XMLDSIG Open Syntax Questions

Donald E. Eastlake 3rd
<dee3@us.ibm.com>

8 November 1999, 46th IETF Meeting

Open Syntax Questions

• Canonicalization - deferred until tomorrow
• Put more dynamic fields first
• No nonce
• Algorithm Parameters
• Unsigned Location/Transforms
• Reliance on xpath/xptr/xslt

Put Dynamic Fields First

• Makes preloading hash less useful
• In SignedInfo
• First: ObjectReference
• Later: SignatureMethod, CanonicalizationMethod
• In ObjectReference
• First: DigestValue
• Later: Location, Transforms, DigestMethod

No Nonce

• All signature algorithms listed in draft do not need a nonce to protect against attacks
• Other signature algorithms can be modified, if necessary, to not need a nonce.

Algorithm Parameters

• Consensus for algorithm specific parameter types
• Is an element needed at all for one parameter algorithms?
• Possibilities

  <Parameter type="http:…hmac-trunctation-length">100</Parameter>

  <foo:Integer xmlns:foo="http:…hmac-trunctation-length">100</Paramter>

Decisions Needed

• Optimize one parameter algorithm?
• Type attribute versus Namespace?
• Generic element name versus Integer / Real / Boolean / String / Binary ?

Unsigned Location/Transforms

• Desire to have data that can move around
• Nested Manifests
• Allow Location outside of SignedInfo
• clever URIs for Location
• iotp:transaction-id#idref
• http://example.com/cgi/finder?doc-1234
• Allow Transforms of SignedInfo to remove Location
• Different locations need different transforms
• Same issues, nested Manifests, move Transforms, clever Transforms, Transform Transform

Nested Manifests

• SignedInfo ObjectReference points to a Manifest which points to a Manifest.
• Depends on application verification behavior.
• Bulky/Kludgy.

Locations Outside of SignedInfo

• Need to also move Transforms out
• Need to link to DigestMethod / DigestValue inside Signed info if multiple ObjectReferences allowed, adding an IDREF and ID.

Clever URIs for Locations

• Not very elegant.
• Implies clever Transforms which is even hairier.

Allow Transform of SignedInfo

• Can simply drop out Locations and/or Transforms.
• Can be dangerous but not much worse than other uses of Transforms.
• <SignedInfo> (ObjectReference)+ CanonicalizationMethod? SignatureMethod (Transforms)? </SignedInfo>

Reliance on xpath/xptr/xslt

• XPath is a subset of both XPointer and XSLT and provides useful filtering functionality.
• Only XSLT is defined to have a mode where it produces XML. Others need additional specification beyond the standard, which is present in XMLDSIG documentation.