XML Digital Signatures Working Group               J. Reagle,
INTERNET-DRAFT                                     W3C/MIT
Expires April 14, 1999

                         XML-Signature Requirements

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

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

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


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   This Working Draft of XML Signature Requirements is a very stable
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   an IETF Informational RFC.

   Please send comments to the editor <reagle@w3.org> and cc: the list
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   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. Introduction
     2. Design Principles and Scope
     3. Requirements
          3.1 Signature Data Model and Syntax
          3.2 Format
          3.3 Cryptography and Processing
          3.4 Coordination
     4. References

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 Design Principles and Scope

    1. The specification must describe how to sign digital content, and
       XML content in particular. The XML syntax used to represent a
       signature (over any content) is described as an XML-signature.
    2. XML-signatures are generated from a hash over the canonical form
       of a signature manifest. (In this document we use the term
       manifest to mean a collection of references to the objects being
       signed. The specifications may use the terms manifest, package or
       other terms differently from this document while still meeting

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       this requirement.) 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 that
       can be addressed using the syntax of XLink locator [XLink]).
    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-critical-path proposals
            defining common semantics (e.g., manifest, 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 below.
       The notation is "definition(inputs):constraints" where definition
       evaluates as true for the given inputs and specified constraints.
       signed-resource(URI-of-resource, content, key, signature): (there
       was some protocol message at a specific time such that
       "GET(URI-of-resource) = content") AND (sign-doc(content, key,
       sign-doc(content, key, signature): signature is the value of a
       strong one-way transformation over content and key that yields
       content integrity/validity and/or key 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
       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 or
       specify the necessary transformations for this process within the
       signature.  [Charter]
    7. XML-signature applications must be conformant with the

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       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 to 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. For any
            resource identification beyond simple URIs (without fragment
            IDs) or fragmentIDs, applications must use XLink locators to
            reference signed 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 Requirements

3.1 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
       one does not wish to sign. This feature allows one to create
       signatures that have document closure [List(Boyer(1)], 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) the
       XML-Fragment or XPointer specifications -- or a requested change
       to those specifications if the functionality is not available. See

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       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. [Berners-Lee]
    7. XML-signatures must be able to apply to the original version of an
       included/encoded resource. [WS-list (Brown/Himes)]

3.2 Format

    1. An XML-signature must be an XML element (as defined by production
       39 of the XML1.0 specification. [XML])
    2. When XML signatures are placed within a document the operation
       must preserve (1) the document's root element tag as root and (2)
       the root's descendancy tree except for the addition of signature
       element(s) in places permitted by the document's content model.
       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. An important 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 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,
       however it isn't where a conflict will be flagged between the
       various function and application layers regardless.
    4. The signature design and specification text must not permit
       implementers to erroneously build weak implementations susceptible

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       to common security weaknesses (such as as downgrade or algorithm
       substitution attacks).

3.4 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 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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