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XML-Signature Core Syntax and Processing

W3C Working Draft 03-February-2000

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Donald Eastlake <>
Joseph Reagle <>
David Solo <>
Mark Bartel <>
John Boyer <>
Barb Fox <
Ed Simon <>
See Acknowledgements


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

Status of this document

This is a Working Draft of the IETF/W3C XML Signature Working Group . This version follows from the January face-to-face meeting. We hope to issue an institutional (IETF/W3C) Last Call within four weeks. This version includes and XML Schema definition and a DTD; both of which are fairly mature but may be buggy.

Please send comments to the editors and cc: the list <>. Publication as a Working Draft does not imply endorsement by the W3C membership or IESG. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite W3C Drafts as other than "work in progress." A list of current W3C working drafts can be found at

Patent disclosures relevant to this specification may be found on the WG's patent disclosure page.

Table of Contents

  1. Introduction
    1. Editorial Conventions
    2. Design Philosophy
    3. Namespaces and Identifiers
    4. Acknowledgements
  2. Signature Overview
    1. The Signature Element
    2. The SignedInfo Element
    3. The Reference Element
    4. The Manifest Element
    5. The SignatureProperties Element
  3. Processing Rules
    1. Signature Generation
    2. Signature Validation
  4. Core Signature Syntax
    1. The Signature element
    2. The SignatureValue Element
    3. The SignedInfo Element
    4. The KeyInfo Element
    5. The Object Element
  5. Additional Signature Syntax
    1. The Manifest Element
    2. The SignatureProperties Element
    3. Other Useful Types
    4. Processing Instructions
    5. Comments in dsig Elements
  6. Algorithms
    1. Algorithm Identifiers and Implementation Requirements
    2. Message Digests
    3. Message Authentication Codes
    4. Signature Algorithms
    5. Canonicalization Algorithms
    6. Transform Algorithms
  7. XML Canonicalization and Syntax Constraint Considerations
    1. XML 1.0, Syntax Constraints, and Canonicalization
    2. DOM/SAX Processing and Canonicalization
  8. Security Considerations
    1. Only What is Signed is Secure
    2. Only What is "Seen" Should be Signed
    3. Check the Security Model
    4. Algorithms, Key Lengths, Etc.
  9. Schema, DTD, and Data Model
  10. Example Syntax
  11. Definitions
  12. References

1.0 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 or other data. Furthermore, 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  referenced externally via a URI.

This document also defines other useful types including methods of referencing collections of resources, and key management and algorithm definitions.

1.1 Editorial Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document 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 keywords 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 formally and unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of other features and behaviors . For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification is described as "REQUIRED."

This document may include editorial comments within the text.

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

No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace  The XML namespace [XML-ns] URI that MUST be used by experimental implementations of this dated specification is:


This namespace is also used as the prefix for algorithm identifiers used by this specification. While applications MUST support XML and XML-namespaces, the use of internal entities [XML] or our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL; we use these facilities so as to provide compact and readable examples.

This specification uses Uniform  Resource Identifiers [URI] to identify resources, algorithms, and semantics. The URI in the namespace declaration above is also used as a prefix for URIs under the control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN] or Uniform Resource Locators [URL] defined by the external specification. If an external specification has not allocated itself a Uniform Resource Identifier we allocate an identifier under our own namespace. For instance:

SignatureProperties is identified and defined by this specification's namespace
XSLT is identified and defined by an external namespace
SHA1 is identified via this specification's namespace and defined via a normative reference

FIPS PUB 180-1. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.

This specification uses both XML Schemas [XML-schema] and DTDs [XML]. Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's "Declaring Elements and Attributes in an XML DTD."

Finally, in order to provide for terse namespace declarations we use XML internal entities [XML]as macros within URIs. For instance:

<?xml version='1.0'?>
<!DOCTYPE Signature SYSTEM "xmldsig-core-schema.dtd" [
<!ENTITY dsig "">
<Signature xmlns="&dsig;">
  <SignedInfo Id="mypage">

Security Comment: XML processors will automatically expand entity declarations prior to signature generation. Consequently, this feature does not permit a substitution attack whereby an attacker replaces the entity declaration with another so as to change the meaning of the signature. However, since this specification presently permits a CanonicalizationMethod of null over SignedInfo, entity declarations may not be expanded in those instances (or where the schema is not present) and we have not completely assessed the security risk.

1.4 Acknowledgements

2.0 Signature Overview

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

In this section, an informal representation is used to describe the structure of the XML signature syntax. This representation omits many attributes and details. The following suffix symbols are used to represent the number of times elements may occur: "?" denotes zero or one occurrence; "+" denotes one or more occurrences; and "*" denotes zero or more occurrences.

2.1 The Signature Element

XML Signatures are very flexible and can sign arbitrary digital content (data objects). An XML Signature is applied 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. While the data object(s) are not directly operated on by a cryptographic signature algorithm, we still refer to the signature as being over the data object(s). Somtimes content is obtained by dereferencing an identified resource. Within an XML document, signatures are related to data objects via IDREFs [XML] and the data can be included within an enveloping signature or can enclose an enveloped signature. Signatures are related to external data objects via URIs [URI] and the signature and data object are said to be detached.

