XMLDSIG
Open Syntax Questions 2

Donald E. Eastlake 3rd

<dee3@us.ibm.com>

9 November 1999, 46th IETF Meeting

Open Syntax Questions 2

• Algorithm Parameters
• Unsigned Location/Transforms
• Composite / Orthogonal (Signature) Algorithms

Algorithm Parameters

• Possibilities

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

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

  • <hmac-truncation-length xmlns="…">100
    </hman-truncation-length>

Generic Element

<SignatureMethod Algorithm="dsig:hmacX">
  <Parameter xmlns="dsig:hmac-truncation-length>
  100
  </Parameter>
  <Parameter xmlns="dsig:other" Encoding="dsig:base64">
  2f/0cb==
  </Parameter>
</SignatureMethod>

Type Element

<SignatureMethod Algorithm="dsig:hmacX">
  <Integer xmlns="dsig:hmac-truncation-length>
  100
  </Integer>
  <Binary xmlns="dsig:other" Encoding="dsig:base64">
  2f/0cb==
  </Binary>
</SignatureMethod>

Algorithm Specific Element 1

<SignatureMethod Algorithm="dsig:hmacX">
  <hmac-truncation-length xmlns="dsig:hmacX">
  100
  </hmac-truncation-length>
  <Other xmlns="dsig:hmacX" Encoding="dsig:base64">
  2f/0cb==
  </Other>
</SignatureMethod>

Algorithm Specific Element 2

<SignatureMethod Algorithm="dsig:hmacX" xmlns:a="dsig:hmacX"> 
  <a:hmac-truncation-length>
  100
  </ hmac-truncation-length>
  <a:Other Encoding="dsig:base64">
  2f/0CbW8zp= =
  </Other>
</SignatureMethod>

Unsigned Location/Transforms

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

Nested Manifests

• SignedInfo ObjectReference points to a Manifest which points to a Manifest.
• Con: Depends on application verification behavior. Bulky.
• Pro: Fits current syntax. 2nd level Manifest can be separately signed.

Location Outside SignedInfo

• Need to also move Transforms out
• Con: Need to drop multiple ObjectReferences or add a link to DigestMethod/Value inside SignedInfo. Need some way to securing Location if you want to.

Clever URIs for Locations

• Con: Are permanent URN's possible?
• Implies clever Transforms which is even hairier.
• Pro: fits in current syntax

Allow Transform of SignedInfo

• Can simply drop out Locations and/or Transforms.
• Con: Increases complexity in signature core. Can be dangerous (but not much worse than other uses of Transforms).
• Pro: very powerful, can satisfy many desires.

Composite / Orthogonal Algorithms

• Acceptance of multiple combinations reduces security to that of the weakest
• Composite: RSA-SHA1-Xpad
  • less verbose
  • combinatorial explosion
• Orhogonal: <RSA><SHA1/><Xpad/></RSA>
  • more verbose
  • fewer names