Last Call Review of draft-ietf-pkix-rfc3161-update-
review-ietf-pkix-rfc3161-update-secdir-lc-cain-2009-12-11-00

I have reviewed this document as part of the security directorate's 
ongoing effort to review all IETF documents being processed by the IESG. 
These comments were written primarily for the benefit of the security 
area directors. Document editors and WG chairs should treat these 
comments just like any other last call comments.

Document Abstract:

   This document updates RFC 3161 [RFC3161]. It allows the use of
   ESSCertIDv2 defined in RFC 5035 [ESSV2] to specify the hash of a
   signer certificate when the hash is calculated with a function other
   than SHA-1 [SHA1].

Comment:

The purpose of this document is laudable.
My only comment/concern is about the 'note' at the end of Section 2.2.1.

"Note: For backwards compatibility, in line with RFC 5035, both
            ESSCertID and ESSCertIDv2 MAY be present. Systems MAY ignore
            ESSCertIDv2 if RFC 5035 has not been implemented."

When RFC3161 was undergoing development, there was a robust discussion about
signing 
A TST multiple times. I seem to recall the output was not to do it. Since
the requestor has to verify the TST when the TSA sends it back, I'm unclear
on how a requestor that "has not implemented RFC5035" is going to do this
verification if both ESSCertID and ESSCertIDV2 are included.
I expect more guidance is needed here since the different signature
algorithms will have differing characteristics and strengths.

It seems to make more sense that a TSA return a TST with *either* an
ESSCertID or ESSCertIDV2, but not both. Is there a specific use case that
was intended here or are we just trying to be polite.

Pat Cain