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XML-Signature Core Syntax and Processing

W3C Working Draft 19-November-1999

This version:
http://www.w3.org/TR/1999/WD-xmldsig-core-19991119  [ietf-ascii]
http://www.ietf.org/internet-drafts/draft-ietf-xmldsig-core-02.txt
Latest version:
http://www.w3.org/TR/xmldsig-core
Previous version:
http://www.w3.org/TR/1999/WD-xmldsig-core-19991022  [ietf-ascii]
http://www.ietf.org/internet-drafts/draft-ietf-xmldsig-core-01.txt
Editor(s)
Donald Eastlake <dee3@torque.pothole.com>
Joseph Reagle <reagle@w3.org>
David Solo <dsolo@alum.mit.edu>
Authors
Mark Bartel <mbartel@JetForm.com>
John Boyer <jboyer@uwi.com>
Barb Fox <bfox@Exchange.Microsoft.com>
Contributors
See Acknowledgements

Abstract

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

Status of this document

This is a public WG Draft that follows the November IETF meeting. Consequently it includes a editoral changes and recrafting though no major design changes. This version includes the experimental use of XML Schema and XML entity references. The XML schema declarations within the specification may contain errors, though the complete WG schema definition does validate to the Schema DTD. We expect the final draft will include a DTD and schema.

Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>. 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 http://www.w3.org/TR

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


1.0 Introduction

This document describes the proposed syntax and processing rules for the XML Digital Signature specification. This specification provides a mechanism for applying digital signatures to XML documents and other Internet resources and encoding those signatures as XML.

The structure allows for both embedded and detached signatures. An embedded signature can include the signature within the signed object or embed the signed object within the signature. A detached signature allows the signature to be independent of the object. The processing structure allows for switching between embedded and detached signatures without necessarily invalidating the signature.

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].

This document includes a list of open issues which are still being addressed by the working group and 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 Namespaces and Identifiers

The XML namespace [XML-namespace] URI that MUST be used by experimental implementations of this dated specification is:

xmlns="http://www.w3.org/1999/11/xmldsig-core"

While applications MUST support XML and XML-namespaces, the use of internal entities 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 specifications namespace
http://www.w3.org/1999/11/dsig-core/SignatureProperties
XSLT is identified and defined by an external namespace
http://www.w3.org/TR/1999/PR-xslt-19991008
SHA1 is identified via this specification's namespace and defined via a normative reference
http://www.w3.org/1999/11/dsig-core/SHA1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.

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

<?xml version="1.0" ?>
<!DOCTYPE Signature SYSTEM "xmldsig.dtd" [
<!ENTITY dsig 'http://www.w3.org/1999/10/signature-core'>]>
  ...
  <SignedInfo>
    <SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
    ...
  </SignedInfo>

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. Furthemore, we define this entity as part of the Signature XML Schema such that one does not have to rely upon an internal subset declaration. However, since this specification presently permits a CanonicalizationMethod of null over SignedInfo, entity declarations will not be expanded in those instances (or where the schema is not present) and we have not completely assessed the security risk.

1.4 Versions

No provision is made for an explicit version number in this syntax. If a future version is needed, it is expected to use a different Namespace.

2.0 Signature Overview

This section provides an overview of XML digital signature syntax and processing. The formal specification is provided in 3. The editors assume the reader is familiar with basic digital signature and XML concepts.

2.1 The Signature Element

XML digital signatures are very flexible and may be used to apply signatures to any type of resource. The resource(s) being signed may be included within the signature, outside the signature in the same document, or completely outside of the document.

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

<Signature>
  (SignedInfo)
  (SignatureValue)
  (KeyInfo)?
  (Object)*
</Signature>

The required SignedInfo element is the information which is actually signed. SignedInfo includes a digest calculated over each of the data objects being signed. The core signature verification includes the verification of these digests. The algorithms used in calculating the SignatureValue are also included in the signed information. The signature can not cover itself so the SignatureValue element is outside SignedInfo.

KeyInfo indicates what key was used to create the signature, such as 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. However, if the signer wishes to bind the keying information to the signature, an ObjectReference can easily identify and include the KeyInfo as part of the signature.

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

Signature properties, such as time of signing, can be included in the SignatureProperties element. (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.

<Signature>
  <SignedInfo>
    (CanonicalizationMethod)?
    (SignatureMethod)
    (ObjectReference)+
  </SignedInfo>
  (SignatureValue)
  (KeyInfo)?
  (Object)*
</Signature>

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.

