Re: [Cfrg] AES GCM SIV analysis
John Mattsson <john.mattsson@ericsson.com> Thu, 19 January 2017 08:30 UTC
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From: John Mattsson <john.mattsson@ericsson.com>
To: Brian Smith <brian@briansmith.org>, Adam Langley <agl@imperialviolet.org>
Thread-Topic: [Cfrg] AES GCM SIV analysis
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Date: Thu, 19 Jan 2017 08:29:59 +0000
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Cc: "Cooley, Dorothy E" <decoole@nsa.gov>, "cfrg@irtf.org" <cfrg@irtf.org>
Subject: Re: [Cfrg] AES GCM SIV analysis
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I think that having the same nonce length as previous IETF AEADs should not be a goal. My view is that the GCM-SIV nonce length should be based purely on math. John On 2017-01-19, 04:53, "Cfrg on behalf of Brian Smith" <cfrg-bounces@irtf.org on behalf of brian@briansmith.org> wrote: >Adam Langley <agl@imperialviolet.org> wrote: >> Brian Smith <brian@briansmith.org> wrote: >>> The actual text in the draft is "Thus with AES-GCM-SIV we recommend >>> that, for a specific key, a nonce not be repeated more than 2^8 >>> times." >>> >>> Is this a meaningful recommendation? How would one go about following >>> this recommendation in a practical implementation? In particular, >>> AES-GCM-SIV is mostly interesting in implementations that cannot >>> reliably and/or consistently save state, and it seems like any attempt >>> to write code to enforce this relies on saving state [snip] > >> [snip] With a random, 96-bit nonce you don't have to worry >> about the probability of having repeated a single value > 2^8 times >> until you have a staggering number of plaintexts: greater than 2^100 >> of them. Since that vastly exceeds our current recommendation for >> number of plaintexts per key (2^50), it's basically not a concern. >> >> If that makes sense, what could we have written to be clearer? > >Perhaps: "We recommend instead that an implementation try to avoid >repeating a nonce for a specific key, just like it would it would do >for an AEAD that isn't nonce-misuse-resistant." This shifts the >emphasis away from the 2^8 number to where it belongs, IMO. Note that >"256" and how it is derived and why it is safe is explained in the >next paragraph anyway. > >> I agree that large AEAD messages have several problems. But I don't >> think that we have any need for a larger nonce (see above). (And the >> nonce is used with a counter only in the KDF phase, so it's unrelated >> to the maximum plaintext size.) > >Is there any way that a larger nonce (e.g. 120 bits) hurts, other than >being inconsistent with existing IETF AEADs? > >Cheers, >Brian >-- >https://briansmith.org/ > >_______________________________________________ >Cfrg mailing list >Cfrg@irtf.org >https://www.irtf.org/mailman/listinfo/cfrg
- [Cfrg] AES GCM SIV analysis Cooley, Dorothy E
- Re: [Cfrg] AES GCM SIV analysis Adam Langley
- Re: [Cfrg] AES GCM SIV analysis Adam Langley
- Re: [Cfrg] AES GCM SIV analysis Brian Smith
- Re: [Cfrg] AES GCM SIV analysis Shay Gueron
- Re: [Cfrg] AES GCM SIV analysis Adam Langley
- Re: [Cfrg] AES GCM SIV analysis Brian Smith
- Re: [Cfrg] AES GCM SIV analysis John Mattsson
- Re: [Cfrg] AES GCM SIV analysis Shay Gueron
- Re: [Cfrg] AES GCM SIV analysis Shay Gueron
- Re: [Cfrg] AES GCM SIV analysis Adam Langley
- Re: [Cfrg] AES GCM SIV analysis Dan Harkins
- Re: [Cfrg] AES GCM SIV analysis Adam Langley
- Re: [Cfrg] AES GCM SIV analysis Alex Cope
- Re: [Cfrg] AES GCM SIV analysis Watson Ladd
- Re: [Cfrg] AES GCM SIV analysis Shay Gueron
- Re: [Cfrg] AES GCM SIV analysis Alex Cope
- Re: [Cfrg] AES GCM SIV analysis Andy Lutomirski