XML digital signatures are represented by the Signature element which has the following structure:


The required SignedInfo element is the information which is actually signed. SignedInfo includes a list of References to data objects and their calculated digest value. The core validation consists of two mandatory processes: validation of the signature over SignedInfo and validation of each Reference digest within SignedInfo. The algorithms used in calculating the SignatureValue are also included in the signed information while the SignatureValue element is outside SignedInfo.

KeyInfo indicates what key is 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, KeyInfo might contain information the signer does not wish to reveal to all signature verifiers. 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.

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

Signature properties, such as time of signing, can be optionally included in a SignatureProperties within Object. (These properties are traditionally called signature "attributes" although that term in that context has no relationship to the XML term "attribute".) SignatureProperties can be included within an Object and signed at the signer's discretion.

2.2 The SignedInfo Element

The SignedInfo element has the structure indicated below.


The CanonicalizationMethod is the algorithm which is used to canonicalize the SignedInfo element before it is digested as part of the signature operation. In the absence of a CanonicalizationMethod element, no canonicalization is done.

The SignatureMethod is the algorithm 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 or HMAC-SHA1. The algorithm names are signed to resist attacks based on substituting a weaker algorithm.

To promote application interoperability we specify mandatory to implement canonicalization, digest, and signature algorithms. We specify additional algorithms as recommended or optional and the signature design permits arbitrary signer algorithm specification.

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

2.3 The Reference Element

The Reference element has the structure indicated below.

   (<Reference (URI=|IDREF=)? Type=?>

The optional URI/IDREF 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.)

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

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

Transforms is an optional ordered list of processing steps that were applied to the resource's content before it is digested. Transforms can include operations such as canonicalization, encoding/decoding (including compression/inflation), XSLT and XPath. XPath transforms permit the signer to derive an XML document that omits portions of the source document. Consequently those excluded portions can change without affecting signature validity (this is how the Working Group satisfied the requirement of signing portions of a document.) For example, if the resource being signed encloses the signature itself, such a transform must be used to exclude the signature value from its own computation. If no Transforms element is present, the resource's content is digested directly.

Arbitrary user specified transforms are permitted. To promote interoperability, we specify mandatory to implement canonicalization and decoding transformation algorithms. Additional canonicalization, coding, XSLT, and XPath based transform algorithms are specified as recommended or optional.

DigestMethod is the algorithm applied to the data, after Transforms is applied if specified, to yield the DigestValue. The signing of the DigestValue is what bind's a resources content to the signer's key.

2.4 The Manifest Element

The Manifest element is provided to meet additional requirements not directly addressed by this document. The level of indirection provided by these elements readily meets these requirements. Two examples 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 achieved by including multiple References 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 References within SignedInfo to undergo reference validation -- the DigestValue elements are checked. Some 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 References. 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 References as valid.

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.

To address these requirements, the Manifest element type has been defined which may be referenced by SignedInfo Reference elements. First, the Manifest element may contain a collection of References, but leaves reference validation up to the application. Thus the first case above can be solved by simply putting one reference inside SignedInfo to a Manifest which references the three data objects. Second, multiple signatures over a large number of References need only point to a single Manifest with the many references.

The structure of Manifest, which reuses the Reference element described above, is as follows:


Manifest may appear as the content of an Object. Note that an application could decide whether to verify a DigestValue in a Manifest based on the Type given in the enclosing Reference.

2.5 The SignatureProperties Element

This specification does not address mechanisms for making statements or assertions. Instead, this whole document simply defines what it means for something to be signed by an XML Signature (message authentication, integrity, and/or signer authentication). Applications that wish to represent other semantics must rely upon other technologies, such as [XML, XML-schema, RDF]. However, we do define a SignatureProperties element type for the inclusion of assertions about the signature itself (e.g., 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.  

  (SignatureProperty Target= )*

The structure of SignatureProperties is shown above. Any content about the signature generation may be located within the SignatureProperty element. The mandatory Target attribute references the element to which the property applies. In particular, target may include a reference to a SignedInfo or Reference element.

3.0 Processing Rules

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

3.1 Generation

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

3.1.1 Reference Generation

For each data object being signed:

  1. Apply the Transforms, as determined by the application, to the data object.
  2. Calculate the digest value over the resulting data object.
  3. Create a Reference element, including the (optional) identification of the data object, any (optional) transform elements, the digest algorithm and the DigestValue.

3.1.2 Signature Generation

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

3.2 Validation

The REQUIRED steps of core validation include (1) reference validation, the verification of the digest contained in each Reference in SignedInfo, and (2) the cryptographic signature validation of the signature calculated over SignedInfo.

3.2.1 Reference Validation

For each Reference in SignedInfo:

  1. Obtain the data object to be digested. (The signature application may rely upon the identification (URI/IDREF) and Transforms provided by the signer in the Reference element, or it may obtain the content through other means such as a local cache.)
  2. Digest the resulting data object using the DigestMethod specified in its Reference specification.
  3. Compare the generated digest value against DigestValue in SignedInfo; if there is any mismatch, validation fails.