The ObjectReference element identifies a resource, specifies any transformations, specifies the digest algorithm, and includes the resulting digest value. A resource is signed by computing the contents digest value and the signature over that value. The signature is later checked via resource (defn) and signature validation (defn).

2.3 The ObjectReference Element

The ObjectReference element has the structure indicated below.

...
<SignedInfo>
   (CanonicalizationMethod)?
   (SignatureMethod)
   <ObjectReference (URI=? | IDREF=?) Type=?>
     (Transforms)?
     (DigestMethod)
     (DigestValue)
   </ObjectReference>+
</SignedInfo>
...

The optional URI/IDREF attribute of ObjectReference idenitifies the signed resource.

Several mechanisms are provided for maintaining signature validity over resources which can not be persistently identified via a URL. First, no pointer to the signed object need be given at all for one ObjectReference in a Signature. Second, objects within ObjectReference need not be identified via URLs, instead location independent URIs (such as a URN or other URI schemes) are permitted -- by definition. Note, if a URL is used to identify an ojbect, this acts as an assertion by the signer that they are signing the content of the dereferenced URL. Third, the ObjectReference may reference a Manifest or the like which references instructions for dereferencing the appropriate content.

The optional Type attribute provides information about the content of the resource identified by URI/IDREF. In particular, it can indicate that an Object contains a SignatureProperties, Manifest, or Package elements.

Transforms is an optional ordered list of processing steps that are applied to the resource's content before it is digested. Transforms can include arbitrary specifications such as canonicalization, encoding/decoding (including compression/inflation), XSLT and XPath. XSLT/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 we address 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 identified by the URI/IDREF is digested directly.

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

DigestMethod is the algorithm applied to the object after Transforms is applied 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 and Package Elements

There are cases where it is efficient to have one signature cover many items. One approach is to include multiple ObjectReferences within SignedInfo. Since the core verification behavior includes verifying the digests of objects referenced within SignedInfo, some applications may need an alternative approach which allows pushing the validation decision to the application. This allows more complex processing to be defined on an application specific basis. For example, it may be sufficient if the signature's validity for n out of m of the items can be verified or there may be a large number of items that it is desired to sign with multiple signature algorithms and / or keys where listing all of the items within the SignedInfo element of each Signature is too verbose.

To answer these requirements, additional object types have been defined which may be referenced by SignedInfo. The Manifest element is provided which similarly contains a collection of references and objects (like SignedInfo), but leaves it entirely up to the application which digest or digests it will verify. Multiple signatures over the possibly large number of items in a Manifest need only point to the Manifest from one ObjectReference in each signature's SignedInfo.

The structure of Manifest, which reuses the ObjectReference and Object elements described above, is as follows:

<Manifest>
  (ObjectReference)+
  (Object)*
</Manifest>

A Package is syntactically identical to a Manifest but asserts the identity of each of its ObjectReference elements after Transforms application.

Manifest and Package may appear as the content of an Object.

2.5 The SignatureProperties Element

Statements or assertions concerning data blocks should be included in those data blocks or in other data blocks signed in parallel with them. Statements about the signature process itself, however, such as time of signing or serial number or hardware used in calculation of the signature, can be included in a SignatureProperties block. Such blocks can be signed, via an ObjectReference, or not, as appropriate.

<SignatureProperties>
  <SignatureProperty Target= >
    (ObjectReference)+
    (Object)*
  </SignatureProperty>*
</Manifest>

The structure of SignatureProperties is shown above. It reuses the ObjectReference and Object elements. The mandatory Target attribute references the element to which the property applies.

3.0 Core Signature Syntax

The general structure of an XML signature is described in section 2 above. This section provides detailed syntax of the core signature features and actual exampes. The syntax is defined via [XML-Schema] with the following XML preamble, declaration, and internal entity:

<?xml version='1.0'?>
<!DOCTYPE schema
SYSTEM 'http://www.w3.org/TR/1999/WD-xmlschema-1-19991105/structures.dtd'
[
<!ENTITY dsig 'http://www.w3.org/1999/10/signature-core'>
]>
<schema targetNS='http://www.w3.org/1999/10/signature-core'
   version='0.1'
   xmlns='http://www.w3.org/1999/XMLSchema'>
<textEntity name="dsig">http://www.w3.org/1999/10/signature-core</textEntity>

