Network Working Group J. Reagle
Request for Comments: 2807 W3C/MIT
Category: Informational July 2000

XML Signature Requirements

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (c) 2000 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.

Abstract

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 .............................................. 1
2. Design Principles and Scope ............................... 2
3. Requirements .............................................. 4
3.1. Signature Data Model and Syntax .................... 4
3.2. Format ............................................. 5
3.3. Cryptography and Processing ........................ 5
3.4 Coordination ........................................ 5
4. Security Considerations ................................... 6
5. References ................................................ 6
6. Acknowledgements .......................................... 8
7. Author's Address .......................................... 8
8. Full Copyright Statement .................................. 9

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 non-repudiability.

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.
[Charter]
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
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, sig))

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
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 element.
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 feature.
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 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
transformations, 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-repudiability) 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
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.
[Charter]
2. XML Linking Working Group: signature referents. [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. Security Considerations

This document lists XML Digital Signature requirements as they relate
to the signature syntax, data model, format, cryptographic
processing, and external requirements and coordination. In that
context much of this document is about security.

5. References

AC Review Misha Wolf. "The Charter should include the I18N WG
in the section on `Coordination with Other
Groups'", http://lists.w3.org/Archives/Team/xml-
dsig-review/1999May/0007.Html

Berners-Lee Axioms of Web Architecture: URIs.
http://www.w3.org/DesignIssues/Axioms.html Web
Architecture from 50,000 feet
http://www.w3.org/DesignIssues/Architecture.html

Brown-XML-DSig Work in Progress. Digital Signatures for XML
http://www.w3.org/Signature/Drafts/xmldsig-
signature-990618.html

Charter XML Signature (xmldsig) Charter.
http://www.w3.org/1999/05/XML-DSig-charter-
990521.html

DOMHASH Maruyama, H., Tamura, K. and N. Uramoto, "Digest
Values for DOM (DOMHASH)", RFC2803, April 2000.

FSML FSML 1.5 Reference Specification
http://www.echeck.org/library/ref/fsml-v1500a.pdf

Infoset-Req XML Information Set Requirements Note.
http://www.w3.org/TR/1999/NOTE-xml-infoset-req-
19990218.html

IOTP Burdett, D., "Internet Open Trading Protocol - IOTP
Version 1.0", RFC2801, April 2000.

IOTP-DSig Davidson, K. and Y. Kawatsura, "Digital Signatures
for the v1.0 Internet Open Trading Protocol
(IOTP)", RFC2802, April 2000.

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

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

Signature WG List http://lists.w3.org/Archives/Public/w3c-ietf-
xmldsig/

URI Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC2396, August 1998.
http://www.ietf.org/rfc/rfc2396.txt

WS
(list, summary) XML-DSig '99: The W3C Signed XML Workshop
http://www.w3.org/DSig/signed-XML99/
http://www.w3.org/DSig/signed-XML99/summary.html

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-C14N XML Canonicalization Requirements.
http://www.w3.org/TR/1999/NOTE-xml-canonical-req-
19990605

XFA XML Forms Architecture (XFA)
http://www.w3.org/Submission/1999/05/

XFDL Extensible Forms Description Language (XFDL) 4.0
http://www.w3.org/Submission/1998/16/

XML-Fragment XML-Fragment Interchange
http://www.w3.org/1999/06/WD-xml-fragment-
19990630.html

XML-namespaces Namespaces in XML
http://www.w3.org/TR/1999/REC-xml-names-19990114

XML-schema XML Schema Part 1: Structures
http://www.w3.org/1999/05/06-xmlschema-1/
XML Schema Part 2: Datatypes
http://www.w3.org/1999/05/06-xmlschema-2/

XPointer XML Pointer Language (XPointer)
http://www.w3.org/1999/07/WD-xptr-19990709

WebData Web Architecture: Describing and Exchanging Data.
http://www.w3.org/1999/04/WebData

6. Acknowledgements

This document was produced as a collaborative work item of the XML
Signature (xmldsig) Working Group.

7. Author's Address

Joseph M. Reagle Jr., W3C
XML Signature Co-Chiar
Massachusetts Institute of Technology
Laboratory for Computer Science
W3C, NE43-350
545 Technology Square
Cambridge, MA 02139

Phone: 1.617.258.7621
EMail: reagle@w3.org
URL: http://www.w3.org/People/Reagle

8. Full Copyright Statement

Copyright (c) 2000 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.

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