Re: [TLS] Pull Request: Removing the AEAD explicit IV

Brian Smith <brian@briansmith.org> Thu, 19 March 2015 20:11 UTC

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Date: Thu, 19 Mar 2015 10:11:40 -1000
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From: Brian Smith <brian@briansmith.org>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Pull Request: Removing the AEAD explicit IV
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Eric Rescorla <ekr@rtfm.com>; wrote:
> On Thu, Mar 19, 2015 at 12:53 PM, Brian Smith <brian@briansmith.org>; wrote:
>> It seems like it would be better, instead, to require that the initial
>> nonces to be calculated from the keyblock established during key
>> agreement,
>
> Is there any reason why these should be derived from the keyblock
> as opposed to from purely public information such as the random
> values?

Intuitively, I expect the attacker to have more difficulty if they
don't know the nonce than if they do. In general, we should not
divulge more than the minimum amount of information in cleartext. And,
in particular, one of the design goals is to encrypt as much of the
handshake as possible, and the nonce selection is part of the
handshake.

I wouldn't be surprised if somebody pointed out a good reason to avoid
deriving them out of the key block, though I don't know of one now.

>> and then have them incremented as counters (with
>> wraparound) in the same fashion as being proposed.
>
> Can you explain why you think they need to change? I note that TLS 1.2
> currently does not behave in this fashion.

I think you interpreted my suggested as <initial-nonce> || <record
sequence number>. I just mean that the per-record nonce should be
calculated as <initial-nonce> + <record sequence number>. It seems
better to start all the initial bits of the nonce in a randomly-chosen
state, instead of just a prefix, if there's no conflicting
considerations to do otherwise.

Cheers,
Brian