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

From: "Guida, Richard [JJCUS]" <[email-masked]>
Date: Fri, 4 Mar 2005 13:31:20 -0500

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:[email-masked]]
Sent: Friday, March 04, 2005 10:46 AM
To: 'Russ Housley'; [email-masked]
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.

---

From: Stephen Farrell <[email-masked]>
Date: Fri, 04 Mar 2005 18:54:37 +0000

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
>     *From:* Robert Zuccherato [mailto:[email-masked]]
>     *Sent:* Friday, March 04, 2005 10:46 AM
>     *To:* 'Russ Housley'; [email-masked]
>     *Subject:* RE: [saag] X.509 certificate collision, via MD5 collisions
> 
>     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.
>

---

From: Dave Engberg <[email-masked]>
Date: Fri, 4 Mar 2005 12:36:26 -0700

Is the location of the randomness (before the public key) relevant for SHA-1, or is this only a consideration for the known MD5 attack?  I.e. would Rich's proposal work fine as long as he was using SHA-1?

From: Stephen Farrell
Sent: Fri 3/4/2005 1:54 PM
To: Guida, Richard [JJCUS]
Cc: [email-masked]
Subject: Re: [saag] X.509 certificate collision, via MD5 collisions

Rich,

Guida, Richard [JJCUS] wrote:

>      > 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.
>