3.1 The Signature element

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

<!DOCTYPE Signature [
<!ENTITY dsig 'http://www.w3.org/1999/10/signature-core'>]>
<Signature xmlns="http://www.w3.org/1999/11/xmldsig-core">
  <SignedInfo>
    <!ENTITY dsig "http://www.w3.org/1999/11/xmldsig-core">
    <CanonicalizationMethod
     Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
    <SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
    <ObjectReference Location="http://www.mypage.com">
      <DigestMethod Algorithm="&dsig;/sha1"/>
      <DigestValue encoding="&dsig;/base64">a23bcd43</DigestValue>
    </ObjectReference>
  </SignedInfo>
  <SignatureValue >dd2323dd</SignatureValue>
  <KeyInfo>
     <keyname>Solo</keyname>
  </KeyInfo>
</Signature>

Note: this example will be revised to include generated hash/signature values that validate.

<element name='Signature'>
  <archetype order='seq' content='elemOnly'>
    <element ref='SignedInfo'/>
    <element ref='SignatureValue'/>
    <element ref='KeyInfo' minOccurs='0' maxOccurs='1' />
    <element ref='Object' minOccurs='0' maxOccurs='*'/>
  </archetype'>
</element>

3.2 The SignatureValue Element

The SignatureValue element contains the actual value of the digital signature. The encoding of this value is determined by the SignatureMethod used. For all SignatureMethods specified herein, that encoding is Base 64 [RFC2045]. The ability to define a SignatureMethod and SignatureValue pair which includes multiple distinct signatures is explicitly permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").

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

3.3 The SignedInfo Element

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

<element name='SignedInfo'>
  <archetype order='seq' content='elemOnly'>
    <element ref='CanonicalizationMethod' minOccurs='0' maxOccurs='1'/>
    <element ref='SignatureMethod'/>
    <element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
    <attribute name='Id' type='ID' />
  </archtype>
</element>

SignedInfo does not include explicit signature properties. If an application needs to associate properties (such as signing time, signing device, etc.) with the signature, it may add an additional Object that includes that data and reference that Object via an ObjectReference. See the SignatureProperties element below.

3.3.1 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 in which a URI is used to identify the algorithm and the contents of the element contain any parameter needed by the algorithm. Possible options may include a minimal algorithm (CRLF and charset normalization), or more extensive operations such as [XML-C14N]. An expected default for this value will be defined once the specification of XML aware canonicalization algorithms are finalized. If the CanonicalizationMethod is omitted, no change is made to SignedInfo..

<element name='CanonicalizationMethod'>
  <archetype>
    <attribute name='Algorithm' type='uri' />
  </archetype>
</element>

3.3.2 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, etc.). This element uses the general structure here for algorithms in which a URI is included as an attribute naming the algorithm and contents of the element contain any parameter needed by the algorithm. While there is a single identifier, that identifier may specify a format containing multiple distinct signature values.

<element name='SignatureMethod'>
  <archetype>
    <attribute name='Algorithm' type='uri' />
  </archetype>
</element>

3.3.3 The ObjectReference Element

ObjectReference is an element that may occur one or more times. It identifies the object being signed, the type of the object, an optional list of transforms to be applied prior to digesting, a digest algorithm and digest value. An optional ID attribute permits an ObjectReference to be easily referenced from elsewhere.

<element name='ObjectReference' minOccurs='1' maxOccurs='*'>
  <archetype order='seq'>
    <element ref='Transforms' minOccurs='0' maxOccurs='*'/>
    <element ref='DigestMethod'/>
    <element ref='DigestValue'/>
    <attribute name='Id' type='ID' />
    <attribute name='URI' type='uri' />
    <attribute name='IDREF' type='IDREF' />
    <attribute name='Type' type='string' />
  </archetype>
</element>

The URI/IDREF attribute identifies the Object using a URI [URI] or IDREF [XML]. We distinguish between URIs and IDREFs so as to provide expositional clarity and ease signature processing in the face of 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 ObjectReferences to identify a fragment of a document that is encoded: the ObjectReference identifies the resource, the first Transform specifies decoding, the second Transform specifies the fragement.

Note that a null URI (URI="") is permitted and identifies the parent document.

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 ommit this attribute given the identity of the object is part of the application context. This attribute may be omitted from at most one ObjectReference in any particular SignedInfo, Manifest, or Package.

The digest algorithm is applied to the content yielded after the URI is dereferenced, decoded, and transformed. If the URI indicates an XML document, the document is assumed to be unparsed prior to the application of Transforms. If there are no Transforms, then the indicated resource is passed to the digest algorithm unmodified.

