Re: [TLS] HSM-friendly Key Computation

Brian Smith <brian@briansmith.org> Fri, 17 April 2015 21:18 UTC

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Date: Fri, 17 Apr 2015 11:18:12 -1000
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From: Brian Smith <brian@briansmith.org>
To: Yoav Nir <ynir.ietf@gmail.com>
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Cc: IETF TLS <tls@ietf.org>
Subject: Re: [TLS] HSM-friendly Key Computation
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Yoav Nir <ynir.ietf@gmail.com> wrote:
>> On Apr 17, 2015, at 10:40 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>> We should defer this issue until we know whether we are generating an IV
>> at all.
>>
> The client_write_iv and server_write_iv are needed, and I’m not aware of any contention about them. In AES-GCM they are used for the “salt” value, and the chacha20-poly1305 draft uses the same terminology for a 32-bit fixed part of the nonce that is derived from the key buffer.
>
> This is not the same as the per-record IV that we *are* considering dropping.

I think you're misunderstanding what is being discussed. As far as I
know, nobody objects to using the record sequence number in a way that
turns the nonce into a counter. The only thing being debated is
whether client_write_iv and server_write_iv are used. See the actual
pull request [1]. The proposal put forth in that pull request is that
the nonce will consist *only* of the per-record sequence number,
padded with zeros, and without the part that was comprised of
client_write_iv and server_write_iv.

Cheers,
Brian


[1] https://github.com/ekr/tls13-spec/commit/0c610a9453ca10bd12d3b979d2faa014a0b12251