could be ds:CryptoBinary. However, if HMAC-SHA1 is the signature algorithm then SignatureValue could have leading zero octets that must be preserved. Thus SignatureValue is generically defined as of type base64Binary.

   Schema Definition:

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

4.2 `Signature`

The Signature element is the root element of an XML Signature. Implementation MUST generate laxly schema valid [XML-schema] Signature elements as specified by the following schema:

   Schema Definition:

   <element name="Signature" type="ds:SignatureType"/>
   <complexType name="SignatureType">
     <sequence> 
       <element ref="ds:SignedInfo"/> 
       <element ref="ds:SignatureValue"/> 
       <element ref="ds:KeyInfo" minOccurs="0"/> 
       <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/> 
     </sequence>  
     <attribute name="Id" type="ID" use="optional"/>
   </complexType>

4.3 `SignatureValue` Element

The SignatureValue element contains the actual value of the digital signature; it is always encoded using base64 [MIME]. While we identify two SignatureMethod algorithms, one mandatory and one optional to implement, user specified algorithms may be used as well.

   Schema Definition:

   <element name="SignatureValue" type="ds:SignatureValueType"/> 
   <complexType name="SignatureValueType">
     <simpleContent>
       <extension base="base64Binary">
         <attribute name="Id" type="ID" use="optional"/>
       </extension>
     </simpleContent>
   </complexType>

4.4 `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 `CanonicalizationMethod` Element

CanonicalizationMethod is a required element that specifies the canonicalization algorithm applied to the SignedInfo element prior to performing signature calculations. Only Canonical XML 2.0 [XML-C14N20] or a later version is allowed here.

The SignedInfo element is presented as a single subtree with no exclusions to the Canonicalization 2.0 algorithm. All the subelements of Canonicalization are presented as parameters.

   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 `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: 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-209-0217]. Signatures MUST be deemed invalid if the truncation length is below half the underlying hash algorithm's output length, or 80 bits, whichever of these two values is greater. 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 `Reference` Element

Reference is an element that may occur one or more times. It specifies a Selection element to select the data object, an optional Canonicalization element to canonicalize the data object, a digest algorithm and digest value. 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 `Transforms` Element

Signature 2.0 does not use the Transform mechanism at all. Instead each reference has a Selection and an Canonicalization. To allow signature 1.x implementations to gracefully ignore this new model, they have been put inside a special Transform whose Algorithm name is "http://www.w3.org/2008/xmlsec/experimental#newTransformModel" All references should have this and only this special transform.

The Transforms element schema allows a list of Transform elements, but it should contain only this special transform. The schema is left unchanged for backwards compatibility.

   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>

Definition for the special transform

   Schema definition 
   
   <xs:complexType name="XMLSignature2_0Transform">
       <xs:sequence>
           <xs:element name="Selection" type="ds2:SelectionType"/>
           <xs:element name="Canonicalization" type="ds2:CanonicalizationType" minOccurs="0"/>
       </xs:sequence>
   </xs:complexType>
   <xs:complexType name="SelectionType">
       <xs:sequence>
           <xs:element name="IncludedXPath" type="xs:string" minOccurs="0"/>
           <xs:element name="ExcludedXPath" type="xs:string" minOccurs="0"/>
           <xs:element name="EnvelopedSignature" type="xs:boolean" minOccurs="0"/>
           <xs:element name="ByteRange" type="xs:string" minOccurs="0"/>
           <xs:any namespace="##any" minOccurs="0"/>
       </xs:sequence>
       <xs:attribute name="type" type="xs:string" use="required"/>
       <xs:attribute name="subtype" type="xs:string" use="optional"/>
       <xs:anyAttribute namespace="##any"/>
   </xs:complexType>
   <xs:complexType name="CanonicalizationType">
       <xs:sequence>
           <xs:element name="ExclusiveMode" type="xs:boolean" minOccurs="0"/>
           <xs:element name="InclusiveNamespacePrefixList" minOccurs="0">
               <xs:simpleType>
                   <xs:list itemType="xs:string"/>
               </xs:simpleType>
           </xs:element>
           <xs:element name="IgnoreComments" type="xs:boolean" minOccurs="0"/>
           <xs:element name="TrimTextNodes " type="xs:boolean" minOccurs="0"/>
           <xs:element name="Serialization" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="XML"/>
                       <xs:enumeration value="EXI"/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="PrefixRewrite" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="none"/>
                       <xs:enumeration value="sequential"/>
                       <xs:enumeration value="derived"/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="SortAttributes" type="xs:boolean" minOccurs="0"/>
           <xs:element name="IgnoreDTD" type="xs:boolean" minOccurs="0"/>
           <xs:element name="ExpandEntities" type="xs:boolean" minOccurs="0"/>
           <xs:element name="XmlBaseAncestors" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="none"/>
                       <xs:enumeration value="inherit"/>
                       <xs:enumeration value="combine"/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="XmlIdAncestors" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="none"/>
                       <xs:enumeration value="inherit"/>
                       <xs:enumeration value=""/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="XmlLangAncestors" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="none"/>
                       <xs:enumeration value="inherit"/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="XmlSpaceAncestors" minOccurs="0">
               <xs:simpleType>
                   <xs:restriction base="xs:string">
                       <xs:enumeration value="none"/>
                       <xs:enumeration value="inherit"/>
                   </xs:restriction>
               </xs:simpleType>
           </xs:element>
           <xs:element name="XsiTypeAware" type="xs:boolean" minOccurs="0"/>
           <xs:any namespace="##any" minOccurs="0"/>
       </xs:sequence>
       <xs:attribute name="algorithm" type="xs:string" use="required"/>
   </xs:complexType>

Example usage

   <Transform Algorithm="http://www.w3.org/2008/xmlsec/experimental#newTransformModel">
     <Selection type="..." subtype="...">
        <IncludedXPath>...</IncludedXPath>
        <ExcludedXPath>...</ExcludedXPath>
        <EnvelopedSignature>...</EnvelopedSignature>
        ...
     </Selection>
     <Canonicalization Algorithm="...c14n20" >
        <Inclusive;>...</Inclusive>
        ...
     </Canonicalization>
     
   </Transform>

4.4.3.2 `Selection` element

The Selection element chooses the data object that is to be signed. The type and subtype attributes specifies what kind of data is being signed. type can be "http://www.w3.org/2008/xmlsec/experimental#xml" or "http://www.w3.org/2008/xmlsec/experimental#binary" or any other user defined value.