The optional Type attribute contains information about the type of object being signed (e.g. manifest, package, signature properties, document). This is represented as a URI. For example:

Type="&dsig;/Manifest"
Type="&dsig;/SignatureProperty"

3.3.3.1 The Transforms Element

Transforms is an optional element that contains one or more operations to be performed on an indicated resource prior to digest calculation. (These operations are different from the CanonicalizationMethod specified in the Signature that id applied over SignedInfo.) If the Transforms element is omitted, the exact data referenced is digested byte for byte.

The Transforms element contains an ordered list of Transform elements. The output of each Transform serves as input to the next Transform. The input to the first Transform is the raw data yielded by dereferencing the resource identifier. The output from the last Transform is the input for the DigestMethod algorithm.

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.

<element name='Transforms' minOccurs='0' maxOccurs='1'>
  <archetype>
    <element ref='Transform'/>
    <attribute name='Algorithm' type='string' />
  </archetype>
</element>

<element name='Transform' minOccurs='1' maxOccurs='*'>
  <archetype>
    <attribute name='Algorithm' type='uri' />
    <attribute name='Encoding' type='uri' />
    <attribute name='Type' type='uri' />
  </archetype>
</element>

Examples of resource transforms include but are not limited to base-64 decoding [RFC2045], canonicalization [XML-c14n], XPath filtering [Xpath], and XSLT [XSLT]. The generic definition of the Transform element also allows application-specific transform algorithms. For example, the transform could be a decompression routine given by a Java class appearing as a base-64 encoded parameter to the Java Transform algorithm. However, applications should refrain from using application-specific transforms whenever possible since the resulting signature will not necessarily be verifiable outside of the application domain. The section Transform Algorithms defines the list of standard transformations.

Implementation Comment: When transformations are applied the signer is not signing the native (original) document but the resulting (transformed) document.

3.3.3.2 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 in which a URI is included as an attribute naming the algorithm and optional contents of the element contain any parameter needed by the algorithm.

<element name='DigestMethod'>
  <archetype>
     <element name='Parameter' minOccurs='0' maxOccurs='*'/>
     <attribute name='Algorithm' type='uri' />
  </archetype>
</element>

3.3.3.3 The DigestValue Element

DigestValue is an element which contains the encoded value of the digest. The optional Encoding attribute gives the encoding method which defaults to Base 64 [RFC2045].

<element name='DigestValue' type='string'>
  <archetype>
    <attribute name='Encoding' type='uri' default="&dsig;/Base64"/ />
  </archetype>
</element>

3.4 The KeyInfo Element

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

<element name='KeyInfo' minOccurs='0' maxOccurs='1'>
  <archetype order='seq' content='any'>
    <element name='KeyName' type='string' />
    <element name='KeyValue' type='string' />
    <element name='SubjectName' type='string' />
    <element name='RetrievalMethod' type='string' />
    <element ref='X509Data' type='string'>
    <element ref='PGPData' type='string'>
    <element name='MgmtData' type='string' minOccurs='0' maxOccurs='1'/>
  </archetype>
</element>

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. This element contains one KeyInfo data element providing information for the recipient(s). 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.

<element name='X509Data' type='string'>
  <archetype order='seq' content='any'>
    <element name='X509IssuerSerial' type='string'>
      <archetype order='seq' content='elemOnly'>
        <element name='X509Name' type='string' />
        <element name='X509SerialNumber' type='string' />
      </archetype>
    </element>
    <element name='X509SKI' type='string' />
    <element name='X509Name' type='string' />
    <element name='X509Certificate' type='string' />
    <element name='X509CRL' type='string' />
  </archetype>
</element>

<element name='PGPData' type='string'>
  <archetype order='seq' content='elemOnly'>
    <element name='PGPKeyID' type='string' />
    <element name='PGPKeyPacket' type='string' />
  </archetype>
</element>

<element name='MgmtData' type='string' minOccurs='0' maxOccurs='1'/>
  </archetype>
</element>

Note:  This section is preliminary. A more detailed version will be included in a subsequent version of this specification.

3.5 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 type, ID, and encoding attributes.

The Object's ID is referenced from the ObjectReference in SignedInfo. This element is used for embedded 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. If the application wishes to exclude the <Object> tags from the digest calculation a transform must be used. (Exclusion of the object tags may be desired for cases where the signature is intended to survive a change between embedded and detached objects 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.)

