XML Digital Signatures Working Group J. Reagle,
INTERNET-DRAFT W3C/MIT
draft-ietf-xmldsig-requirements-03.txt
Expires August 01, 2000
XML Signature Requirements
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This document is a production of the joint IETF/W3C XML Signature
Working Group.
http://www.w3.org/Signature
The comparable html draft of this version may be found at
http://www.w3.org/TR/2000/xmldsig-requirements-20000104/
The latest version of this document series may be found at:
http://www.w3.org/TR/xmldsig-core
This Working Draft of XML Signature Requirements is a very stable
result of this Working Draft that has been advanced through W3C Last
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security considerations section.
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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
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. Security Considerations
5. References
6. Acknowledgements
7. Author's Address
8. Full Copyright Statements
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. Design Principles and Scope
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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
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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 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
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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
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
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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 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. 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.'"
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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://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
Internet Draft. Digest Values for DOM (DOMHASH)
http://www.ietf.org/internet-drafts/draft-ietf-trade-
hiroshi-dom-hash-03.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
http://www.ietf.org/internet-drafts/draft-ietf-trade-iotp-v1.0-
protocol-07.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-05.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
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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
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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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
E-Mail: reagle@w3.org
URL: http://www.w3.org/People/Reagle
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