At 1:33 PM -0500 12/30/08, Jeffrey Hutzelman wrote:
This is a practical application of an approach that I remember being brought
up during discussions about MD5 at a saag meeting some time ago. I also
recall someone mentioning at the time that many/most CA's were already issuing
certificates with random rather than sequential serial numbers, which would
have thwarted this particular attack.
Your recollection may be off. I believe I was the person who brought up the
serial number hack at the mic, and I'm pretty sure I said "some", not "many"
(and certainly not "most"!). When I looked at a handful of popular CAs earlier
this week, I only found a few who are using randomization in their serial
numbers.
Regardless of that, the authors of the MD5 paper are correct: trust anchors
signed with MD5 are highly questionable as of today (well, actually, since they
published their last paper). Hopefully, the maintainers of the popular trust
anchor repositories (Microsoft, Mozilla, etc.) will yank out the trust anchors
signed with MD5 (and MD2!) as soon as possible.
At 3:10 PM -0500 12/30/08, Russ Housley wrote:
RFC 5280 does not include this advice. It is sound advice that was discussed
in PKIX and other venues. Perhaps a BCP is in order.
Man, that is really stretching the definition of "best".
For one, it is only needed in signatures that use known-attackable hash
functions. A "best practice" in that case is to use a better hash function in
the signature. Also, it relies on the ability of the software using the random
number to be sure that the result is a positive integer in ASN.1, which seems
overly optimistic.
If the IETF feels that adding randomization to signatures is important, we
should define randomized signature functions. We could start with NIST Draft SP
800-106
(<http://csrc.nist.gov/publications/drafts/800-106/2nd-Draft_SP800-106_July2008.pdf>).
However, I think that doing so is sending the wrong message: we should instead
be encouraging the use of non-broken hash functions.
--Paul Hoffman, Director
--VPN Consortium