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

Stephen Kent <kent@bbn.com> Mon, 14 March 2011 15:57 UTC

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Date: Mon, 14 Mar 2011 11:41:19 -0400
To: Nikos Mavrogiannopoulos <nmav@gnutls.org>
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 8:20 AM +0100 3/11/11, Nikos Mavrogiannopoulos wrote:
>...
>  > 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.
>
>This sounds pretty awkward decision because HMAC per record is full
>(e.g. 160-bits on SHA-1), but the MAC on the handshake message
>"signature" is truncated to 96-bits. Why wasn't the record MAC
>truncated as well? In any case saving few bytes per handshake
>is much less of value than saving few bytes per record. Was
>there any other rationale for truncation?

I think you lost the context here.  I was explaining why IPsec chose 
to truncate the hash, not TLS.

Steve