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XML Digital Signatures Working Group J. Reagle,
INTERNET-DRAFT W3C/MIT
draft-ietf-xmldsig-requirements-01.txt
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.
http://www.w3.org/Signature
The latest version of this draft series may be found at:
http://www.w3.org/TR/1999/xml-dsig-requirements
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
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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
http://www.w3.org/TR
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. 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
non-repudiatability.
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>.
or
<html><title>pricelist</title>...<dsig xmlns="http://..."> ...
</dsig></html>
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.
[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 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. 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
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 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.
[Charter]
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.'"
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
Internet Draft. Digital Signatures for XML
http://search.ietf.org/internet-drafts/draft-ietf-xmldsig-signa
ture-00.txt
Charter
XML Signature (xmldsig) Charter.
http://www.w3.org/1999/05/XML-DSig-charter-990521.html
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DOMHASH
Internet Draft. Digest Values for DOM (DOMHASH)
http://search.ietf.org/internet-drafts/draft-hiroshi-dom-hash-0
1.txt
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
Internet Open Trading Protocol v1.0
draft-ietf-trade-iotp-v1.0-protocol-04.txt
IOTP-DSig
Internet Draft. Digital Signatures for the Internet Open
Trading Protocol
http://www.ietf.org/internet-drafts/draft-ietf-trade-iotp-v1.0-
dsig-00.txt
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
Uniform Resource Identifiers (URI): Generic Syntax
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
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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
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