type and subtype Parameters Meaning

type and subtype	Parameters	Meaning
`type="...xml"`	`URI`, `IncludedXPath`, `ExcludedXPath`, `EnvelopedSignature`	Used for identifying XML data objects. Selects a set of subtrees, with complete subtree exclusions. This selection model is compatible with the Canonical XML 2.0 model. For example `URI="#chapter1"`indicates that complete subtree identified by the ID "chapter1", in current document is being signed. `IncludedXPath="/book/chapter"` indicates that all the subtrees indicated by "/book/chapter" in the current document are signed `URI="#chapter1"` and `ExcludedXPath="price"` indicates the subtree identified by the ID "chapter1" minus any subtrees with "price" element are being signed. `URI="http://example.com/bar.xml"` indicates that the entire external document bar.xml is signed `URI="http://example.com/bar.xml"` and `IncludedXPath="/book/chapter"`indicates that the /book/chapter subtrees of the external document bar.xml are signed. `URI="http://example.com/bar.xml#chapter1"` and `ExcludedXPath="price"`indicates that the subtree identifiedby ID "chapter1" in the external document bar.xml is signed, but from that subtree any subtrees with "price" element are removed. Note: it is illegal to use a URI with a fragment identifier in conjunction with IncludedXPath. If the EnvelopedSignature parameter is set, the subtree for this signature should be excluded.
`type = "...binary"` and `subtype = "...fromURI"`	`URI`, `ByteRange`	This indicates that binary data directly fetched from an external URI is signed. The URI should be an external URI without any fragment ID. The optional byte range parameter can be used to indicate that only a portion of the binary data should be signed. E.g. `byteRange="0-20,220-270,320-"` indicates that the first 20 bytes, then bytes 220 to 270, and finally bytes 320 to end of file are included.
`type = "...binary"` and `subtype = "...fromBase64Node"`	`URI`, `IncludedXPath`, `ByteRange`	This indicates that binary data which is present in the XML as a base64 text node is being signed. Just like the `type="...xml"` an combination of URI and IncludedXPath attributes can be used to identify an element have text node children. These text nodes will be coalesced and then base64 decoded, to get the binary data. This is similar to Base64Decode transform, however Base64Decode transform works with nodeset containing multiple element nodes, but this one is only defined for a single element node. The optional byte range parameter can be used to indicate that only a portion of the binary data should be signed.

type="...xml"

URI,

IncludedXPath,

ExcludedXPath,

EnvelopedSignature

Used for identifying XML data objects. Selects a set of subtrees, with complete subtree exclusions. This selection model is compatible with the Canonical XML 2.0 model. For example

URI="#chapter1"indicates that complete subtree identified by the ID "chapter1", in current document is being signed.
IncludedXPath="/book/chapter" indicates that all the subtrees indicated by "/book/chapter" in the current document are signed
URI="#chapter1" and ExcludedXPath="price" indicates the subtree identified by the ID "chapter1" minus any subtrees with "price" element are being signed.
URI="http://example.com/bar.xml" indicates that the entire external document bar.xml is signed
URI="http://example.com/bar.xml" and IncludedXPath="/book/chapter"indicates that the /book/chapter subtrees of the external document bar.xml are signed.
URI="http://example.com/bar.xml#chapter1" and ExcludedXPath="price"indicates that the subtree identifiedby ID "chapter1" in the external document bar.xml is signed, but from that subtree any subtrees with "price" element are removed.

Note: it is illegal to use a URI with a fragment identifier in conjunction with IncludedXPath.

If the EnvelopedSignature parameter is set, the subtree for this signature should be excluded.

type = "...binary" and subtype = "...fromURI"

URI,

ByteRange

This indicates that binary data directly fetched from an external URI is signed. The URI should be an external URI without any fragment ID.

The optional byte range parameter can be used to indicate that only a portion of the binary data should be signed. E.g. byteRange="0-20,220-270,320-" indicates that the first 20 bytes, then bytes 220 to 270, and finally bytes 320 to end of file are included.

type = "...binary" and subtype = "...fromBase64Node"

URI,

IncludedXPath,

ByteRange

This indicates that binary data which is present in the XML as a base64 text node is being signed. Just like the type="...xml" an combination of URI and IncludedXPath attributes can be used to identify an element have text node children. These text nodes will be coalesced and then base64 decoded, to get the binary data. This is similar to Base64Decode transform, however Base64Decode transform works with nodeset containing multiple element nodes, but this one is only defined for a single element node.

The optional byte range parameter can be used to indicate that only a portion of the binary data should be signed.

4.4.3.3 `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 [XML-schema]. 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 [HTTP] (e.g., 302, 305 and 307 redirects are followed to obtain the entity-body of a 200 status code response). Applications should also be cognizant of the fact that protocol parameter and state information, (such as HTTP cookies, HTML device profiles or content negotiation), may affect the content yielded by dereferencing a URI.

If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease). (See the 3.2 Core Validation (section 3.2.1) for a further information on reference processing.)

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 .

XML Signature applications MUST support the null URI and shortname XPointer [XPointer-Framework] (e.g. URI="#foo"). We RECOMMEND support for the same-document XPointers '#xpointer(/)' and '#xpointer(id('ID')) as this was RECOMMENDED in the 1.x specification. All other support for XPointers was OPTIONAL in 1.x, but is dropped from 2.0, because it is not compatible with the new Reference processing model. The IncludedXPath can be used instead.

4.4.3.4 Same-Document URI-References

Dereferencing a same-document reference MUST result in a subtree. Dereferencing a null URI (URI="") MUST result in subtree pointing to the document root. In a fragment URI, the characters after the number sign ('#') character conform to the XPointer syntax [XPointer-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 [XPointer-Framework] were a subtree rooted at the element node pointed to by the XPointer.

4.4.3.5 Subset of XPath for performance

The XPath mentioned in the IncludedXPath and ExcludedXPath are "normal" XPath, i.e. it is not like the XPath in XPath Filter transform which is evaluated as a binary expression. Instead this XPath is a path to the root of the subtree being included or excluded. E.g. /book/chapter refers to the all chapter children of all book children of root node. The IncludedXPath element should only select Element nodes, whereas the ExcludedXPath element can choose element or attribute nodes. Again this is consistent with the C14N 2.0 data model.

We have identified a profile of XPath, with the following goals in mind.

The profile should produce results that are compatible with the C14N 2.0 data model. I.e it should only result in element nodes or attribute nodes (but not xml: attribute and namespace attributes).
It should possible to execute this XPath in a streamable XPath implementation. The capabilities of various streamable XPath implementations vary. Some XPaths cans be executed by even the most rudimentary streaming XPath implementations e.g. /book/chapter, whereas some can be executed by none of them e.g. child::para[position()=last()-1]. Streaming XPath implementation by definition do a forward only pass over the document, so knowing that a particular element's position is last but one, requires it to reach the end and then backtrack.

Streaming parsers read the document one "event" at a time. Usually the entire element tag including all the attributes are read in a single event, however text nodes can are often split up into separate events because some text nodes can be very large. So any XPath that requires comparison of text node value may not work in streaming XPath implementations.
This XPath subset should include some of the known usages of XPath in XML Signatures.
1. ebXML messages require a signature to exclude elements whose @SOAP:actor attribute matches a certain value. Refer section 4.1.3 of [ebXML-Msg]
2. UK government specification requires signature to include the GovTalkMessage/Body subtree, but exclude the GovTalkMessage/Body/IRevenvelope/IRHeader/IRmark subtree. [HMRMC]

This subset can be expressed precisely using the following grammar, which is still under development.