<element name='Object' minOccurs='0' maxOccurs='*'>
  <archetype content='any'>
    <attribute name='Id' type='ID' />
    <attribute name='Type' type='uri' />
    <attribute name='Encoding' type='uri' />
  </archetype>
</element>

4.0 Additional Signature Syntax

This section describes the optional to implement Manifest and Package elements and describes the handling of XML Processing Instructions and Comments.

4.1 The Manifest and Package Elements

The Manifest element provides a list of ObjectReferences. 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 verification 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.

A Package has the same syntax as a Manifest but also asserts the equality of each of its referenced objects, after any transforms. The testing of this equality and action if it fails is also entirely at the discretion of the applicaiton.

<element name='Manifest'>
  <archetype>
    <element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
    <element ref='Object' minOccurs='0' maxOccurs='*'/>
    <attribute name='Id' type='id' />
  </archetype>
</element>

<element name='Package'>
  <archetype>
    <element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
    <element ref='Object' minOccurs='0' maxOccurs='*'/>
    <attribute name='Id' type='id' />
  </archetype>
</element>

4.2 The SignatureProperties Element

Additional information items concerning the signature or particular ObjectReferences can be placed in SignatureProperty elements within a SignatureProperties element within an Object. This should be such information as signing time or the serial number of crypto hardware used. An additional information concerning data being signed should be with that data.

<element name='SignatureProperties' >
  <archetype >
    <element ref='SignatureProperty' minOccurs='1' maxOccurs='*'>
    <attribute name='Target' type='idref' />
  </archetype>
</element>

4.3 Processing Instructions

TDB - will specify the use, if any, of XML processing instructions by this specification and the handling of PIs appearing within elements specified in this document.

4.4 Comments in dsig Elements

TDB - will specify the use, if any, and handling of XML comments appearing within elements specified in this document.

5.0 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.

5.1 Algorithm Identifiers, Parameters, 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 they 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 description element name, which is frequently algorithm specific, and MUST be in 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
Digest      
  SHA1 REQUIRED &dsig;/sha1
Encoding      
  Base64 REQUIRED &dsig;/base64
  QuotedPrintable RECOMMENDED &dsig;/qp
MAC      
  HMAC-SHA1 REQUIRED &dsig;/hmac-sha1
Signature      
  DSAwithSHA1 (DSS) REQUIRED &dsig;/dsa
  RSAwithSHA1 RECOMMENDED &dsig;/rsa-sha1
  ECDSAwithSHA1 OPTIONAL &dsig;/ecdsa
Canonicalization      
  minimal REQUIRED &dsig;/minimal
  XML-Canonicalization RECOMMENDED http://www.w3.org/1999/07/WD-xml-c14n-19990729
Transform      
  XSLT RECOMMENDED http://www.w3.org/TR/1999/PR-xslt-19991008
  XPath RECOMMENDED http://www.w3.org/TR/1999/PR-xpath-19991008
  XPointer RECOMMENDED http://www.w3.org/1999/07/WD-xptr-19990709
  Java OPTIONAL urn:ECMA-org:java

5.2 Message Digests

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

Digest algorithms take as an implicit parameter a byte string to be digested.

5.2.1 SHA-1

The SHA-1 algorithm  [SHA-1] identifier is &dsig;/sha1. The SHA-1 algorithm takes no explicit parameters. An example of an SHA-1 DigestAlg element is

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

An 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

<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>

5.3 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.

5.3.1 HMAC

The HMAC algorithm [HMAC] identifiers are &dsig;/hmac-sha1. The HMAC algorithm takes the truncation length in bits as a parameter (parameter identifier urn:ietf-org:hmac-outputlength). An example of an HMAC SignatureMethod element:

<SignatureMethod Algorithm="&dsig;/hmac-sha1">
  <hmac-outputlength xmlns="&dsig;/hmac-sha1">
     128
  </hmac-outputlength>
</SignatureMethod>

The output of the HMAC algorithm is ultimately the output (possibly truncated) of the chosen digest algorithm. This value shall be base64 encoded in the same straightforward fashion as the output of the digest algorithms. Example: the SignatureValue element for the HMAC-MD5 digest

9294727A 3638BB1C 13F48EF8 158BFC9D

from the test vectors in [RFC 2104] would be

<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>

5.4 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.

