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.
This Working Draft of XML Signature Requirements is a very stable result of this Working Draft having been advanced through W3C Last Call. Relatively small changes have been made to clarify the stated requirements during that period. This document will now be advanced as an IETF Informational RFC.
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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."
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
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
The WG may specify security requirements that constrain the operation of these dependencies to ensure consistent and secure signature generation and operation. [Oslo]
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
(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.
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.
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.