Grammar	Explanation
[0a] IncludedXPath ::= ( LocationPath '\|' )* LocationPath [0b] ExcludedXPath ::= ( LocationPath '\|' )* LocationPath	The Included and Excluded Xpath do not use the generic XPath Expr. Instead they are just a union of LocationPath. There is a slight difference between IncludedXPath and ExcludedXPath, ExcludedXpath can select attributes and element, whereas IncludedXPath can only select elements.
[1] LocationPath ::= ~~RelativeLocationPath \|~~ AbsoluteLocationPath [2] AbsoluteLocationPath ::= '/' RelativeLocationPath? \| AbbreviatedAbsoluteLocationPath [3] RelativeLocationPath ::= ~~Step \| RelativeLocationPath '/' Step~~ ( StepNoPredicate '/')* Step \| AbbreviatedRelativeLocationPath	RelativeLocationPath is not allowed, only Absolute is allowed. This is because if you use relative, it would probably mean relative to the <Signature> element, and then you would have to support the ancestor axis to reach the other nodes, but that axis is not streamable. Only the last Step can have a predicate. So I have created a non-terminal "StepNoPredicate"
[4] Step ::= AxisSpecifier NodeTest Predicate* \| AbbreviatedStep [4a] StepNoPredicate ::= AxisSpecifier NodeTest \| AbbreviatedStep [4b] StepAttributeOnly ::= 'attribute' '::' NameTest \| '@' '::' NameTest	Added two new versions of Step. One is a Step with no Predicate, and the other is a step attribute only e.g. in this XPath expression: `/doc/chapter[@type="warning"]` `/doc` is StepNoPredicate `chapter[@type="warning"]` is Step `@type` is StepAttributeOnly
[5] AxisSpecifier ::= AxisName '::' \| AbbreviatedAxisSpecifier	unchanged
[6] AxisName ::= ~~'ancestor'~~ ~~\| 'ancestor-or-self'~~ \| 'attribute' \| 'child' \| 'descendant' \| 'descendant-or-self' ~~\| 'following'~~ ~~\| 'following-sibling'~~ ~~\| 'namespace'~~ ~~\| 'parent'~~ ~~\| 'preceding'~~ ~~\| 'preceding-sibling'~~ \| 'self'	All the non streamable axes have been removed - ancestor, ancestor-or-self, following, following-sibling, namespace, parent, preceding, preceding-sibling
[7] NodeTest ::= NameTest \| NodeType '(' ')' ~~\| 'processing-instruction' '(' Literal ')'~~	processing instruction test is not allowed. only the `node()` node test is allowed, not `comment()`, `text()` and `processing-instruction()`
[8] Predicate ::= '[' PredicateExpr ']' [9] PredicateExpr ::= Expr	unchanged but the definition of Expr has changed, so it is only a additive/relative expressions of StepAttributeOnly and Literals.
[10] AbbreviatedAbsoluteLocationPath ::= '//' RelativeLocationPath [11] AbbreviatedRelativeLocationPath ::= RelativeLocationPath '//' Step [12] AbbreviatedStep ::= '.' \| '..' [13] AbbreviatedAxisSpecifier ::= '@'?	unchanged
[14] Expr ::= OrExpr [15] PrimaryExpr ::= VariableReference \| '(' Expr ')' \| Literal \| Number \| FunctionCall	unchanged
[16] FunctionCall ::= FunctionName '(' ( Argument ( ',' Argument )* )? ')' [17] Argument ::= Expr	unchanged
~~[18] UnionExpr ::=~~ ~~PathExpr \| UnionExpr '\|' PathExpr~~ ~~[19] PathExpr ::=~~ ~~LocationPath~~ ~~\| FilterExpr~~ ~~\| FilterExpr '/' RelativeLocationPath~~ ~~\| FilterExpr '//' RelativeLocationPath~~ ~~[20] FilterExpr ::= PrimaryExpr~~ ~~\| FilterExpr Predicate~~	UnionExpr, PathExpr and FilterExpr have been removed.
[21] OrExpr ::= AndExpr \| OrExpr 'or' AndExpr [22] AndExpr ::= EqualityExpr \| AndExpr 'and' EqualityExpr [23] EqualityExpr ::= RelationalExpr \| EqualityExpr '=' RelationalExpr \| EqualityExpr '!=' RelationalExpr [24] RelationalExpr ::= AdditiveExpr \| RelationalExpr '<' AdditiveExpr \| RelationalExpr '>' AdditiveExpr \| RelationalExpr '<=' AdditiveExpr \| RelationalExpr '>=' AdditiveExpr	unchanged
[25] AdditiveExpr ::= MultiplicativeExpr \| AdditiveExpr '+' MultiplicativeExpr \| AdditiveExpr '-' MultiplicativeExpr [26] MultiplicativeExpr ::= UnaryExpr \| MultiplicativeExpr MultiplyOperator UnaryExpr \| MultiplicativeExpr 'div' UnaryExpr \| MultiplicativeExpr 'mod' UnaryExpr [27] UnaryExpr ::= ~~UnionExpr~~ PrimaryExpr \| StepAttributeOnly \| '-' UnaryExpr	The unaryExpr is changed to only allow a PrimaryExpr or StepAttributeOnly
[28] ExprToken ::= '(' \| ')' \| '[' \| ']' \| '.' \| '..' \| '@' \| ',' \| '::' \| NameTest \| NodeType \| Operator \| FunctionName \| AxisName \| Literal \| Number \| VariableReference [29] Literal ::= '"' [^"]* '"' \| "'" [^']* "'" [30] Number ::= Digits ('.' Digits?)? \| '.' Digits [31] Digits ::= [0-9]+ [32] Operator ::= OperatorName \| MultiplyOperator \| '/' \| '//' \| '\|' \| '+' \| '-' \| '=' \| '!=' \| '<' \| '<=' \| '>' \| '>=' [33] OperatorName ::= 'and' \| 'or' \| 'mod' \| 'div' [34] MultiplyOperator ::= '' [35] FunctionName ::= QName - NodeType [36] VariableReference ::= '$' QName [37] NameTest ::= '' \| NCName ':' '*' \| QName [38] NodeType ::= ~~'comment'~~ ~~\| 'text'~~ ~~\| 'processing-instruction'~~ \| 'node' [39] ExprWhitespace ::= S	unchanged, expect for the NodeTest
Node set functions ~~last()~~ ~~position()~~ ~~count(nodeset)~~ ~~id()~~ local-name(nodeset) namespace-uri(nodeset) name(nodeset) String functions string(object) concat(string, string, string*) starts-with(string, string) contains(string, string) substring-before(string, string) substring-after(string, string) substring(string, number, number) string-length(string?) normalize-space(string?) Boolean functions boolean(object) true() false() lang(string) Number functions number(object?) sum(node-set) floor(number) ceiling(number) round(number)	Note: As mentioned before, only the last Step can have a Predicate, and this predicate's expression can only involve attribute nodes of the current element. Functions can only be used inside this last step's predicate, and this function can only accept a single attribute as an argument. There is no way to use element names, text nodes, comments and processing instructions in functions. The "string-value" become just the attributes value. All functions involving context position and context size are not supported i.e.. last, position, count or their shortcut versions e.g. foo[1]. the streaming parser cannot maintain counts. String, number and boolean functions are all supported.

Here is an algorithm for Streaming XPath. For simplicity this algorithm assumes that excludedXPath is not present:

For parsing:

Split up the union expression by "|". i.e. break up the locationPath | locationPath | .. into individual location paths.
Split up each location paths to get individual steps and the final predicate. i.e. break up the / step / step / step .. / step [ predicate ] to get the steps and optional predicate. Two slashes together indicates descendant axis.
The predicate will have an expression involving attribute names e.g. @a = "foo" and @b > "bar" You need to have an expression parsing and evaluating engine to do this.

For executing:

A streaming XML Parser (e.g. StAX), reads an XML document and produces "events" like StartElement, EndElement, TextNode etc. At any point this parser only remembers the current node. If the current node is an start element, then it also reads all the attributes for that element. To execute a streaming XPath you maintain a stack of ancestor element names, i.e whenever you get a StartElement tag, you need to push the element QName onto this stack, and when you get an EndElement tag you need to pop it off.
As you stream through the nodes, you need to execute this XPath expression for every node. I.e. utilize the current element, the current element's attributes and the stack of ancestors to evaluate the XPath expression.

