Re: [TLS] PRF digest function for ChaCha20-Poly1305 cipher suites

Nikos Mavrogiannopoulos <nmav@redhat.com> Thu, 24 December 2015 10:36 UTC

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Date: Thu, 24 Dec 2015 05:36:39 -0500 (EST)
From: Nikos Mavrogiannopoulos <nmav@redhat.com>
To: Hubert Kario <hkario@redhat.com>
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Thread-Topic: PRF digest function for ChaCha20-Poly1305 cipher suites
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Cc: Adam Langley <agl@imperialviolet.org>, tls@ietf.org
Subject: Re: [TLS] PRF digest function for ChaCha20-Poly1305 cipher suites
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----- Original Message -----
> I'm not convinced about SHA-512, but yes, they probably should use
> SHA-384 at the very least. And given that the algorithm for SHA-384 and
> SHA-512 is essentially the same, using just different IVs, that should
> be usable for highly restricted hardware, wouldn't it?
> I would be against SHA-512 as that would be the very first cipher that
> uses SHA-512 PRF in TLS1.2, making its addition/implementation much more
> invasive to the underlying library, OTOH, we have multiple ciphers which
> use SHA-384 PRF. I think I just need to remind the delay after which NSS
> added support for SHA-384 compared to introduction to AES-128-GCM TLS
> ciphers...

I agree. SHA384 also aligns it with the AES-256-GCM ciphers. I believe an
implementation mixing chacha20 with AES is a much more common scenario than
the one described by Brian.

regards,
Nikos