5.4.1 DSA

The DSA algorithm [DSA] identifier is &dsig;/dsa. The DSA algorithm 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 PKCS #1 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

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

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 DssKeyValue (P, Q, G, Y, J?, (seed, pgenCounter)?) >
<!-- Each of these fields consists a CDATA where the data is base64 encoded -->

5.4.2 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 identifiers are &dsig;/rsa-sha1 and urn:rsasdi-com:rsa-md5. 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 RSAKeyValue ( Modulus, Exponent ) > <!-- Each field contains a CDATA which is the value for that item base64 encoded -->

5.4.3 ECDSA

The expression ECDSA  [ECDSA] as used in this document refers to the signature algorithms specified in ANSI X9.62.  Additional details are to be provided.

5.5 Canonicalization Algorithms

5.5.1 Null Canonicalization

Null canonicalization, i.e., no modification whatsoever, can be achieved for signed data by simply not putting any canonicalization in the Transforms element (omitting it entirely if no other tranforms are needed) for a data object or omitting CanonicalizationMethod for SignedInfo.

5.5.2 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:

This algorithm is only applicable to XML resources.

5.5.3 Canonical XML

The algorithm identifier for XML canonicalization is http://www.w3.org/1999/07/WD-xml-c14n-19990729. An example of an XML canonicalization element is

<CanonicalizationMethod Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>

See the Canonical XML specification.

5.6 Transform Algorithms

A Transform algorithm has three implicit parameters. The first is the byte stream from the ObjectReference URI/IDREF 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 severe 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.

5.6.1 Canonicalization

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

5.6.2 Base-64 and Quoted-Printable Decoding

The Algorithm values for the base 64 and quoted-printable decoding transforms [RFC2045] are &dsig;/base64 and &dsig;/qp.

The base-64 Transform element has no content. The input (from the URI/IDREF or from the previous Transform) 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.

5.6.3 XPath Filtering

The Algorithm value for the XPath filtering transform is "http://www.w3.org/TR/1999/PR-xpath-19991008"

The Transform element content MUST conform to the XML Path Language (XPath) syntax.

XPath assumes that an XML processor has processed the input resource. So, for example, entity reference expansion, normalization of linefeeds and attribute values are normalized, and CDATA section replacement are expected. As well, XPath joins all consecutive text characters into a single text nodes.

The input resource MUST be a well-formed XML document. The result of applying the XPath to the input resource MUST be a node-set (as defined in XPath). The output of this transform is a new XML document with the following characteristics:

  1. The output document has the XML declaration of the input resource (see rule 23 XMLDecl in XML specification). If the encoding is UTF-16, the output document has the same byte order mark as the input resource.
  2. The output document contains the nodes in the node-set identified by the XPath, and excludes the nodes of the input resource that are not not in the node-set identified by the XPath.
  3. The nodes in the output document appear in the document order (as defined in XPath) of the input resource.
  4. The output document has all of the input resource's entity references expanded, except that characters corresponding to illegal XML are reencoded as character references (XML rule 66) except the ampersand and less than symbol, which are encoded using &amp; and &lt;, respectively.
  5. Attribute values are normalized in accordance with the rules for a validating XML processor (even if the implementation did not use a validating XML processor to parse the input resource).

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).

5.6.4 XPointer Filtering

The Algorithm value for the XPointer filtering transform is "http://www.w3.org/1999/07/WD-xptr-19990709".

The Transform element content MUST conform to the XML Pointer Language (XPointer) syntax.

The processing rules for XPointer filtering are identical to those for XPath filtering (stated above), except that the additional functionality offered by XPointer can be utilized in constructing the output node-set.

The XPointer filter is particularly important if the input resource is processed by a validating XML processor since the XPointer barename shortcut could then be used to implement the well-known fragment identification by ID attribute.

NOTE: In application environments with severe resource limitations, applications MAY constrain XPointer support to barename processing and also to determination of the ID attribute by means other than a validating XML processor. In fact, the use of an XML processor for barename resolution is OPTIONAL. However, the output expectations of this transform MUST be supported by the application.

5.6.5 XSLT Transform

The Algorithm value for the XSLT transform is "http://www.w3.org/TR/1999/PR-xslt-19991008"

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.

5.6.6 Java Transform

The Algorithm value for the Java transform is urn:ECMA-org:java.

Details to be determined.

Although the Algorithm attribute of a Transform can take application-specific values, having a Java transform seems to be the most reasonable way to allow application-specific transforms that can be processed outside of the application domain.

6.0 Processing rules

These sections describe the operations to be performed as part of signature generation and validation. The description is of a logical behavior and does not specify an order of execution, nor specify discrete steps.