3.2.2 Signature Validation

  1. Canonicalize the SignedInfo element based on the CanonicalizationMethod, if any, in SignedInfo.
  2. Obtain the keying information from KeyInfo or from an external source.
  3. Use the specified SignatureMethod to validate the SignatureValue over the (optionally canonicalized) SignedInfo element.

4.0 Core Signature Syntax

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

Schema Definition:

<?xml version='1.0'?> 
<!DOCTYPE schema 
   SYSTEM '' 
   <!ENTITY dsig ''> 

<schema targetNamespace='&dsig;' 

4.1 The Signature element

The Signature element is the root element of a XML Signature. A simple example of a complete signature follows:

Dummy Example:

<!DOCTYPE Signature [
<!ENTITY dsig ''>]>
<Signature xmlns="">
     Algorithm="" />
    <SignatureMethod Algorithm="&dsig;dsaWithSHA-1"/>
    <Reference Location="">
      <DigestMethod Algorithm="&dsig;sha1"/>

Schema Definition:

   <element name='Signature'> 
     <type content='elementOnly'>
       <group order='seq' minOccurs='1' maxOccurs='1'> 
   	  <element ref='ds:SignedInfo' minOccurs='1' maxOccurs='1'/> 
   	  <element ref='ds:SignatureValue' minOccurs='1' maxOccurs='1'/> 
   	  <element ref='ds:KeyInfo' minOccurs='0' maxOccurs='1'/> 
   	  <element ref='ds:Object' minOccurs='0' maxOccurs='*'/> 
   	<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*)  >
<!ATTLIST Signature  
	xmlns	CDATA	#FIXED ''

4.2 The SignatureValue Element

The SignatureValue element contains the actual value of the digital signature. The encoding of this value is determined by SignatureMethod. Base64 [MIME] is the encoding method for all SignatureMethods specified within this specification. The ability to define a SignatureMethod and SignatureValue pair which includes multiple distinct signatures is explicitly permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").

Schema Definition:

<element name='SignatureValue' type='string'/> 

<!ELEMENT SignatureValue  (#PCDATA)  >

4.3 The SignedInfo Element

The structure of SignedInfo includes the canonicalization algorithm (if any), 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.

Schema Definition:

<element name='SignedInfo'> 
  <type content='elementOnly'>
    <group order='seq' minOccurs='1' maxOccurs='1'> 
	  <element ref='ds:CanonicalizationMethod' minOccurs='0' maxOccurs='1'/> 
	  <element ref='ds:SignatureMethod' minOccurs='1' maxOccurs='1'/> 
	  <element ref='ds:Reference' minOccurs='1' maxOccurs='*'/> 
	<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT SignedInfo (CanonicalizationMethod?, 
	SignatureMethod,  Reference+)  >
<!ATTLIST SignedInfo  
	Id	 ID 	 #IMPLIED >

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 found within an Object element.

4.3.1 The CanonicalizationMethod Element

CanonicalizationMethod is an optional element which specifies the canonicalization algorithm applied to the SignedInfo element prior to performing signature calculations. This element uses the general structure here for algorithms described in section 5.1: Algorithm Identifiers. Possible options may include a minimal algorithm (CRLF and charset normalization), or more extensive operations such as [XML-C14N]. If the CanonicalizationMethod is omitted, no change is made to SignedInfo before digesting. (Note this may lead to interoperability failures as other applications may not serialize it as the creators application did by default.)

Schema Definition:

<element name='CanonicalizationMethod'> 
  <type content='elementOnly'>
    <attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> 

<!ELEMENT CanonicalizationMethod ANY > 
<!ATTLIST CanonicalizationMethod 
          Algorithm CDATA #REQUIRED > 

4.3.2 The SignatureMethod Element

SignatureMethod is a required element which 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 5.1.   While there is a single identifier, that identifier may specify a format containing multiple distinct signature values.

Schema Definition:

<element name='SignatureMethod'> 
  <type content='elementOnly'> 
    <attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> 

<!ELEMENT SignatureMethod ANY > 
<!ATTLIST SignatureMethod 
          Algorithm CDATA #REQUIRED > 

4.3.3 The Reference Element

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

Schema Definition:

<element name='Reference'> 
  <type content='elementOnly'>
    <group order='seq' minOccurs='1' maxOccurs='1'> 
	  <element ref='ds:Transforms' minOccurs='0' maxOccurs='1'/> 
	  <element ref='ds:DigestMethod' minOccurs='1' maxOccurs='1'/> 
	  <element ref='ds:DigestValue' minOccurs='1' maxOccurs='1'/> 
	<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 
	<attribute name='URI' type='uri' minOccurs='0' maxOccurs='1'/> 
	<attribute name='IDREF' type='IDREF' minOccurs='0' maxOccurs='1'/> 
	<attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/> 

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

The URI/IDREF attribute identifies a data object using a URI [URI] or IDREF [XML]. We distinguish between URIs and IDREFs so as to provide expositional clarity and ease signature processing. Note there is some popular confusion about URIs and fragment identifiers. As specified by RFC2396 [URI], URIs can be used in conjunction with a fragment identifier by use of a separating pound symbol '#', but the URI proper does not include the fragment identifier. (The meaning of the fragment is defined by the resource's MIME type). URI/IDREF only permits a 'clean' URI or IDREF; fragment identification is specified under Transforms. This choice permits References to identify a fragment of a document that is encoded: the Reference identifies the resource, the first Transform could specify decoding, and the second Transform could specify the fragment.

