XML Digital Signatures Working Group               J. Reagle,
INTERNET-DRAFT                                     W3C/MIT
Expires March 20, 1999

                         XML-Signature Requirements

Copyright Notice

   Copyright (c) 1999 The Internet Society & W3C (MIT, INRIA, Keio), All
   Rights Reserved.

IETF Status of this Memo

   This document is a production of the joint IETF/W3C XML Signature
   Working Group.


   The latest version of this draft series may be found at:


   This document is an Internet-Draft and is in full conformance with all
   provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering Task
   Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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W3C Status of this document

   This is a Last Call XML Signature Requirements public Working Draft.
   This report is not expected to be advanced to Recommendation. Instead,
   this Last Call designation is (1) a representation of WG consensus,
   (2) an invitation for comments that will affect the future course of
   the technical specification, and (3) an opportunity to identify and
   obtain commitments regarding WG dependencies. This document will be
   referred to at least the W3C XML Plenary Interest Group and W3C Chairs
   Working Group. Last Call period ends when  dependencies between WGs
   have been acknowledged and the Signature Chairs have procured
   commitments of review. This is expected to take six weeks from the
   date of publication.

   This document attempts to capture the Working Group's consensus though

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   it contains points which are still uncertain or not well
   specified. Issues which are still being actively discussed during the
   publication of this document are of class="discuss" and rendered in
   navy by style sheet compliant applications.

   Please send comments to the editor <reagle@w3.org> and cc: the list
   <w3c-ietf-xmldsig@w3.org>. Publication as a Working Draft does not
   imply endorsement by the W3C membership. 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


   This document lists the design principles, scope, and requirements for
   the XML Digital Signature specification. It includes requirements as
   they relate to the signature syntax, data model, format, cryptographic
   processing, and external requirements and coordination.

Table of Contents

     1. 1. Introduction
     2. 2. Design Principles and Scope
     3. 3. Requirements
          3.1 1. Signature Data Model and Syntax
          3.2 2. Format
          3.3 3. Cryptography and Processing
          3.4 4. Coordination
     4. 4. References

1 1. Introduction

   The XML 1.0 Recommendation [XML] describes the syntax of a class of
   data objects called XML documents. The mission of this working group
   is to develop a XML syntax used for representing signatures on digital
   content and procedures for computing and verifying such signatures.
   Signatures will provide data integrity, authentication, and/or

   This document lists the design principles, scope, and requirements
   over three things: (1) the scope of work available to the WG, (2)  the
   XML signature specification, and (3) applications that implement the
   specification. It includes requirements as they relate to the
   signature syntax, data model, format, cryptographic processing, and
   external requirements and coordination. Those things that are required
   are designated as "must," those things that are optional are
   designated by "may," those things that are optional but recommended
   are designated as "should."

2 2. Design Principles and Scope

    1. The specification must describe how to a sign digital content, and

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       XML content in particular. [Charter]
    2. XML-signatures are generated from a hash over the canonical form
       of a signature manifest. The manifest must support references to
       Web resources, the hash of the resource content (or its
       canonicalized form), and (optionally) the resource content type.
       [Brown, List(Solo)] Web resources are defined as any digital
       content content that can be addressed using the syntax of XLink
       locator [XLink]).
       Comment: Scenarios are being explored which examine the ability to
       sign without requiring a manifest whereas the scope of the signed
       content is designated by the relative placement of signature
       elements in the XML stream/tree. For instance:
       <html> .....</body><dsig xmlns="http://..." referent=""><html>.
       <html><title>pricelist</title>...<dsig xmlns="http://..."> ...
    3. The meaning of a signature is simple:  The XML-signature syntax
       associates the content of resources listed in a manifest with a
       key via a strong one-way transformation.
         1. The XML-signature syntax must be extensible such that it can
            support arbitrary application/trust semantics and assertion
            capabilities -- that can also be signed.
            [Charter(Requirement1&4), List(Bugbee, Solo)]
         2. The WG is not chartered to specify trust semantics, but
            syntax and processing rules necessary for communicating
            signature validity (authenticity, integrity and
            non-repudiation).  [Charter(Requirement1)] At the Chairs'
            discretion and in order to test the extensibility of the
            syntax, the WG may produce non-standard-track proposals
            defining common semantics (e.g., package, timestamps,
            endorsement, etc.) relevant to signed assertions about Web
            resources in a schema definition [XML, RDF] or link type
            definition [XLink].
       Comment: A more formal definition of a signed resource is the
       following evaluates as true "definition(inputs):constraints" where
       R is a resource., I is a resource identifier (URI), and C is
       content (sequence-of-octects).
       signed-resource(I, C, key, sig): there was some request R such
       that GET(R) = C and address(R) = I and sign-doc(C, key, sig)
       sign-doc(C, key, sig): sig is the value of a strong one-way
       function over content and key that yields C integrity/validity and
       K non-repudiability
    4. The specification must not specify methods of confidentiality
       though the Working Group may report on the feasibility of such
       work in a future or rechartered activity. [List(Bugbee)]
    5. The specification must only require the provision of key
       information essential to checking the validity of the
       cryptographic signature. For instance, identity and key recovery
       information might be of interest to particular applications, but
       they are not within the class of required information defined in
       this specification. [List(Reagle)]
    6. The specification must define or reference at least one method of
       canonicalizing and hashing the signature syntax (i.e., the

