Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx

Stephen Kent <kent@bbn.com> Thu, 10 March 2011 23:28 UTC

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Date: Thu, 10 Mar 2011 18:28:47 -0500
To: Eric Rescorla <ekr@rtfm.com>
From: Stephen Kent <kent@bbn.com>
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Subject: Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx
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At 5:08 PM -0800 3/8/11, Eric Rescorla wrote:
>On Tue, Mar 8, 2011 at 3:55 PM, Peter Gutmann 
><pgut001@cs.auckland.ac.nz> wrote:
>>
>>  Martin Rex <mrex@sap.com> writes:
>>
>>>Truncating HMACs and PRFs may have become first popular in the IETF within
>>>IPSEC.
>>
>>  It wasn't any "may have become first popular", there was only room 
>>for 96 bits
>>  of MAC data in the IP packet, so MD5 was truncated to that size.
>
>This is an odd claim, since:
>
>(a) RFC 1828 (http://tools.ietf.org/html/rfc1828) originally specified
>not HMAC but a keyed MD5 variant
>with a 128-bit output.
>(b) The document that Martin points to has MACs > 96 bits long.
>
>Can you please point to where in IP there is a limit that requires a
>MAC no greater than 96 bits.
>
>-Ekr

What Peter probably meant to say was that IPsec chose to truncate the HMAC
value to 96 bits because that preserved IPv4 and IPv6 byte-alignment for
the payload.  Also, as others have noted, the hash function used here is
part of an HMAC calculation, and any collisions have to be real-time 
exploitable to be of use to an attacker.  Thus 96 buts was viewed as 
sufficient.

Steve