Re: [saag] Further MD5 breaks: Creating a rogue CA certificate

[Paul Hoffman <paul.hoffman@xxxxxxxx>]

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