Note that a null URI (URI="") is permitted and identifies the document the reference is in (the root element).

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

The digest algorithm is applied to the data octets being secured. Typically that is done by locating (possibly using the URI/IDREF if provided) the data and transforming it. If the data is an XML document, the document is assumed to be unparsed prior to the application of Transforms. If there are no Transforms, then the data is passed to the digest algorithm unmodified.

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


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

The optional Transforms element contains an ordered list of  Transform elements; these describe how the signer obtained the data object that was digested. The output of each Transform (octets) serve as input to the next Transform. The input to the first Transform is the source data. The output from the last Transform is the input for the DigestMethod algorithm. When transforms are applied the signer is not signing the native (original) document but the resulting (transformed) document [section 7.2: Only What is "Seen" Should be Signed].

Each Transform consists of an Algorithm attribute, optional MimeType and Charset attributes, and content parameters, if any, appropriate for the given algorithm. The Algorithm attribute value specifies the name of the algorithm to be performed, and the Transform content provides additional data to govern the algorithm's processing of the input resource.

The optional MimeType and Charset (IANA registered character set) attributes are made available to algorithms which need and are otherwise unable to deduce that information about the data they are processing.

Schema Definition:

<element name='Transforms' > 
  <type content='elementOnly'> 
    <element ref='ds:Transform' minOccurs='1' maxOccurs='*'/> 

<element name='Transform'> 
  <type content='elementOnly'>
    <attribute name='Algorithm' type='string' minOccurs='1' maxOccurs='1'/> 
    <attribute name='MimeType' type='string' minOccurs='0' maxOccurs='1'/> 
    <attribute name='Charset' type='string' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT Transforms (Transform+)>

<!ELEMENT Transform ANY>
<!ATTLIST Transform 
          Algorithm    CDATA     #REQUIRED
          MimeType     CDATA    #IMPLIED 
          Charset      CDATA    #IMPLIED >
<!--      The Type conforms to the 
          productions specified by [URI] --> 

Examples of transforms include but are not limited to base-64 decoding [MIME], canonicalization [XML-c14n], XPath filtering [XPath], and XSLT [XSLT]. The generic definition of the Transform element also allows application-specific transform algorithms. For example, the transform could be a decompression routine given by a Java class appearing as a base-64 encoded parameter to a Java Transform algorithm. However, applications should refrain from using application-specific transforms if they wish their signatures to be verifiable outside of their application domain. Section 5-6: Transform Algorithms defines the list of standard transformations. The DigestMethod Element

DigestMethod is a required element which identifies the digest algorithm to be applied to the signed object. This element uses the general structure here for algorithms specified in section 5.1: Algorithm Identifiers.

Schema Definition:

<element name='DigestMethod'> 
  <type content='elementOnly'> 
	<attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> 

<!ELEMENT DigestMethod ANY >
<!ATTLIST DigestMethod  
	Algorithm		CDATA	#REQUIRED > The DigestValue Element

DigestValue is an element which contains the encoded value of the digest. The digest is always encoded using Base 64 [MIME].

Schema Definition:

<element name='DigestValue' type='ds:encoded'/> 

<!ELEMENT DigestValue  (#PCDATA)  >
<!-- base64 encoded signature value --> 

4.4 The KeyInfo Element

KeyInfo may contain keys, names, certificates and other public key management information (such as in-band key distribution or agreement data or data supporting any other method.) This specification defines a few simple types but applications may place their own key identification and exchange semantics within this element through the XML-namespace facility. [XML-namespace]

Schema Definition:

<element name='KeyInfo'> 
  <type content='elementOnly'>
    <group order='choice' minOccurs='1' maxOccurs='*'>       
	<element name='KeyName' type='string'/> 
	<element name='KeyValue' type='string'/> 
[[	<element name='SubjectName' type='string'/> ]]
	<element name='RetrievalMethod' type='uri'/> 
	<element ref='ds:X509Data'/> 
	<element ref='ds:PGPData'/> 
	<element name='MgmtData' type='string' minOccurs='0' maxOccurs='1'/>
	<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT KeyInfo ((KeyName | KeyValue | SubjectName | 
          RetrievalMethod | X509Data | PGPData  | MgmtData)*)  >
<!ATTLIST KeyInfo  
        Type	CDATA    #IMPLIED>
<!ELEMENT KeyName  (#PCDATA) >
<!ELEMENT KeyValue  (#PCDATA)  >
[[<!ELEMENT SubjectName  (#PCDATA)  >]]
<!ELEMENT RetrievalMethod  (#PCDATA)  >

KeyInfo is an optional element which enables the recipient(s) to obtain the key(s) needed to validate the signature. If omitted, the recipient is expected to be able to identify the key based on application context information. Multiple declarations within KeyInfo refer to the same key. Applications may define and use any mechanism they choose through inclusion of elements from a different namespace.