For each locationPath, match up the steps to the ancestor stack, If they match, evaluate the predicate with the current element's expression. If that passes too, this element and all its descendants are included.

4.4.3.6 Reference Processing Model

The processing of these Selection depends on the type and subtype attributes.

For type="...xml"
1. If the URI is a same document reference, compute the subtree pointed to by this reference. If it is an external reference, fetch the document and use an xml parser to parse it into a complete document tree
2. If present, evaluate the IncludedXPath with the context set at the root of subtree, or the root of the complete document. The result of this will be a list of included subtrees.
3. Similarly evaluate the ExcludedXPath to get a list of excluded subtrees or excluded attributes
4. If the EnvelopedSignature is "true", set add the current signature subtree to the list of excluded subtrees.
5. Get the canonicalization parameters from the Canonicalization element.
6. Send this list of included subtrees, excluded subtrees, excluded attributes, and canonicalization parameter for Canonical XML 2.0 processing. The result of canonicalization will be an octet stream
7. Compute the digest of the octets.
For type = "...binary" and subtype = "...fromURI"
1. The URI is expected to be an external reference. Fetch the document as an octet stream.
2. If there is a ByteRange parameter, create a new octet stream with a subset of the bytes fetched. Alternatively this can be combined with the first step to intelligently fetch only the bytes in the byte range.
3. Compute the digest of the octets.
For type = "...binary" and subtype = "...fromBase64Node"
1. If the URI is a same document reference, compute the subtree pointed to by this reference. If it is an external reference, fetch the document and use an xml parser to parse it into a complete document tree
2. If present, evaluate the IncludedXPath with the context set at the root of subtree, or the root of the complete document. The result of this should be a single subtree.
3. At this point the subtree should consist of a single element, with only text node children.
4. Coalesce all the text node values into one large string and Base64 Decode that string to obtain an octet stream
5. If there is a ByteRange parameter, create a new octet stream with a subset of the bytes decoded.
6. Compute the digest of the octets.

4.4.3.7 `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 6.1 Algorithm Identifiers and Implementation Requirements (section 6.1).

The digest algorithm is applied to the data octets of the octet stream that results from canonicalization of the data object in the Reference. Note if the data object is an octet stream then canonicalization is optional, in which case the data object is directly digested.

   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>

4.4.3.8 `DigestValue`Element

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

   Schema Definition:

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

4.5 `KeyInfo` Element

This section will be derived from XML Signature 1.1 but has not yet been added. Please refer to XML Signature 1.1 [XMLDSIG11]. Note the Transforms inside the RetrievalMethod is deprecated.

4.6 `Object` Element

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

Object is an optional element that may occur one or more times. When present, this element may contain any data. The Object element may include optional MIME type, ID, and encoding attributes.

The Object's Encoding attributed may be used to provide a URI that identifies the method by which the object is encoded (e.g., a binary file).

The MimeType attribute is an optional attribute which describes the data within the Object (independent of its encoding). This is a string with values defined by [MIME]. For example, if the Object contains base64 encoded PNG, the Encoding may be specified as 'http://www.w3.org/2000/09/xmldsig#base64' and the MimeType as 'image/png'. This attribute is purely advisory; no validation of the MimeType information is required by this specification. Applications which require normative type and encoding information for signature validation should specify 4.4.3.1 Transforms Element with well defined resulting types and/or encodings.

The Object's Id is commonly referenced from a Reference in SignedInfo, or Manifest. This element is typically used for enveloping signatures where the object being signed is to be included in the signature element. The digest is calculated over the entire Object element including start and end tags.

Note, if the application wishes to exclude the <Object> tags from the digest calculation the Reference must identify the actual data object (easy for XML documents) or a transform must be used to remove the Object tags (likely where the data object is non-XML). Exclusion of the object tags may be desired for cases where one wants the signature to remain valid if the data object is moved from inside a signature to outside the signature (or vice versa), or where the content of the Object is an encoding of an original binary document and it is desired to extract and decode so as to sign the original bitwise representation.

   Schema Definition:

   <element name="Object" type="ds:ObjectType"/> 
   <complexType name="ObjectType" mixed="true">
     <sequence minOccurs="0" maxOccurs="unbounded">
       <any namespace="##any" processContents="lax"/>
     </sequence>
     <attribute name="Id" type="ID" use="optional"/> 
     <attribute name="MimeType" type="string" use="optional"/>
     <attribute name="Encoding" type="anyURI" use="optional"/> 
   </complexType>

5 Additional Signature Syntax

This section describes the optional to implement Manifest and SignatureProperties elements and describes the handling of XML processing instructions and comments. With respect to the elements Manifest and SignatureProperties this section specifies syntax and little behavior -- it is left to the application. These elements can appear anywhere the parent's content model permits; the Signature content model only permits them within Object.

5.1 `Manifest`Element

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

The Manifest element provides a list of References. The difference from the list in SignedInfo is that it is application defined which, if any, of the digests are actually checked against the objects referenced and what to do if the object is inaccessible or the digest compare fails. If a Manifest is pointed to from SignedInfo, the digest over the Manifest itself will be checked by the core signature validation behavior. The digests within such a Manifest are checked at the application's discretion. If a Manifest is referenced from another Manifest, even the overall digest of this two level deep Manifest might not be checked.

   Schema Definition:

   <element name="Manifest" type="ds:ManifestType"/> 
   <complexType name="ManifestType">
     <sequence>
       <element ref="ds:Reference" maxOccurs="unbounded"/> 
     </sequence>  
     <attribute name="Id" type="ID" use="optional"/> 
   </complexType>

5.2 The `SignatureProperties` Element

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

Additional information items concerning the generation of the signature(s) can be placed in a SignatureProperty element (i.e., date/time stamp or the serial number of cryptographic hardware used in signature generation).

   Schema Definition:

   <element name="SignatureProperties" type="ds:SignaturePropertiesType"/> 
   <complexType name="SignaturePropertiesType">
     <sequence>
       <element ref="ds:SignatureProperty" maxOccurs="unbounded"/> 
     </sequence>
     <attribute name="Id" type="ID" use="optional"/> 
   </complexType>

      <element name="SignatureProperty" type="ds:SignaturePropertyType"/> 
      <complexType name="SignaturePropertyType" mixed="true">
        <choice maxOccurs="unbounded">
          <any namespace="##other" processContents="lax"/>
          <!-- (1,1) elements from (1,unbounded) namespaces -->
        </choice>
        <attribute name="Target" type="anyURI" use="required"/> 
        <attribute name="Id" type="ID" use="optional"/> 
      </complexType>

5.3 Processing Instructions in Signature Elements

No XML processing instructions (PIs) are used by this specification.

Note that PIs placed inside SignedInfo by an application will be signed unless the CanonicalizationMethod algorithm discards them. (This is true for any signed XML content.) All of the CanonicalizationMethods identified within this specification retain PIs. When a PI is part of content that is signed (e.g., within SignedInfo or referenced XML documents) any change to the PI will obviously result in a signature failure.

5.4 Comments in Signature Elements

XML comments are not used by this specification.

