Re: [openpgp] AEAD Chunk Size

Jon Callas <joncallas@icloud.com> Wed, 27 February 2019 22:37 UTC

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From: Jon Callas <joncallas@icloud.com>
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Date: Wed, 27 Feb 2019 14:37:20 -0800
Cc: Jon Callas <joncallas@icloud.com>, Vincent Breitmoser <look@my.amazin.horse>, "openpgp@ietf.org" <openpgp@ietf.org>, "Neal H. Walfield" <neal@walfield.org>
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Subject: Re: [openpgp] AEAD Chunk Size
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> On Feb 27, 2019, at 1:34 PM, Bart Butler <bartbutler=40protonmail.com@dmarc.ietf.org> wrote:
> 
> Hi all,
> 
> We (Sunny Rajan and I) definitely agree with reducing the range of allowed sizes, because both 4 exabyte chunks and large messages composed of 64-byte chunks are clearly not optimal.
> 
> In our AEAD implementation for OpenPGP.js we settled on a default `c` of 12 rather than 8 after some performance benchmarking showed that 256 kiB chunks were more performant for the most common message sizes. Our result was specific to the web crypto API, and therefore specific to OpenPGP.js, but in general libraries may want to use different default sizes depending on implementation details like this.
> 
> So ideally we’d prefer to keep the size byte, but to shrink its range in both directions. For example, the RFC could state that the chunk SHOULD be 16 kiB (or 256 kiB, hint hint), but decryption MUST be available for `c` values between 8-12 inclusive. This would also allow us to be backwards-compatible with messages that have already been created following the current version of the draft, which do exist given the benefit that AEAD provides and that OpenPGP.js has supported the current draft in experimental mode for most of the last year. 

I think this is a reasonable way to have it to put an upper limit with a SHOULD.

I believe that if we limit it to 16K, we *will* regret that, for performance or other reasons. And while some of the upper sizes are presently absurdly large, one never knows what happens a couple decades from now. 

Moreover, forbidding it says that we state now that no one could ever have a reasonable use; my experience is that categorical statements like that are just asking the fates to bite us in an uncomfortable place. Amazon S3 increased their max object size to 5TB a few years ago. I’m not saying it should be that large, but I think this is a pretty convincing argument that 16K is too small.

So I think that a SHOULD is the right way to put it. I care less about what the SHOULD limit should be, but a small hard limit sounds like a bad idea.

	Jon