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

Title: Message

Yes, the two public keys are produced so that after they are processed there is a collision in the hash function chaining variable. Anything added after this point will similarly produce a collision, as long as the same data is added to both. Thus, even if the CA includes randomness after the public key in the TBS blob, an identical certificate with the second public key will produce the same hash, and thus the same signature.

The reason why the randomness in the serial number is able to prevent this particular attack is that the attacker cannot predict it's value and thus cannot predict the value of the hash function chaining variable at the point the public keys begin to get processed. This appears to prevent the construction of collisions in the public keys.

Robert.

-----Original Message-----
From: Guida, Richard [JJCUS] [mailto:RGuida@xxxxxxxxxxxxx]
Sent: March 4, 2005 1:31 PM
To: Robert Zuccherato; 'Russ Housley'; ietf-pkix@xxxxxxx
Subject: RE: [saag] X.509 certificate collision, via MD5 collisions

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.