Note that unless CanonicalizationMethod removes comments within SignedInfo or any other referenced XML (which [XML-C14N] does), they will be signed. Consequently, if they are retained, a change to the comment will cause a signature failure. Similarly, the XML signature over any XML data will be sensitive to comment changes unless a comment-ignoring canonicalization/transform method, such as the Canonical XML [XML-C14N], is specified.

6 Algorithms

This section will be derived from XML Signature 1.1 but has not yet been added. Please refer to XML Signature 1.1 [XMLDSIG11].

6.1 Algorithm Identifiers and Implementation Requirements

6.2 Message Digests

6.3 Message Authentication Codes

6.4 Canonicalization Algorithms

7 XML Canonicalization and Syntax Constraint Considerations

Digital signatures only work if the verification calculations are performed on exactly the same bits as the signing calculations. If the surface representation of the signed data can change between signing and verification, then some way to standardize the changeable aspect must be used before signing and verification. For example, even for simple ASCII text there are at least three widely used line ending sequences. If it is possible for signed text to be modified from one line ending convention to another between the time of signing and signature verification, then the line endings need to be canonicalized to a standard form before signing and verification or the signatures will break.

XML is subject to surface representation changes and to processing which discards some surface information. For this reason, XML digital signatures have a provision for indicating canonicalization methods in the signature so that a verifier can use the same canonicalization as the signer.

Throughout this specification we distinguish between the canonicalization of a Signature element and other signed XML data objects. It is possible for an isolated XML document to be treated as if it were binary data so that no changes can occur. In that case, the digest of the document will not change and it need not be canonicalized if it is signed and verified as such. However, XML that is read and processed using standard XML parsing and processing techniques is frequently changed such that some of its surface representation information is lost or modified. In particular, this will occur in many cases for the Signature and enclosed SignedInfo elements since they, and possibly an encompassing XML document, will be processed as XML.

Similarly, these considerations apply to Manifest, Object, and SignatureProperties elements if those elements have been digested, their DigestValue is to be checked, and they are being processed as XML.

The kinds of changes in XML that may need to be canonicalized can be divided into four categories. There are those related to the basic [XML], as described in 7.1 XML 1.0, Syntax Constraints, and Canonicalization below. There are those related to [DOM], [SAX], or similar processing as described in 7.2 DOM/SAX Processing and Canonicalization below. Third, there is the possibility of coded character set conversion, such as between UTF-8 and UTF-16, both of which all [XML] compliant processors are required to support, which is described in the paragraph immediately below. And, fourth, there are changes that related to namespace declaration and XML namespace attribute context as described in 7.3 Namespace Context and Portable Signatures below.

Any canonicalization algorithm should yield output in a specific fixed coded character set. All canonicalization 6.4 Canonicalization Algorithms identified in this document use UTF-8 (without a byte order mark (BOM)) and do not provide character normalization. We RECOMMEND that signature applications create XML content (Signature elements and their descendants/content) in Normalization Form C [NFC], [NFC-Corrigendum] and check that any XML being consumed is in that form as well; (if not, signatures may consequently fail to validate). Additionally, none of these algorithms provide data type normalization. Applications that normalize data types in varying formats (e.g., (true, false) or (1,0)) may not be able to validate each other's signatures.

7.1 XML 1.0, Syntax Constraints, and Canonicalization

XML 1.0 [XML] defines an interface where a conformant application reading XML is given certain information from that XML and not other information. In particular,

line endings are normalized to the single character #xA by dropping #xD characters if they are immediately followed by a #xA and replacing them with #xA in all other cases,
missing attributes declared to have default values are provided to the application as if present with the default value,
character references are replaced with the corresponding character,
entity references are replaced with the corresponding declared entity,
attribute values are normalized by
1. replacing character and entity references as above,
2. replacing occurrences of #x9, #xA, and #xD with #x20 (space) except that the sequence #xD#xA is replaced by a single space, and
3. if the attribute is not declared to be CDATA, stripping all leading and trailing spaces and replacing all interior runs of spaces with a single space.

Note that items (2), (4), and (5.3) depend on the presence of a schema, DTD or similar declarations. The Signature element type is laxly schema valid [XML-schema], consequently external XML or even XML within the same document as the signature may be (only) well-formed or from another namespace (where permitted by the signature schema); the noted items may not be present. Thus, a signature with such content will only be verifiable by other signature applications if the following syntax constraints are observed when generating any signed material including the SignedInfo element:

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

7.2 DOM/SAX Processing and Canonicalization

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

If an XML Signature is to be produced or verified on a system using the DOM or SAX processing, a canonical method is needed to serialize the relevant part of a DOM tree or sequence of SAX events. XML canonicalization specifications, such as [XML-C14N], are based only on information which is preserved by DOM and SAX. For an XML Signature to be verifiable by an implementation using DOM or SAX, not only must the 7.1 XML 1.0, Syntax Constraints, and Canonicalization be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octet stream that was signed.

7.3 Namespace Context and Portable Signatures

In [XPath] and consequently the Canonical XML data model an element has namespace nodes that correspond to those declarations within the element and its ancestors:

An element E has namespace nodes that represent its namespace declarations as well as

any namespace declarations made by its ancestors that have not been overridden in E's declarations,
the default namespace if it is non-empty, and
the declaration of the prefix xml.

[XML-C14N]

When serializing a Signature element or signed XML data that's the child of other elements using these data models, that Signature element and its children, may contain namespace declarations from its ancestor context. In addition, the Canonical XML and Canonical XML with Comments algorithms import all xml namespace attributes (such as xml:lang) from the nearest ancestor in which they are declared to the apex node of canonicalized XML unless they are already declared at that node. This may frustrate the intent of the signer to create a signature in one context which remains valid in another. For example, given a signature which is a child of B and a grandchild of A:

   <A xmlns:n1="&foo;">
     <B xmlns:n2="&bar;">
       <Signature xmlns="&dsig;">   ...
         <Reference URI="#signme"/> ...
       </Signature>
       <C ID="signme" xmlns="&baz;"/>
     </B>
   </A>

when either the element B or the signed element C is moved into a [SOAP] envelope for transport:

   <SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
     ...
     <SOAP:Body>
       <B xmlns:n2="&bar;">
         <Signature xmlns="&dsig;">
           ...
         </Signature>
         <C ID="signme" xmlns="&baz;"/>
       </B>
     </SOAP:Body>
   </SOAP:Envelope>

The canonical form of the signature in this context will contain new namespace declarations from the SOAP:Envelope context, invalidating the signature. Also, the canonical form will lack namespace declarations it may have originally had from element A's context, also invalidating the signature. To avoid these problems, the application may:

Rely upon the enveloping application to properly divorce its body (the signature payload) from the context (the envelope) before the signature is validated. Or,
Use a canonicalization method that "repels/excludes" instead of "attracts" ancestor context. [XML-C14N] purposefully attracts such context.

8 Schema

XML Signature Schema Instance

xmldsig-core-schema.xsd

Valid XML schema instance based on [XML-schema].

XML Signature 1.1 Schema Instance

xmldsig11-schema.xsd

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

9 Security Considerations

9.1 Check the Security Model

10 Differences from 1.x version

The main change in Signature 2.0 is the replacement of the Transform model of Signature 1.x. The 1.x transform model is very general and open-ended, which complicates implementation and allows for misuse, leading to performance and security difficulties. Instead it has been replaced by a more declarative syntax, that clearly separates selection and canonicalization.

