Re: [Cfrg] AES GCM SIV analysis

[John Mattsson <[e-mail]>]

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"
<[e-mail] on behalf of [e-mail]> wrote:

>Adam Langley <[e-mail]> wrote:
>> Brian Smith <[e-mail]> 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/
>
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