Subject: RE: [RPSEC] Hash functions (was: Re: Minutes and Presentations fromParis are now Posted... )

From: "Vishwas Manral" <[email-masked]>
Date: Thu, 18 Aug 2005 20:51:12 -0700

Hi Paul/ Steve/ Tony,

Based on the discussion and seeing the state of "Cryptographic Algorithm
Implementation Requirement" in Routing Protocols, I agree, we can change
the recommendations in routing protocols (OSPF/ IS-IS/ RIP) to implement
HMAC-SHA1 or HMAC-MD5 to MUST and probably MD5 to MAY.

Does that sound correct?

Thanks,
Vishwas
-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Paul Jakma
Sent: Friday, August 19, 2005 12:15 AM
To: Tony Tauber
Cc: [email-masked]; Stephen Kent; Steven M. Bellovin
Subject: Re: [RPSEC] Hash functions (was: Re: Minutes and Presentations
fromParis are now Posted... )

On Thu, 18 Aug 2005, Tony Tauber wrote:

> functions?  For instance, I'd expect that if a PDF of my credit 
> card bill comes every month and I'm used to the format of it (e.g. 
> layout and logo graphics) it'd be very hard to produce a file with 
> the same format but just some of the numbers changed. (Perhaps the 
> analogy isn't valid, but hopefully I'm being clear in what I'm 
> asking.)

 	http://www.cits.rub.de/MD5Collisions/

Note however that the deviant actor must prepare /both/ inputs, and 
further must make use of some tricks in the format of the input to 
hide some of the differences from cursory inspection. In this case 
it's in the details of the postscript, but you could theoretically 
exploit similarity of glyphs in UTF-8, or, in the below example, 
special terminal control characters (on linux console):

http://www.derkeiler.com/Mailing-Lists/securityfocus/bugtraq/2004-12/006
7.html

MD5 does not /yet/ appear to be vulnerable to finding a collision for 
a /specific/ (fixed, eg cause it's someone else's packet) message.. 
And even if MD5 were, you still have about 28 bits of protection due 
to the HMAC usage mode (the secret key which is not included in the 
message), which is typically the case in routing protocols (OSPF).

As someone pointed out in Paris, if someone guesses your key as part 
of trying to attack the MD5 HMAC, then you're out of luck anyway - 
the attacker no longer needs to find MD5 collisions ;).

IANAC, i'm just rehashing information I gleaned from others.

regards,
-- 
Paul Jakma	[email-masked]	[email-masked]	Key ID: 64A2FF6A
Fortune:
It appears that PL/I (and its dialects) is, or will be, the most widely
used higher level language for systems programming.
 		-- J. Sammet

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