Compliant versions implementing KeyInfo MUST implement KeyValue, and SHOULD implement RetrievalMethod.

Schema Definition

<element name='X509Data'> 
  <type content='elementOnly'> 
    <group order='seq' minOccurs='1' maxOccurs='1'> 
      <group order='choice' minOccurs='1' maxOccurs='1'> 
	  <element ref='ds:X509IssuerSerial'/>
	  <element name='X509SKI' type='string'/> 
	  <element name='X509SubjectName' type='string'/> 
	<element name='X509Certificate' type='string' minOccurs='0' maxOccurs='*'/> 
	<element name='X509CRL' type='string' minOccurs='0' maxOccurs='*'/> 

<element name='X509IssuerSerial'> 
   <type content='elementOnly'> 
      <group order='seq' minOccurs='1' maxOccurs='1'> 
   	 <element name='X509IssuerName' type='string' minOccurs='1' maxOccurs='1'/> 
  	 <element name='X509SerialNumber' type='string' minOccurs='1' maxOccurs='1'/> 
<element name='PGPData'> 
  <type content='elementOnly'> 
    <group order='seq' minOccurs='1' maxOccurs='1'>
	 <element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/> 
	 <element name='PGPKeyPacket' type='string' minOccurs='1' maxOccurs='1'/> 

<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName), 
          X509Certificate*, X509CRL*)>
<!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber)  >
<!ELEMENT X509IssuerName (#PCDATA)  >
<!ELEMENT X509SubjectName  (#PCDATA)  >
<!ELEMENT X509SerialNumber  (#PCDATA)  >
<!ELEMENT X509Certificate  (#PCDATA)  >

<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?)  >
<!ELEMENT PGPKeyPacket  (#PCDATA)  >

4.5 The Object Element

Object is an optional element which 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 MimeType attribute is an optional attribute which describes the data within the Object. This is a string with values defined by [MIME]. For example, if the Object contains XML, the MimeType would be text/xml. This attribute is purely advisory, no validation of the MimeType informatin is required by this document.

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 document. 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 content='mixed'>
     <any namespace='##targetNamespace'/>
	<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 
	<attribute name='MimeType' type='[[uri]]string' minOccurs='0' maxOccurs='1'/> 
	<attribute name='Encoding' type='uri' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT Object (#PCDATA) >
<!ATTLIST Object  
	Encoding	CDATA	#IMPLIED >

5.0 Additional Signature Syntax

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

5.1 The Manifest Element

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 application 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 content='elementOnly'>
    <group order='seq' minOccurs='1' maxOccurs='1'>
	<element ref='ds:Reference' minOccurs='1' maxOccurs='*'/> 
	<element ref='ds:Object' minOccurs='0' maxOccurs='*'/> 
  <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 

<!ELEMENT Manifest ((Reference | Object)+)  >
<!ATTLIST Manifest  

5.2 The SignatureProperties Element

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 content='elementOnly'> 
    <element ref='ds:SignatureProperty' minOccurs='1' maxOccurs='*'/> 
    <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> 

<element name='SignatureProperty'> 
  <type content='mixed'> 
    <any namespace='##other'/>
    <attribute name='Target' type='IDREF' minOccurs='1' maxOccurs='1'/> 
<!ELEMENT SignatureProperties (SignatureProperty[[+]]*)  >
<!ATTLIST SignatureProperties  

<!ELEMENT SignatureProperty (#PCDATA)  >
<!ATTLIST SignatureProperty  
	Target 	IDREF	 #REQUIRED  >

5.3 Other Useful Types

We define the following URIs for use in identifying XML resources that include non-core but signature related semantics.
designates that the referenced resource is a Signature Object element type.
designates that the referenced resource is a statement about the referring signature.
designates that the referenced resource is a collection of other resources.

5.5 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 specified within this document 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.

5.5 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, they will be signed. Consequently, 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-canonicalization], is specified.

6.0 Algorithms

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

6.1 Algorithm Identifiers and Implementation Requirements

Algorithms are identified by URIs that appear as an attribute to the element that identifies the algorithms' role (DigestMethod, Transform, SignatureMethod, or CanonicalizationMethod). All algorithms used herein take parameters but in many cases the parameters are implicit. For example, a SignatureMethod is implicitly given two parameters: the keying info and the output of CanonicalizationMethod (or SignedInfo directly if there is no CanonicalizationMethod). Explicit additional parameters to an algorithm appear as content elements within the algorithm role element. Such parameter elements have a descriptive element name, which is frequently algorithm specific, and MUST be in the XML Signature namespace or an algorithm specific namespace.

This specification defines a set of algorithms, their URIs, and requirements for implementation. Requirements are specified over implementation, not over requirements for signature use. Furthermore, the mechanism is extensible, alternative algorithms may be used by signature applications.

Algorithm Type Algorithm Requirements Algorithm URI
  Base64 REQUIRED 
  QuotedPrintable RECOMMENDED 
  minimal REQUIRED
  XML-Canonicalization RECOMMENDED

Note that the normative identifier is the complete URIs in the table though they are frequently abbreviated in XML syntax as "&dsig;base64" or the like.