Although this approach takes away a the flexibility that was possible with Transforms, loss of flexibility is actually a good thing for reducing the attack surface. Often signature verification is performed on completely untrusted messages, and these message might contain attacks exploiting the expressiveness of transforms. The verifier needs to be able to reject these attack messages very quickly. The new declarative way of expressing the selection allows does not require the verifier to run any processing on the message to determine what was signed, thereby reducing the likelihood of denial of service attacks.

But these modifications are done in such a way that the older Transform model is still allowed - it is just deprecated. It is expected that implementations support both models for some time, and then gradually migrate to the new model. Many 1.0 signatures can be expressed in the 2.0 syntax, so a new implementation which supports only 2.0 style signatures, can also process 1.x signatures by converting them into the 2.0 syntax. The reverse is also true, many 2.0 signatures can be expressed in 1.0 syntax as well.

1.3 Versions, Namespaces and Identifiers :

Introduces new namespace for dsig2:.

2 Signature Overview and Examples :

In Signature 1.x, data objects were referenced by URI, but in 2.x they are referenced by "data selector" which is a combination of URI and other properties.

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

The simple example has been modified to show the <Selection> and <Canonicalization> objects.
The Algorithm specified in <CanonicalizationMethod> must be C14N 2.0 or later.

2.1.1 More on Reference :

This example has also been modified to show the <Selection> and <Canonicalization> elements.
The explanation mentions that this spec only defines two types of data "xml" and "binary", but it allows user specified types e.g. "database-rows".

2.2 Extended Example (Object and SignatureProperty) :

This example has also been modified to show the <Selection> and <Canonicalization> elements.
In Signature 1.x 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 is not used any more in Signature 2.0. Instead the type attribute in Selection provides information about the resource identified by the URI. However this is a different kind of information, Selection@type doesn't distinguish between Object, Manifest and SignatureProperty, instead it distinguishes between xml and binary content.

2.3 Extended Example (Object and Manifest) :

This example has also been modified to show the <Selection> and <Canonicalization> elements.

3.1.1 Reference Generation :

Unlike Signature 1.x, there is no default canonicalization algorithm, it needs to be explicitly specified.

3.2 Core Validation :

The code validation steps have been rearranged in order of execution complexity to ensure that the verifier rejects invalid signatures as quickly as possible, which would minimize denial of service attacks. Specifically Signature 1.x has Reference Validation was the first step, but this a very expensive step and it can potentially have many attack points, so in 2.0 it has been moved to the last step.

3.2.1 Selection Validation :

Added this new kind of validation "Selection Validation" as a distinct step to check what the signature is actually signing

3.2.2 Signature Validation :

Signature 1.x asked to convert the SignatureMethod to canonical form, but this is removed in 2.0, because canonicalization to bytes and then parsing those bytes back to xml are expensive operations. Canonicalization 2.0 should not change any URIs, so there is no need to perform this expensive step.
In Signature 1.x canonicalization of SignedInfo was part of the Reference Validation, but in 2.x it has been moved to Signature validation.

3.2.3 Reference Validation :

Does not have execution of Transforms any more. That is replaced by Selection and Canonicalization steps.

4.4.1 CanonicalizationMethod Element :

Unlike Signature 1.x, other alternative canonicalization algorithms are not allowed. Canonical XML 2.0 must be used.
Security concerns about other canonicalization algorithms rewriting the URIs or massively transforming the SignedInfo are not applicable any more. Text based canonicalization algorithms are not allowed either.
Since there is only a fixed Canonicalization algorithm, the way in which the SignedInfo is presented to the Canonicalization algorithm is clearly defined. Canonical XML 2.0 expects subtrees and the whole SignedInfo goes in as a subtree.

4.4.3 Reference Element :

Reference does not have a list of Transforms, instead has a Selection and Canonicalization element.
Reference does not use use the Type attribute, although it is still present in the schema for backwards compatibility.

4.4.3.2 Selection element :

The new Selection element.

4.4.3.3 URI Attribute :

The Type attribute of Reference is no longer used. Instead there is a type attribute in Selection. In 1.x. "The Type attribute applies to the item being pointed at, not its contents.", but the Selection@type attribute applies to the contents, not to the item.
Apart from #xpointer(/)' and '#xpointer(id('ID'))' all other forms of Xpointer are disallowed in 2.0. In 1.x they were "discouraged".

4.4.3.4 Same-Document URI-References :

Dereferencing a same document reference doesn't result in a nodeset, but in a subtree.
In 1.x there was a difference between URI="" and URI='#xpointer(/)', the latter did not strip out comments. In 2.0 this distinction is gone - both of the them produce the same result - the complete document, which includes all comments
Similarly URI="#foo" and URI="#xpointer(id('foo'))" produce exactly the same results - a subtree rooted at the element with ID foo, including all comments.

4.4.3.1 Transforms Element :

Includes the schema definition of the new Transform.

4.4.3.6 Reference Processing Model :

The reference processing model is completely changed, Transforms do not exist any more, instead they have been broken up into two distinct steps Selection which specifies how the data object is selection, and Canoncalization which converts the data object to octet stream.
Unlike Signature 1.x, Signature 2.x does not use the XPath data model at all. There is conversion in between XPath nodeset and octet stream to match impedance between transforms.

