Re: [openpgp] AEAD Chunk Size

Jon Callas <joncallas@icloud.com> Thu, 28 March 2019 19:33 UTC

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From: Jon Callas <joncallas@icloud.com>
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Date: Thu, 28 Mar 2019 12:33:39 -0700
Cc: Jon Callas <joncallas@icloud.com>, Tobias Mueller <muelli@cryptobitch.de>, Werner Koch <wk@gnupg.org>, "openpgp@ietf.org OpenPGP" <openpgp@ietf.org>, "Neal H. Walfield" <neal@walfield.org>, Vincent Breitmoser <look@my.amazin.horse>
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To: Derek Atkins <derek@ihtfp.com>
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Subject: Re: [openpgp] AEAD Chunk Size
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> On Mar 27, 2019, at 8:50 AM, Derek Atkins <derek@ihtfp.com> wrote:
> 
> In my mind, each chunk is its own AEAD ciphertext.  So the chunking is
> happening *during* AEAD encryption, and not after encryption.  I.e., the
> chunking and AEAD encryption should be tied together such that the chunk
> header is part of the AEAD protection and the chunk data is the AEAD
> encrypted data.
> 
> This approach does, IMHO, map directly into the RFC definition.

This is exactly what I presumed would be done — each chunk is an AEAD segment. I presumed that one would probably put a chunk number as Additional Data, and that the nonce context would carry over from one chunk to the next in some reasonable way.

That’s directly analogous to the present chunking mechanism, which uses CFB as a stream.

	Jon