6.2 Message Digests

Only one digest algorithm is defined herein. However, it is expected that one or more additional strong digest algorithms will be developed in connection with the US Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptography have cast doubt on its strength.

6.2.1 SHA-1


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

<DigestMethod Algorithm="&dsig;sha1"/>

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

A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D

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


6.3 Message Authentication Codes

MAC algorithms take two implicit parameters, their keying material determined from KeyInfo and the byte stream output by CanonicalizationMethod or SignedInfo directly if there is no CanonicalizationMethod. MACs and signature algorithms are syntactically identical but a MAC implies a shared secret key.

6.3.1 HMAC


The HMAC algorithm [RFC2104:HMAC] takes the truncation length in bits as a parameter. An example of an HMAC SignatureMethod element:

<SignatureMethod Algorithm="&dsig;hmac-sha1">
  <hmac-outputlength xmlns="&dsig;hmac-sha1"> <!-- need to have a DTD for this -->
  </hmac-outputlength> <!-- define length -->

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

9294727A 3638BB1C 13F48EF8 158BFC9D

from the test vectors in [HMAC] would be


6.4 Signature Algorithms

Signature algorithms take two implicit parameters, their keying material determined from KeyInfo and the byte stream output by CanonicalizationMethod or SignedInfo directly if there is no CanonicalizationMethod. Signature and MAC algorithms are syntactically identical but a signature implies public key cryptography.

Note: the schema and DTD declarations within this section are not yet part of section 9: schemas.

6.4.1 DSA


The DSA algorithm [DSA] takes no explicit parameters. An example of a DSA SignatureMethod element is:

<SignatureMethod Algorithm="&dsig;dsa"/>

The output of the DSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value shall consist of the base64 encoding of the concatenation of two octet-streams that respectively result from the octet-encoding of the values r and s. Integer to octet-stream conversion shall be done according to the I2OSP operation defined in the RFC 2437 [RSA] specification with a k parameter equal to 20. Example: the SignatureValue element for a DSA signature (r, s) with values specified in hexadecimal

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

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


DSA key values have the following set of fields: P, Q, G and Y are mandatory when appearing as a key value, J, seed and pgenCounter are optional but SHOULD be present. (The seed and pgenCounter fields MUST both either appear or be absent). All parameters are encoded as base64 values.


<element name='DSAKeyValue'> 
  <type content='elementOnly'> 
    <group order='seq' minOccurs='1' maxOccurs='1'>
      <element name='ds:P' type='string' minOccurs='1' maxOccurs='1'/> 
      <element name='ds:Q' type='string' minOccurs='1' maxOccurs='1'/> 
      <element name='ds:G' type='string' minOccurs='1' maxOccurs='1'/> 
      <element name='ds:Y' type='string' minOccurs='1' maxOccurs='1'/> 
      <element name='ds:J' type='string' minOccurs='0' maxOccurs='1'/> 
    <group order='seq' minOccurs='0' maxOccurs='1'>
      <element name='ds:Seed' type='string' minOccurs='1' maxOccurs='1'/> 
      <element name='ds:PgenCounterQ' type='string'   minOccurs='1' maxOccurs='1'/> 

<!ELEMENT DSAKeyValue (P, Q, G, Y, J?, (Seed, PgenCounter)?) > 
<!ELEMENT PgenCounter (#PCDATA) >
<!-- Each of these fields consists a PCDATA 
     where the data is base64 encoded --> 

6.4.2 RSA


The expression "RSA algorithm" as used in this document refers to the RSASSA-PKCS1-v1_5 algorithm described in  RFC 2437 [RSA]. The RSA algorithm takes no parameters. An example of an RSA SignatureMethod element is:

<SignatureMethod Algorithm="&dsig;rsa-sha1"/>

The output of the RSA algorithm is an octet string. The SignatureValue content for an RSA signature shall be the base64 encoding of this octet string. Example: TBD

RSA key values have two fields: Modulus and Exponent.


<element name='RSAKeyValue'> 
  <type content='elementOnly'> 
    <element name='ds:Modulus' type='string' minOccurs='1' maxOccurs='1'/> 
    <element name='ds:Exponent' type='string' minOccurs='1' maxOccurs='1'/> 

<!ELEMENT RSAKeyValue (Modulus, Exponent) > 
<!ELEMENT Modulus (#PCDATA)  >
<!ELEMENT Exponent (#PCDATA)  >
<!-- Each field contains a CDATA which is the 
     value for that item base64 encoded --> 

6.5 Canonicalization Algorithms

6.5.1 Minimal Canonicalization


The algorithm identifier for the minimal canonicalization is &dsig;minimal. An example of a minimal canonicalization element is:

<CanonicalizationMethod Algorithm="&dsig;minimal"/>

The minimal canonicalization algorithm:

6.5.1 Canonical XML


An example of an XML canonicalization element is:

<CanonicalizationMethod Algorithm=""/>

The normative specification of Canonical XML is [XML-c14n].

6.6 Transform Algorithms

A Transform algorithm has three implicit parameters. The first is a byte stream from the Reference or as the output of an earlier Transform. The second and third are the optional MimeType and Charset attributes that can be specified on the Transform element.