11 Definitions

Authentication Code(Protected Checksum): A value generated from the application of a shared key to a message via a cryptographic algorithm such that it has the properties of message authentication (and integrity) but not signer authentication. Equivalent to protected checksum, "A checksum that is computed for a data object by means that protect against active attacks that would attempt to change the checksum to make it match changes made to the data object." [SEC]
Authentication, Message: The property, given an authentication code/protected checksum, that tampering with both the data and checksum, so as to introduce changes while seemingly preserving integrity, are still detected. "A signature should identify what is signed, making it impracticable to falsify or alter either the signed matter or the signature without detection." [Digital Signature Guidelines, ABA].
Authentication, Signer: The property that the identity of the signer is as claimed. "A signature should indicate who signed a document, message or record, and should be difficult for another person to produce without authorization." [Digital Signature Guidelines, ABA] Note, signer authentication is an application decision (e.g., does the signing key actually correspond to a specific identity) that is supported by, but out of scope, of this specification.
Checksum: "A value that (a) is computed by a function that is dependent on the contents of a data object and (b) is stored or transmitted together with the object, for the purpose of detecting changes in the data." [SEC]
Core: The syntax and processing defined by this specification, including core validation. We use this term to distinguish other markup, processing, and applications semantics from our own.
Data Object (Content/Document): The actual binary/octet data being operated on (transformed, digested, or signed) by an application -- frequently an HTTP entity [HTTP]. Note that the proper noun Object designates a specific XML element. Occasionally we refer to a data object as a document or as a resource's content. The term element content is used to describe the data between XML start and end tags [XML]. The term XML document is used to describe data objects which conform to the XML specification [XML].
Integrity: "The property that data has not been changed, destroyed, or lost in an unauthorized or accidental manner." [SEC] A simple checksum can provide integrity from incidental changes in the data; message authentication is similar but also protects against an active attack to alter the data whereby a change in the checksum is introduced so as to match the change in the data.
Object: An XML Signature element wherein arbitrary (non-core) data may be placed. An Object element is merely one type of digital data (or document) that can be signed via a Reference.
Resource: "A resource can be anything that has identity. Familiar examples include an electronic document, an image, a service (e.g., 'today's weather report for Los Angeles'), and a collection of other resources.... The resource is the conceptual mapping to an entity or set of entities, not necessarily the entity which corresponds to that mapping at any particular instance in time. Thus, a resource can remain constant even when its content---the entities to which it currently corresponds---changes over time, provided that the conceptual mapping is not changed in the process." [URI] In order to avoid a collision of the term entity within the URI and XML specifications, we use the term data object, content or document to refer to the actual bits/octets being operated upon.
Signature: Formally speaking, a value generated from the application of a private key to a message via a cryptographic algorithm such that it has the properties of integrity, message authentication and/or signer authentication. (However, we sometimes use the term signature generically such that it encompasses Authentication Code values as well, but we are careful to make the distinction when the property of signer authentication is relevant to the exposition.) A signature may be (non-exclusively) described as detached, enveloping, or enveloped.
Signature, Application: An application that implements the MANDATORY (REQUIRED/MUST) portions of this specification; these conformance requirements are over application behavior, the structure of the Signature element type and its children (including SignatureValue) and the specified algorithms.
Signature, Detached: The signature is over content external to the Signature element, and can be identified via a URI or transform. Consequently, the signature is "detached" from the content it signs. This definition typically applies to separate data objects, but it also includes the instance where the Signature and data object reside within the same XML document but are sibling elements.
Signature, Enveloping: The signature is over content found within an Object element of the signature itself. The Object (or its content) is identified via a Reference (via a URI fragment identifier or transform).
Signature, Enveloped: The signature is over the XML content that contains the signature as an element. The content provides the root XML document element. Obviously, enveloped signatures must take care not to include their own value in the calculation of the SignatureValue.
Transform: The processing of a data from its source to its derived form. Typical transforms include XML Canonicalization, XPath, and XSLT.
Validation, Core: The core processing requirements of this specification requiring signature validation and SignedInfo reference validation.
Validation, Reference: The hash value of the identified and transformed content, specified by Reference, matches its specified DigestValue.
Validation, Signature: The SignatureValue matches the result of processing SignedInfo with CanonicalizationMethod and SignatureMethod as specified in Core Validation (section 3.2).
Validation, Trust/Application: The application determines that the semantics associated with a signature are valid. For example, an application may validate the time stamps or the integrity of the signer key -- though this behavior is external to this core specification.

