Re: [TLS] padding bug

"Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu> Tue, 24 September 2013 12:48 UTC

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From: "Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu>
To: "'pgut001@cs.auckland.ac.nz'" <pgut001@cs.auckland.ac.nz>, "'tls@ietf.org'" <tls@ietf.org>
Date: Tue, 24 Sep 2013 08:48:32 -0400
Thread-Topic: [TLS] padding bug
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Subject: Re: [TLS] padding bug
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Guys,

Isn't it enough already? Sufficient evidence, including publications by respected cryptographers, has been presented to adequately demonstrate that truncating MAC has security advantages over including the entire output of the hash function. That was the primary reason/justification for truncating MAC in IPsec. 

FWIW, the reasoning of that paper applies to SHA3 as well, though arguably that extra security measure is unnecessary there.

P.S. Peter, I admire your ability to keep arguing well past the logical defeat. <half-smile>

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----- Original Message -----
From: Peter Gutmann [mailto:pgut001@cs.auckland.ac.nz]
Sent: Tuesday, September 24, 2013 08:31 AM
To: <tls@ietf.org> <tls@ietf.org>
Subject: Re: [TLS] padding bug

Nikos Mavrogiannopoulos <nmav@gnutls.org> writes:

>Since you are the one claiming that MAC truncation isn't necessary when all
>existing protocols do it (even TLS does it on the Finished message MAC), it
>may be better for you to present evidence that this isn't necessary.

Before that, you'd have to present evidence that "all existing protocols do
it".  As I've already pointed out, S/MIME doesn't, PGP doesn't, TLS doesn't,
SSH doesn't, ... .  In fact of the major IETF application-level security
protocols that I can think of, only IPsec does, and AFAIK (meaning based on
long-ago discussions with one of the IPsec folks) that was done in order to
fit the MAC into the fixed-size AH header (although there seems to be some
disagreement on the details).

As I've also already pointed out, TLS has for some years now provided for MAC
truncation to a range of sizes.  If anyone's really worried about this, they
can request truncation to whatever size they prefer.

Peter.
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