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

Eric Rescorla <ekr@rtfm.com> Tue, 08 March 2011 17:32 UTC

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Date: Tue, 08 Mar 2011 09:33:59 -0800
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From: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx
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On Tue, Mar 8, 2011 at 9:20 AM, Martin Rex <mrex@sap.com> wrote:
> 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.


I don't understand this analysis. Consider two ideal PRFs:

* R-160 with a 160-bit output
* R-256 with a  256-bit output

Now, consider the function R-256-Reduced, which takes the first 160
bits of R-256.
Are you arguing that R-256-Reduced is weaker than R-160? If so, why?

-Ekr

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