12 References

http://www.w3.org/TR/2008/REC-xml-c14n11-20080502/
1363: IEEE 1363: Standard Specifications for Public Key Cryptography. August 2000.
ABA: Digital Signature Guidelines. http://www.abanet.org/scitech/ec/isc/dsgfree.html
ANSI-X9-62: ANSI X9.62:2005. ANSI X9.62:2005 Temporarily here... 2005. http://nist.gov/
CVE-209-0217: CVE-2009-0217, Common Vulnerabilities and Exposures List
DOM: Document Object Model (DOM) Level 1 Specification. W3C Recommendation. V. Apparao, S. Byrne, M. Champion, S. Isaacs, I. Jacobs, A. Le Hors, G. Nicol, J. Robie, R. Sutor, C. Wilson, L. Wood. October 1998. http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/
DSS: FIPS PUB 186-3. Digital Signature Standard (DSS). U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
HMAC: RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H. Krawczyk, M. Bellare, R. Canetti. February 1997. http://www.ietf.org/rfc/rfc2104.txt
HMRMC: HM Revenue and customs Her Majesty's Revenue and Customs. Sample response message with XML signature
HTTP: RFC 2616. Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. June 1999. http://www.ietf.org/rfc/rfc2616.txt
KEYWORDS: RFC 2119. Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. March 1997. http://www.ietf.org/rfc/rfc2119.txt
LDAP-DN: RFC4514 . Lightweight Directory Access Protocol : String Representation of Distinguished Names. K. Zeilenga, Ed. June 2006. http://www.ietf.org/rfc/rfc2253.txt http://www.ietf.org/rfc/rfc4514.txt
MD5: RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. April 1992. http://www.ietf.org/rfc/rfc1321.txt
MIME: RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. N. Freed & N. Borenstein. November 1996. http://www.ietf.org/rfc/rfc2045.txt
NFC: TR15, Unicode Normalization Forms. M. Davis, M. Dürst. Revision 18: November 1999. http://www.unicode.org/unicode/reports/tr15/tr15-18.html.
NFC-Corrigendum: Normalization Corrigendum. The Unicode Consortium. http://www.unicode.org/unicode/uni2errata/Normalization_Corrigendum.html.
OCSP: RFC 2560. X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP. M. Myers, R. Ankney, A. Malpani S. Galperin, C. Adams. June 1999. http://www.ietf.org/rfc/rfc2560.txt
PGP: RFC 2440. OpenPGP Message Format. J. Callas, L. Donnerhacke, H. Finney, R. Thayer. November 1998. http://www.ietf.org/rfc/rfc2440.txt
PKCS1: RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0. B. Kaliski, J. Staddon. October 1998. http://www.ietf.org/rfc/rfc2437.txt
RANDOM: RFC 1750. Randomness Recommendations for Security. D. Eastlake, S. Crocker, J. Schiller. December 1994. http://www.ietf.org/rfc/rfc1750.txt
RDF: Resource Description Framework (RDF) Schema Specification 1.0. W3C Candidate Recommendation. D. Brickley, R.V. Guha. March 2000. http://www.w3.org/TR/2000/CR-rdf-schema-20000327/ Resource Description Framework (RDF) Model and Syntax Specification. W3C Recommendation. O. Lassila, R. Swick. February 1999. http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/
RFC3279: RFC 3279Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. W. Polk, R. Housley, L. Bassham. April 2002.http://www.ietf.org/rfc/rfc3279.txt
RFC4050: RFC 4050. Using the Elliptic Curve Signature Algorithm (ECDSA) for XML Digital Signatures. S. Blake-Wilson, G. Karlinger, T. Kobayashi, Y. Wang. April 2005. http://www.ietf.org/rfc/rfc4050.txt
RFC5280: RFC 5280. Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. D. Cooper, et. al. May 2008. http://www.ietf.org/rfc/rfc5280.txt
RFC5480: RFC 5480. Elliptic Curve Cryptography Subject Public Key Information. S. Turner, et. al. March 2009. http://www.ietf.org/rfc/rfc5480.txt
SAX: SAX: The Simple API for XML. D. Megginson, et al. May 1998. http://www.megginson.com/downloads/SAX/
SEC: RFC 2828. Internet Security Glossary. R. Shirey. May 2000. http://www.faqs.org/rfcs/rfc2828.html
SECG: SEC 1. SEC 1: Elliptic Curve Cryptography, Version 1.0, Standards for Efficient Cryptography Group, September 2000. http://www.secg.org/download/aid-385/sec1_final.pdf
SECG2: SEC 2. SEC 2: Elliptic Curve Cryptography, Version 1.0, Standards for Efficient Cryptography Group, September 2000. http://www.secg.org/download/aid-386/sec2_final.pdf
SHA-1: FIPS PUB 180-3. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf
SHA-1-Analysis: McDonald, C., Hawkes, P., and J. Pieprzyk. SHA-1 collisions now 2^52 , EuroCrypt 2009 Rump session. http://eurocrypt2009rump.cr.yp.to/837a0a8086fa6ca714249409ddfae43d.pdf
SHA-256: FIPS PUB 180-3. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf
SHA-384: FIPS PUB 180-3. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf
SHA-512: FIPS PUB 180-3. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf
SOAP: Simple Object Access Protocol (SOAP) Version 1.1. W3C Note. D. Box, D. Ehnebuske, G. Kakivaya, A. Layman, N. Mendelsohn, H. Frystyk Nielsen, S. Thatte, D. Winer. May 2001. http://www.w3.org/TR/2000/NOTE-SOAP-20000508/
SP800-57: SP 800-57. Recommendation for Key Management – Part 1: General (Revised). U.S. Department of Commerce/National Institute of Standards and Technology. http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57-Part1-revised2_Mar08-2007.pdf
URI: RFC 3986. Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. January 2005. http://www.ietf.org/rfc/rfc3986.txt
URL: RFC 1738. Uniform Resource Locators (URL). T. Berners-Lee, L. Masinter, and M. McCahill. December 1994. http://www.ietf.org/rfc/rfc1738.txt
URN: RFC 2141. URN Syntax. R. Moats. May 1997. http://www.ietf.org/rfc/rfc2141.txt RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D. van Gulik, R. Iannella, P. Falstrom. June 1999. http://www.ietf.org/rfc/rfc2611.txt
URN-OID: RFC 3061. A URN Namespace of Object Identifiers. M. Mealling. February 2001. http://www.ietf.org/rfc/rfc3061.txt
UTF-16: RFC 2781. UTF-16, an encoding of ISO 10646. P. Hoffman , F. Yergeau. February 2000. http://www.ietf.org/rfc/rfc2781.txt
UTF-8: RFC 2279. UTF-8, a transformation format of ISO 10646. F. Yergeau. January 1998. http://www.ietf.org/rfc/rfc2279.txt
X509v3: ITU-T Recommendation X.509 version 3 (1997). "Information Technology - Open Systems Interconnection - The Directory Authentication Framework" ISO/IEC 9594-8:1997.
XHTML: XHTML(tm) 1.0: The Extensible Hypertext Markup Language. W3C Recommendation. S. Pemberton, D. Raggett, et al. January 2000. http://www.w3.org/TR/2000/REC-xhtml1-20000126/
XLink: XML Linking Language. W3C Recommendation. S. DeRose, E. Maler, D. Orchard. June 2001. http://www.w3.org/TR/2001/REC-xlink-20010627/
XML: Extensible Markup Language (XML) 1.0 (Fourth Edition). W3C Recommendation T. Bray, E. Maler, J. Paoli, C. M. Sperberg-McQueen, F.Yergeau. 16 August 2006, edited in place 29 September 2006. http://www.w3.org/TR/2006/REC-xml-20060816/
XML-C14N: Canonical XML. W3C Recommendation. J. Boyer. March 2001. http://www.w3.org/TR/2001/REC-xml-c14n-20010315 http://www.ietf.org/rfc/rfc3076.txt
XML-C14N11: Canonical XML 1.1. W3C Recommendation. J. Boyer, G. Marcy. 2 May 2008. http://www.w3.org/TR/2008/REC-xml-c14n11-20080502/
XML-C14N20: Canonical XML 2.0, J. Boyer, G. Marcy, P. Datta, F. Hirsch. W3C Working Draft 22 October 2009. http://www.w3.org/TR/2009/WD-xml-c14n2-20091022/
XML-Enc: XML Encryption Syntax and Processing.W3C Recommendation. D. Eastlake 3rd., J. Reagle. December 2002.http://www.w3.org/TR/xmlenc-core/
XML-Japanese: XML Japanese Profile. W3C Note. M. Murata. April 2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-20000414/
XML-MT: RFC 2376 . XML Media Types. E. Whitehead, M. Murata. July 1998. http://www.ietf.org/rfc/rfc2376.txt
XML-Signature-RD: RFC 2807. XML Signature Requirements. W3C Working Draft. J. Reagle, April 2000. http://www.w3.org/TR/1999/WD-xmldsig-requirements-19991014.html http://www.ietf.org/rfc/rfc2807.txt http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
XML-exc-C14N: Exclusive XML Canonicalization Version 1.0 W3C Recommendation. J. Boyer, D. Eastlake 3rd., J. Reagle. July 2002. http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/
XML-ns: Namespaces in XML 1.0 (Second Edition). W3C Recommendation. T. Bray, D. Hollander, A. Layman, R. Tobin. 16 August 2006. http://www.w3.org/TR/2006/REC-xml-names-20060816/
XML-schema: XML Schema Part 1: Structures. W3C Recommendation. H. Thompson,D. Beech, M. Maloney, N. Mendelsohn. October 2004. http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/ XML Schema Part 2: Datatypes W3C Recommendation. P. Biron, A. Malhotra. May 2001. http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/
XMLDSIG-2002: XML-Signature Syntax and Processing. D. Eastlake, J. Reagle, and D. Solo. W3C Recommendation, February 2002. http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
XMLDSIG11: XML Signature Syntax and Processing 1.1. D. Eastlake, J. Reagle, et al. Working Draft, 30 July 2009. This is a draft and subject to change. http://www.w3.org/TR/xmldsig-core1/
XPTR-2001: XML Pointer Language (XPointer). W3C Candidate Recommendation. S. DeRose, R. Daniel, E. Maler. January 2001. http://www.w3.org/TR/2001/CR-xptr-20010911/
XPath: XPath XML Path Language (XPath) Version 1.0. W3C Recommendation. J. Clark, S. DeRose. October 1999. http://www.w3.org/TR/1999/REC-xpath-19991116
XPath-Filter-2: XML-Signature XPath Filter 2.0. W3C Recommendation. J. Boyer, M. Hughes, J. Reagle. November 2002. http://www.w3.org/TR/2002/REC-xmldsig-filter2-20021108/
XPointer-Element: XPointer element() Scheme. W3C Recommendation. P. Grosso, E. Maler, J. Marsh, N. Walsh. March 2003. http://www.w3.org/TR/2003/REC-xptr-element-20030325/
XPointer-Framework: XPointer-Framework XPointer Framework. W3C Recommendation. P. Grosso, E. Maler, J. Marsh, N. Walsh. March 2003. http://www.w3.org/TR/2003/REC-xptr-framework-20030325/
XPointer-xmlns: XPointer xmlns() Scheme. W3C Working Recommendation. S. DeRose, R. Daniel, E. Maler, J. Marsh. March 2003. http://www.w3.org/TR/2003/REC-xptr-xmlns-20030325/
XPointer-xpointer: XPointer xpointer() Scheme. W3C Working Draft. S. DeRose, E. Maler, R. Daniel. December 2002. http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/
XSL: Extensible Stylesheet Language (XSL). W3C Recommendation. S. Adler, A. Berglund, J. Caruso, S. Deach, T. Graham, P. Grosso, E. Gutentag, A. Milowski, S. Parnell, J. Richman, S. Zilles. October 2001. http://www.w3.org/TR/2001/REC-xsl-20011015/
XSLT: XSL Transforms (XSLT) Version 1.0. W3C Recommendation. J. Clark. November 1999. http://www.w3.org/TR/1999/REC-xslt-19991116.html
ebXML-Msg: ebXML Messsage Service Specification Version 2.0 OASIS ebXML Messaging Services Technical Committee
rfc4051: RFC 4051. Additional XML Security Uniform Resource Identifiers. D. Eastlake 3rd. April 2005. http://www.ietf.org/rfc/rfc4051.txt