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

Martin Rex <mrex@sap.com> Tue, 08 March 2011 17:19 UTC

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From: Martin Rex <mrex@sap.com>
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To: ekr@rtfm.com
Date: Tue, 08 Mar 2011 18:20:31 +0100
In-Reply-To: <AANLkTinHFPBWZdManf+awYiH1ExDhpRBjf=_HRi6EF8Z@mail.gmail.com> from "Eric Rescorla" at Mar 8, 11 08:29:14 am
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Subject: Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx
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Eric Rescorla wrote:
> 
> I don't understand this reasoning. Why does the output size of the
> pre-truncated PRF
> influence the desirable length of the verify_data (provided that the
> output size is > than
> the length of the verify_data of course).

One of the purposes of a cryptographic hash function is to protect
from collisions (both random and fabricated collisions).

Cutting down the SHA-384 output from 48 to 12 octets significantly impairs
its ability to protect from collisions.  It's comparable to
truncating the SHA-1 output from 20 to 5 octets.

Unless you have _a_very_good_reason_ to truncate a hash output
so severely, you very probably should not do it.


-Martin

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