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       manifest and signature blocks). [Oslo] The specification must not
       specify methods of canonicalizing resource content [Charter],
       though it may specify security requirements over such methods.
       [Oslo] Such content is normalized by specifying an appropriate
       content C14N (canonicalization) algorithm [DOMHASH, XML-C14N].
       Applications are expected to normalize application specific
       semantics prior to handing data to a XML-signature application.
    7. XML-signature applications must be conformant with the
       specifications as follows:
         1. XML-namespaces [XML-namespaces] within its own signature
            syntax. Applications may choose C14N algorithms which do or
            do not process namespaces within XML content. For instance,
            some C14N algorithms may opt remove all namespace
            declarations, others may rewrite namespace declarations to
            provide for context independent declarations within every
         2. XLink [Xlink] within its own signature syntax. Applications
            must use XLink locators within the signature manifest to
            reference resources. Signature applications must not embed or
            expand XLink references in signed content, though
            applications may choose C14N algorithms which provide this
         3. XML-Pointers [XPointer] within its own signature syntax. If
            applications reference/select parts of XML documents, they
            must use XML-Pointer within an XLink locator. [WS-list(1)]
       The WG may specify security requirements that constrain the
       operation of these dependencies to ensure consistent and secure
       signature generation and operation. [Oslo]
    8. XML-signatures must be developed as part of the broader Web design
       philosophy of decentralization, URIs, Web data,
       modularity/layering/extensibility, and assertions as statements
       about statements. [Berners-Lee, WebData] In this context, existing
       cryptographic provider (and infrastructure) primitives should be
       taken advantage of. [List(Solo)]

3 3. Requirements

3.1 1. Signature Data Model and Syntax

    1. XML-signature data structures must be based on the RDF data model
       [RDF] but need not use the RDF serialization syntax. [Charter]
    2. XML-signatures apply to any resource addressable by a locator --
       including non-XML content. XML-signature referents are identified
       with XML locators (URIs or fragments) within the manifest that
       refer to external or internal resources (i.e., network accessible
       or within the same XML document/package). [Berners-Lee, Brown,
       List(Vincent), WS, XFDL]
    3. XML-signatures must be able to apply to a part or totality of a
       XML document.  [Charter, Brown]
       Comment: A related requirement under consideration is requiring
       the specification to support the ability to indicate those
       portions of a document one signs via exclusion of those portions

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       one does not wish to sign. This feature allows one to create
       signatures that have document closure, retain ancestor
       information, and retain element order of non-continuous regions
       that must be signed. We are considering implementing this
       requirement via (1) a special <dsig:exclude> element, (2) an
       exclude list accompanying the resource locator, or (3) a request
       to change the XML-Fragment or XPointer specifications to yield
       this functionality. See List(Boyer(1,2)) for further discussion of
       this issue.
    4. Multiple XML-signatures must be able to exist over the static
       content of a Web resource given varied keys, content
       transormations, and algorithm specifications (signature, hash,
       canonicalization, etc.). [Charter, Brown]
    5. XML-signatures are first class objects themselves and consequently
       must be able to be referenced and signed. [Berners-Lee]
    6. The specification must permit the use of varied digital signature
       and message authentication codes, such as symmetric and asymmetric
       authentication schemes as well as dynamic agreement of keying
       material. [Brown] Resource or algorithm identifier are a first
       class objects, and must be addressable by a URI. [Beners-Lee]
    7. XML-signatures must be able to apply to the original version of an
       included/encoded resource. [WS-list (Brown/Himes)]