6.1 Generation

  1. apply Transforms determined by application to each object being signed.
  2. calculate digest over each transformed object (including start and end tags)
  3. create ObjectReference element(s) including location of object, digest, digest algorithm, and transform elements, if required.
  4. create SignedInfo element with SignatureMethod, CanonicalizationMethod if required, and ObjectReference(s).
  5. canonicalize and calculate signature over SignedInfo based on algorithms in step 4.
  6. construct signature document with SignedInfo, Object (s) (if desired, encoding may be different than that used for signing), KeyInfo (if required), and SignatureValue.

6.2 Signature Validation

  1. locate object and apply Transforms to the specified resource based on each ObjectReference(s) in the SignedInfo element. Each transform is applied in order from left to right to the object with the output of each transform being the input to the next.
  2. calculate digest over each transformed signed object(s) (including start and end tags) based on the algorithm in ObjectReference(s).
  3. compare value against DigestValue in SignedInfo for each reference (if any mismatch, validation fails).
  4. canonicalize the SignedInfo element based on the CanonicalizationMethod, if any, in SignedInfo.
  5. obtain the validation keying info from KeyInfo or externally.
  6. validate the SignatureValue based on the SignatureMethod in the SignedInfo element, the key obtained in step 5, and the results of step 4. - Digest calculation is performed over the SignedInfo element including start and end tags.

Any processing beyond cryptographic validation (e.g. certificate validation, applicability decisions, time related processing) is outside the scope of this specification.

7.0 Security Considerations

The XML digital signature standard provides a very flexible mechanism. In designing a system to make use of it, due consideration should be given to the threat model being defended against and to the factors covered in the subsections below.

7.1 Only What is Signed is Secure

The flexible Transforms mechanism, including canonicalization and explicit filtering and extraction, permit securing only a subset of data in an object. This is good for many applications where a limited portion of an object must change after the signature or different signatures secure different parts or the application modifies aspects of the object that are not significant and can be omitted from signature coverage or the like. Keep in mind that whenever this is done, those aspects that are not signed can be arbitrarily modified and the signature will still validate.

7.2 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.

7.3 Check the Security Model

This standard specifies public key signatures and secret key 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 secret key can create signatures. The number of holders of the secret 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 secret key is an important issue, usually addressed by certificate or on line 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 "key" which is a physical characteristic of the user and thus can not be changed the way public or secret keys can be and may have other security model differences.

7.4 Algorithms, Key Lengths, 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 [RFC 1750] and the size of the key, the security of key and certificate authentication and distribution mechanisms, protection of all 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.

8.0 Example syntax

<Signature xmlns="http://www.w3.org/1999/11/xmldsig-core">
  <SignedInfo ID="5">
    <CanonicalizationMethod
     Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
    <SignatureMethod Algorithm="&dsig;/dsa"/>
    <ObjectReference URI="http://www.mypage.com">
      <Transforms>
         <Transform Algorithm="&dsig;/null">
         <Encoding Algorithm="&dsig;/base64"/>
      </Transforms>
      <DigestMethod Algorithm="&dsig;/sha1"/>
      <DigestValue>a23bcd43</DigestValue>
    </ObjectReference>
    <ObjectReference IDREF="timestamp"
       Type="&dsig;/signatureattributes">
      <Transforms>
         <CanonicalizationMethod name="http://..."/>
      </Transforms>
      <DigestMethod Algorithm="&dsig;/sha1"/>
      <DigestValue>a53uud43</DigestValue>
    </ObjectReference>
  </SignedInfo>
  <SignatureValue>dd2323dd</SignatureValue>
  <Object ID="timestamp"
   type="&dsig;/SignatureAttributes" >
    <timestamp about="5"
     xmlns="http://www.ietf.org/rfcXXXX.txt">
      <date>19990908</date>
      <time>14:34:34:34</time>
    </timestamp>
  </Object>
  <KeyInfo>
     <keyname>Solo</keyname>
  </KeyInfo>
</Signature>

9.0 Schema

[TBD: xmldsig-core-schema]

10 Definitions

[needs work]

Authentication, Message
?
Authentication, Signer
?
Data
?
Object
An XML element defined by this specification for embedding resources within a signature.
Resource
"A resource can be anything that has identity." [URI]
Validation, Resource
When the value of hash over the transformed content yielded from the dereferenced URI matches the DigetsValue in SignedInfo.
Validation, Signature
When the SignatureValue matches the result of processing SignedInfo with  CanonicalizationMethod and SignatureMethod as specified in 6.2, including the resource validation of SignedInfo ObjectReferences.
Validation, Trust
When the application determines that the semantics associated with the signature are valid. For example, the validation of time stamps or confirming the integrity of the signer key.

