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Re: [Cfrg] Call for adoption for draft-wood-cfrg-aead-limits

Martin Thomson <mt@lowentropy.net> Mon, 27 July 2020 04:31 UTC

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Date: Mon, 27 Jul 2020 14:31:28 +1000
From: Martin Thomson <mt@lowentropy.net>
To: cfrg@irtf.org
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Subject: Re: [Cfrg] Call for adoption for draft-wood-cfrg-aead-limits
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Hi Dan,

On Sat, Jul 25, 2020, at 12:46, Dan Harkins wrote:
>    But as I mentioned on the virtual meeting I'd like to see some 
> analysis on
> the impact that the amount of AAD has on the integrity (and if 
> applicable, the
> encryption) bound. I know this draft is motivated by the use of AEAD 
> algorithms
> with QUIC and TLS where the AAD will be a fraction of the ciphertext, but it
> should address other uses of AEAD algorithms that may do things differently.

This is good feedback.  Counting the AAD alongside the plaintext was a simplification that made sense, but I agree that it isn't always that way.  From memory, CCM was one that had clear separation, and that was a factor of 2 difference (because the limits count applications of the block cipher).  Generally, I would not concern myself with a mere factor of 2, but the margins on some CCM variants are pretty dismal, so could probably benefit from the extra margin.

Chris opened an issue so that we don't forget this feedback: https://github.com/chris-wood/draft-wood-cfrg-aead-limits/issues/16

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