Re: [openpgp] AEAD Chunk Size

Bart Butler <bartbutler@protonmail.com> Sat, 30 March 2019 16:42 UTC

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Date: Sat, 30 Mar 2019 16:42:19 +0000
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To: Benjamin Kaduk <kaduk@mit.edu>
From: Bart Butler <bartbutler@protonmail.com>
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Subject: Re: [openpgp] AEAD Chunk Size
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Hi Ben,

> One concern that I have (and is only tangentially related to this quoted
> part) is that I want to make it easy for implementations to "do the right
> thing" when ciphertext is modified, i.e., return an error, and specifically
> to return an error without releasing any plaintext that originates from the
> modified ciphertext. The current openpgp ecosystem does not seem to be
> very compliant to that desired behavior, and part of that may be due to a
> lack of philosophical support/help from the spec.
> 


If you mean 'modified ciphertext' to equal 'modified chunk', and are OK with releasing previously unauthenticated chunks, then I completely agree. The alternative is is just no streaming.

> I'm still not sure I understand the point of very large chunks, since once
> they get really big an implementation is choosing between streaming
> plaintext from potentially modified ciphertext or return an error without
> even attempting to process the chunk. I'm not convinced that the second
> will win out in implementations if we alow very large chunks.
> 


Agreed. Part of the rationale with a smaller chunk limit is not forcing implementations to make this choice. The guidance becomes very simple--never release unauthenticated chunks, full stop.