Application developers are strongly encouraged to support all transforms listed in this section as RECOMMENDED unless the application environment has resource constraints that would make such support impractical. The Working Group goal is to maximize application interoperability on XML signatures, and the working group expects ubiquitous availability of software to support these transforms that can be incorporated into applications without extensive development.

6.6.1 Canonicalization

Any canonicalization algorithm that can be used for CanonicalizationMethod can be used as a Transform.

6.6.2 Base-64 and Quoted-Printable Decoding


The normative specification for base 64 and quoted-printable decoding transforms is [MIME]. The base-64 Transform element has no content. The input is base-64 decoded by this algorithm. This transform is useful if an application needs to sign the raw data associated with base-64 encoded content of an element.

6.6.3 XPath Filtering


The Transform element content MUST conform to the XML Path Language [XPath] syntax. XPath is a language for addressing parts of an XML document. Hence, an XPath expression MUST be applied to an entire well-formed XML document.

Note: The current output of a Reference's IDREF cannot be used as input to an XPath transform. The XPath transform could be defined to provide an XML declaration when one is found not to exist since the encoding attribute could be set equal to the XPath transform's Charset attribute. However, there is currently no way to communicate the correct byte order mark to the transform. For security reasons, a default cannot be selected.

The XPath transform applies the W3C XML canonicalization [XML-C14N] to the input resource. This ensures all entity reference substitutions and attribute normalizations are performed in a manner consistent with a validating XML processor. Linefeeds are normalized, and CDATA sections are eliminated. The types of quotes around attributes are normalized, and the order of attributes is defined. Namespace attributes are created in descendant elements that use namespace definitions. All of these modifications are necessary to achieve a consistent interpretation of the XPath expression and a consistent output of the XPath transform.

Finally, the XPath expression is evaluated assuming that the entity references created by canonicalization have been replaced by the corresponding entity values and that each block of consecutive text characters has been replaced by a single text node.

The result of the XPath is a string, boolean, number, or node-set. If the result of the XPath expression is a string, then the string is the output of the XPath transform. If the XPath result is a boolean or number, then the result is converted to a string using the XPath string() function. If the result of the XPath expression is a node-set, then the output of the transform is a string containing the text rendering of the nodes in the node-set. The nodes are selected for rendering based on the document order (as defined in [XPath]) of the canonicalized input resource. The text rendering is performed in accordance with [XML-C14N].

It is RECOMMENDED that the XPath be constructed such that the result of this operation is a well-formed XML document. This should be the case if root element of the input resource is included by the XPath (even if a number of its descendant elements and attributes are omitted by the XPath).

6.6.4 XSLT Transform


The Transform element content MUST conform to the XSL Transforms [XSLT] language syntax. The processing rules for the XSLT transform are stated in the XSLT specification [XSLT].

7.0 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 document we distinguish between the canonicalization of a Signature data object 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 three categories. There are those related to the basic [XML], as described in 7.1 below. There are those related to [DOM], [SAX], or similar processing as described in 7.2 below. And, third, there is the possibility of character set conversion, such as between UTF-8 and UTF-16, both of which all XML standards compliant processors are required to support. Any canonicalization algorithm should yield output in a specific fixed character set. For both the minimal canonicalization defined in this document and the W3C Canonical XML [XML-c14n], that character set is UTF-8.

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,

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

Note that items (2), (4), (5C), and (6) depend on specific Schema, DTD, or similar declarations. In the general case, such declarations will not be available to or used by the signature verifier. Thus, to interoperate between different XML implementations, the following syntax contraints MUST be observed when generating any signed material to be processed as XML, including the SignedInfo element:

  1. attributes having default values be explicitly present,
  2. all entity references (except "amp", "lt", "gt", "apos", and "quot" which are pre-defined) be expanded,
  3. attribute value white space be normalized, and
  4. insignificant white space not be generated within elements having element content.

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 the 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 syntax constraints given in section 7.1 be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same byte sequence that was signed.

8.0 Security Considerations

The XML Signature specification provides a very flexible digital signature mechanism. Implementors must give consideration to their application threat models and to the following factors.

8.1 Only What is Signed is Secure

A requirement of this specification is to permit signatures to "apply to a part or totality of a XML document." [3.1.3 XML-Signature-RD] The Transforms mechanism meets this requirement by permitting one to sign data derived from processing the content of the identified resource. For instance, applications that wish to sign a form, but permit users to enter limited field data without invalidating the form itself might use XPath [XPath] to exclude those portions the user needs to change. Transforms may be arbitrarily specified and may include canonicalization instructions or even XSLT transformations. Of course, signatures over such a dervied document do not secure any information discarded by the Transforms.

Furthermore, core validation behavior does not confirm that the signed data was obtained by applying each step of the indicated transforms. (Though it does check that the digest of the resulting content matches that specified in the signature.)  For example, some application may be satisfied with verifying an XML signature over a cached copy of already transformed data. Other application might require that content be freshly dereferenced and transformed.

