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Re: [TLS] comparison of draft-josefsson-salsa20-tls-02 and draft-agl-tls-chacha20poly1305-02

Nikos Mavrogiannopoulos <nmav@gnutls.org> Wed, 23 October 2013 16:34 UTC

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Date: Wed, 23 Oct 2013 18:34:47 +0200
From: Nikos Mavrogiannopoulos <nmav@gnutls.org>
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Cc: "tls@ietf.org" <tls@ietf.org>, Joachim Strömbergson <joachim@secworks.se>
Subject: Re: [TLS] comparison of draft-josefsson-salsa20-tls-02 and draft-agl-tls-chacha20poly1305-02
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On 10/23/2013 04:58 PM, Adam Langley wrote:
> On Wed, Oct 23, 2013 at 10:51 AM, Nikos Mavrogiannopoulos
> <nmav@gnutls.org> wrote:
>> As far as I understand you use chacha to generate the keystream for
>> poly1305. Thus you carry state between records (chacha is a stream
>> cipher). I don't know if I have missed anything there, but I don't see
>> resetting chacha with a new IV per MAC calculation.
> 
> There is no state carried between records: "ChaCha20 is run with the
> given key and nonce and with the two counter words set to zero. The
> first 32 bytes of the 64 byte output are saved to become the one-time
> key for Poly1305." (The nonce is the sequence number of the record.)
> (http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-00#section-5)

I had assumed that you used the Poly1305-AES construction but with
Chacha in place of AES. Clearly this isn't the case, and even the
attacks described may not apply to your construction. I have not seen
this construction before. It looks pretty elegant. Has it been used
somewhere else?

regards,
Nikos

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