3.2 2. Format

    1. An XML-signature must be an XML element (as defined by production
       39 of the XML1.0 specification. [XML])
    2. An XML document of a certain type must still be recognizable as
       its original type when signed. For example, an XML form, when
       signed, should still be recognizable as a XML form to its
       application after it has been signed. [WS-summary]
    3. XML-signature must provide a mechanism that facilitates the
       production of composite documents -- by addition or deletion --
       while preserving the signature characteristics (integrity,
       authentication, and non-repudiatability) of the consituent parts.
       [Charter, Brown, List(Bugbee)]
    4. A key use of XML-signatures will be detached Web signatures.
       However, signatures may be embedded within or encapsulate XML or
       encoded content. [Charter] This WG must specify a simple method of
       packaging and encapsulation if no W3C Recommendation is available.

3.3 3. Cryptography and Processing

    1. The specification must permit arbitrary cryptographic signature
       and message authentication algorithms, symmetric and asymmetric
       authentication schemes, and key agreement methods. [Brown]
    2. The specification must specify at least one mandatory to implement
       signature canonicalization, content canonicalization, hash, and
       signature algorithm.
    3. In the event of redundant attributes within the XML Signature
       syntax and relevant cryptographic blobs, XML Signature
       applications prefer the XML Signature semantics.
       Comment: Another possibility is that an error should be generated,

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       however it isn't where a conflict will be flagged between the
       various function and application layers regardless.

3.4 4. Coordination

    1. The XML Signature specification should meet the requirements of
       the following applications:
         1. Internet Open Trading Protocol v1.0 [IOTP]
         2. Financial Services Mark Up Language v2.0 [Charter]
         3. At least one forms application [XFA, XFDL]
    2. To ensure that all requirements within this document are
       adequately addressed, the XML Signature specification must be
       reviewed by a designated member of the following communities:
         1. XML Syntax Working Group: canonicalization dependencies.
         2. XML Linking Working Group: signature referants. [Charter]
         3. XML Schema Working Group: signature schema design. [Charter]
         4. Metadata Coordination Group: data model design. [Charter]
         5. W3C Internationalization Interest Group:  [AC Review]
         6. XML Package Working Group: signed content in/over packages.
         7. XML Fragment Working Group: signing portions of XML content.
       Comment: Members of the WG are very interested in signing and
       processing XML fragments and packaged components. Boyer asserts
       that [XML-fragment] does not "identify non-contiguous portions of
       a document in such a way that the relative positions of the
       connected components is preserved." Packaging is a capability
       critical to XML-Signature applications, but it is clearly
       dependent on clear trust/semantic definitions, package application
       requirements, and even cache-like application requirements. It is
       not clear how this work will be addressed.

4 4. References

   AC Review
          Misha Wolf. "The Charter should include the I18N WG in the
          section on 'Coordination with Other Groups.'"

          Axioms of Web Architecture: URIs.
          Web Architecture from 50,000 feet

          Internet Draft. Digital Signatures for XML

          XML Signature (xmldsig) Charter.

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          Internet Draft. Digest Values for DOM (DOMHASH)

          FSML 1.5 Reference Specification

          XML Information Set Requirements Note.

          Internet Open Trading Protocol v1.0

          Internet Draft. Digital Signatures for the Internet Open
          Trading Protocol

          Minutes of the XML Signature WG Sessions at  IETF face-to-face
          meeting in Oslo.

          RDF Schema
          RDF Model and Syntax

   Signature WG List

          Uniform Resource Identifiers (URI): Generic Syntax

   WS (list, summary)
          XML-DSig '99: The W3C Signed XML Workshop

          XML Linking Language

          Extensible Markup Language (XML) Recommendation.

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          XML Canonicalization Requirements.

          XML Forms Architecture (XFA)

          Extensible Forms Description Language (XFDL) 4.0

          XML-Fragment Interchange

          Namespaces in XML

          XML Schema Part 1: Structures
          XML Schema Part 2: Datatypes

          XML Pointer Language (XPointer)

          Web Architecture: Describing and Exchanging Data.

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