8.2 Only What is "Seen" Should be Signed

If signing is intended to convey the judgment or consent of an automated mechanism or person concerning some information, then it is normally necessary to secure as exactly as practical the information that was presented to that mechanism or person. Note that this can be accomplished by literally signing what was presented, for example the screen images shown a user. However, this may result in data which it is difficult for subsequent software to manipulate. It can be effective instead to secure the full data along with whatever filters, style sheets, or the like were used to control the part of the information that was presented.

8.3 Check the Security Model

This standard specifies public key signatures and keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified additions which may have other models.

With public key signatures, any number of parties can hold the public key and verify signatures while only the parties with the private key can create signatures. The number of holders of the private key should be minimized and preferably be one. Confidence by verifiers in the public key they are using and its binding to the entity or capabilities represented by the corresponding private key is an important issue, usually addressed by certificate or online authority systems.

Keyed hash authentication codes, based on secret keys, are typically much more efficient in terms of the computational effort required but have the characteristic that all verifiers need to have possession of the same key as the signer. Thus any verifier can forge signatures.

This standard permits user provided signature algorithms and keying information designators. Such user provided algorithms may have further different security models. For example, methods involving biometrics usually depend on a physical characteristic of the authorized user which can not be changed the way public or secret keys can be and may have other security model differences.

8.4 Key Lengths, Algorithms, Certificates, Etc.

The strength of a particular signature depends on all links in the security chain. This includes the signature and digest algorithms used, the strength of the key generation [RANDOM] and the size of the key, the security of key and certificate authentication and distribution mechanisms, certificate chain valiation policy, protection of cryptographic processing from hostile observation and tampering, etc. The security of an overall system would also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. The factors listed in this paragraph, while critical to the overall security of a system, are mostly beyond the scope of this document.

9.0 Schema, DTD, and Data Model

XML Signature Schema Instance
XML schema instance validates to 19991207 Schema DTD [XML-Schema].
XML Signature DTD
XML DTD that could still use improvement.
RDF Data Model

10.0 Example syntax

XML Signature Example
Well formed XML that validates under the schema and DTD. The non-normative cryptographic values were generated by Ed Simon and Tamura Kent. The text below was edited for readability.
<Signature xmlns=""> 
  <SignedInfo Id="mypage"> 
    <SignatureMethod Algorithm=""> 
    <Reference URI=""> 
        <Transform Algorithm=""/>
        <Transform Algorithm=""/>
      <DigestMethod Algorithm=""> 
    <Reference URI=""> 
        <Transform Algorithm=""/> 
      <DigestMethod Algorithm=""> 
    <KeyValue xmlns:java=""

11.0 Definitions

Authentication, Message
"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
"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] See non-repudiation.
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].
The inability to change a message without also changing the signature value. See message authentication.
The inability of a key holder to assert that their key was not associated with a message given a strong signature algorithm. (This definition speaks nothing of the number of key holders, the key length, whether the key is comprised, whether the signature was coerced, etc.) See signer authentication.
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.
"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 being operated upon.
A value generated from the application of a key to a message via a cryptographic algorithm such that it has the properties of signer authentication, integrity, and non-repudiation. A signature may be (non-exclusively) described as detached, enveloping, or enveloped.
Signature, Detached
The signature is over content external to the Signature element, and can be identified via a URI, IDREF, 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 the Object element of the signature itself. The Object(or its content) is identified via a Reference (via IDREF or transform). The enveloping Signature element is used to provide the root document element.
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.
The processing of a byte stream from source content to derived content. 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 section 6.2.
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.

12.0 References

Digital Signature Guidelines.
Document Object Model (DOM) Level 1 Specification.
Internet Draft. Digest Values for DOM (DOMHASH) .
FIPS PUB 186-1. Digital Signature Standard (DSS). U.S. Department of Commerce/National Institute of Standards and Technology.
RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H. Krawczyk, M. Bellare, R. Canetti. INFORMATIONAL.
RFC 2616.Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee.
RFC2119 -- Key words for use in RFCs to Indicate Requirement Levels.
RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. INFORMATIONAL.
RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. N. Freed & N. Borenstein. DRAFT STANDARD.
RFC1750 -- Randomness Recommendations for Security.
RDF Schema
RDF Model and Syntax
RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0. B. Kaliski, J. Staddon. INFORMATIONAL.
David Megginson et. al. SAX: The Simple API for XML May 1998.
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.
RFC2396 - Uniform Resource Identifiers (URI): Generic Syntax
RFC1738. Uniform Resource Locators (URL). Berners-Lee, T., Masinter, L., and M. McCahill . December 1994.
RFC 2141. URN Syntax. R. Moats. PROPOSED STANDARD.
RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D. van Gulik, R. Iannella, P. Falstrom. BEST CURRENT PRACTICE.
XML Linking Language
Extensible Markup Language (XML) Recommendation.
Canonical XML. W3C Working Draft
Namespaces in XML
XML Path Language (XPath)Version 1.0. W3C Proposed Recommendation
XML Pointer Language (XPointer). W3C Working Draft.
XML Schema Part 1: Structures
XML Schema Part 2: Datatypes
XML-Signature Requirements
Extensible Stylesheet Language (XSL) W3C Working Draft
XSL Transforms (XSLT) Version 1.0. W3C Proposed Recommendation
Web Architecture: Describing and Exchanging Data.