11.0 Other Useful Types (normative)

We define the following types for use in identifying XML resources that include Signture semantics.

http://www.w3.org/1999/11/xmldsig-core/SignatureProperties
designates that the referenced resource is a statement about the referring signature.
http://www.w3.org/1999/11/xmldsig-core/Manifest
designates that the referenced resource is a collection of other resources.
http://www.w3.org/1999/11/xmldsig-core/Package
designates that the referenced resources is a collection of other resources and the creator of that collection asserts that the specified resources, when transformed as specified, yield the same exact content.

12.0 References

DOMHASH
Internet Draft. Digest Values for DOM (DOMHASH)
http://search.ietf.org/internet-drafts/draft-hiroshi-dom-hash-01.txt .
DSS
FIPS PUB 186-1. Digital Signature Standard (DSS). U.S. Department of Commerce/National Institute of Standards and Technology.
http://www.ietf.org/rfc/rfc2104.txt
ECSDA
?ANSI X9.62
HMAC
RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H. Krawczyk, M. Bellare, R. Canetti. INFORMATIONAL.
MD5
RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. INFORMATIONAL.
http://www.ietf.org/rfc/rfc1321.txt
RDF
RDF Schema
http://www.w3.org/TR/1999/PR-rdf-schema-19990303

RDF Model and Syntax
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222
RFC1750
RFC1750 -- Randomness Recommendations for Security.
http://www.ietf.org/rfc/rfc1750.txt
RFC2045
RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. N. Freed & N. Borenstein. DRAFT STANDARD.
http://www.ietf.org/rfc/rfc2045.txt
RFC2119
RFC2119 -- Key words for use in RFCs to Indicate Requirement Levels.
http://www.ietf.org/rfc/rfc2119.txt
RSA
RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0. B. Kaliski, J. Staddon. INFORMATIONAL.
http://www.ietf.org/rfc/rfc2432.txt
SHA-1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.
http://csrc.nist.gov/fips/fip180-1.pdf
URI
RFC2396 - Uniform Resource Identifiers (URI): Generic Syntax
http://www.ietf.org/rfc/rfc2396.txt
URL
RFC1738. Uniform Resource Locators (URL). Berners-Lee, T., Masinter, L., and M. McCahill . December 1994.
http://www.ietf.org/rfc/rfc1738.txt
URN
RFC 2141. URN Syntax. R. Moats. PROPOSED STANDARD.
ftp://ftp.isi.edu/in-notes/rfc2141.txt
RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D. van Gulik, R. Iannella, P. Falstrom. BEST CURRENT PRACTICE.
ftp://ftp.isi.edu/in-notes/rfc2611.txt
XLink
XML Linking Language
http://www.w3.org/1999/07/WD-xlink-19990726
XML
Extensible Markup Language (XML) Recommendation.
http://www.w3.org/TR/1998/REC-xml-19980210
XML-Canonicalization
Canonical XML. W3C Working Draft
http://www.w3.org/1999/07/WD-xml-c14n-19990729
XML-namespace
Namespaces in XML
http://www.w3.org/TR/1999/REC-xml-names-19990114
XPath
XML Path Language (XPath)Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xpath-19991008
XPointer
XML Pointer Language (XPointer). W3C Working Draft.
http://www.w3.org/1999/07/WD-xptr-19990709
XML-schema
XML Schema Part 1: Structures
http://www.w3.org/TR/1999/WD-xmlschema-1-19991105/
XML Schema Part 2: Datatypes
http://www.w3.org/TR/1999/WD-xmlschema-2-19991105/
XML-Signature-RD
XML-Signature Requirements
http://www.w3.org/1999/08/WD-xmldsig-requirements-990820
XSL
Extensible Stylesheet Language (XSL) W3C Working Draft
http://www.w3.org/TR/1999/WD-xsl-19990421
XSLT
XSL Transforms (XSLT) Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xslt-19991008
WebData
Web Architecture: Describing and Exchanging Data.
http://www.w3.org/1999/04/WebData
 

13.0 Acknowledgements (non-normative)

14.0 Open Issues (non-normative)

  1. More detail for KeyInfo types, based on IETF'46, we need proposals for the actual XML'ized algorithm parameters.
  2. Make sure we are consistent with respect to types, algorithm IDs, URIs, etc.
  3. The signature data structures specified in this document are not yet associated with a data model.