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Re: [saag] X.509 certificate collision, via MD5 collisions



Rich,

I believe that randomness after the public key isn't much
use, but others'll know better.

However, instead of including an extension with a random value,
you could include an extension with a better hash over the
rest of the TBS-cert, (e.g. using sha256 or better). If you
just don't have any other hash algorithm available for some
reason, then you could add an extension which contains a
random value ("V") and H("V"||TBS-cert).

In both cases a client that can process the extension can
detect problems. You can also detect bad certs via filtering
on e.g. an ldap server. Not sure how much of this'd be
worthwhile though.

In any case, I'd agree with others who've said that its a
bit too soon to be deciding how to fix this.

Next week's meetings should be interesting anyway!
Stephen.

Guida, Richard [JJCUS] wrote:
Robert - can you help me understand the substance of your last sentence? We are indeed considering adding a proprietary extension which would be populated with just random data (for certs issued by our enterprise PKI). Is the issue that the extension appears after the public key in the TBS blob - and the attack thus would work regardless of that random data? Thanks very much. Wish I had more time to understand the details of the attack myself, but at my age I am lucky to remember the definition of the Fibonacci sequence..... 
*Richard A. Guida*
*Director, Information Security*

*/Johnson & Johnson/*//
Room GS8217
410 George Street
New Brunswick, New Jersey  08901
Phone:  732 524 3785

    -----Original Message-----
    *From:* Robert Zuccherato [mailto:robert.zuccherato@xxxxxxxxxxx]
    *Sent:* Friday, March 04, 2005 10:46 AM
    *To:* 'Russ Housley'; ietf-pkix@xxxxxxx
    *Subject:* RE: [saag] X.509 certificate collision, via MD5 collisions

    I'd like to comment on the following suggestion from Russ:

     > In the past, I have recommended the use of large serial numbers
     > where the first part is a monotonically increasing integer and
    the second
> part is random. A 64-bit random value should thwart this attack. 
    This idea has a number of good points.  It seems that a CA that
    includes randomness in its serial number would be able to prevent
    collisions like the one produced by Lenstra,

    Wang and de Wegner.  Including the randomness in the serial number
    in the way that has been suggested is also nice because it's done in
    such a way that PKIX conformant applications shouldn't choke on
    them.  A CA can also continue to issue serial numbers using
    monotonically increasing values, or any other scheme that they wish
to use, in the non-random part. 
    However, I really wonder if we should be recommending that people
    change their implementations at this point.  I don't think we know
    enough yet about the potential attacks on MD5, SHA-1 to say with any
    certainty that any particular counter-measures are worth
    implementing.  This infrastructure was built assuming that people
    will use strong hash functions.  I see no reason at this point to
    change that assumption.  People should have stopped using MD5 a long
    time ago.  Over the next couple of years they will likely have to
    stop using SHA-1 as well.  I think that is the real advice that we
    should be giving people at this time.

    There are also some practical problems with overloading the serial
    number.  CRLs will, in most circumstances, increase in size.  Also,
    OCSP responders that pre-compute responses may have trouble
    pre-computing "good" responses if they cannot predict which serial
    numbers have been used.  This issue would come up with responders
    that work from CRLs and assume that a certificate is "good" if it's
    serial number doesn't appear on a CRL.

    I'd also like to point out that the serial number proposal would
    only help X.509 certificates and not CRLs, RFC 3161 time stamp
    tokens (there was already a message to the CFRG list today showing
    how to extend the MD5 X.509 work to 3161 tokens), OCSP responses,
    etc.  One might consider defining a non-critical extension that
    would simply contain a random integer of an appropriate size that is
    freshly generated by the signer at signing time.  But, I don't
    believe that would solve the problem.  Once the collision has been
    produced using the public keys, anything added to the certificate
    after that (i.e., an extension) will also produce a collision.

            Robert Zuccherato.