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Re: [openpgp] AEAD Chunk Size

Jon Callas <joncallas@icloud.com> Tue, 16 April 2019 01:24 UTC

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From: Jon Callas <joncallas@icloud.com>
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Date: Mon, 15 Apr 2019 18:23:56 -0700
Cc: Jon Callas <joncallas@icloud.com>, Jon Callas <joncallas=40icloud.com@dmarc.ietf.org>, Bart Butler <bartbutler@protonmail.com>, "openpgp@ietf.org" <openpgp@ietf.org>, Justus Winter <justuswinter@gmail.com>, "Neal H. Walfield" <neal@walfield.org>, Peter Gutmann <pgut001@cs.auckland.ac.nz>
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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 Apr 15, 2019, at 5:16 PM, Derek Atkins <derek@ihtfp.com> wrote:
> 
> Jon,
> 
> I would rather 256K not be the One Required Size.  It's larger than the
> RAM size on some of my systems.  I would still prefer 16K (or even 8K).

Bart, Neal? What do you guys think?

I spent part of the afternoon verifying GnuPG's chunk size. I started off using --list-packets to get the size of something, and then moved to using pgpdump. gpg --list-packets reports the combined length of all the partials as if it was one big thing, but pgpdump shows different. I found that out after I sent my last thing. I thought it was using single-chunk lengths even with some moderately large files. Only when it was showing single size for something over a gigabyte did I go grab pgpdump. 

Playing around, GnuPG aggressively uses 8K chunks, and even when I encrypted something below 8K in length, it used a 4K chunk, a 2K chunk, and then tied off the rest. 

Why not pick 8K?

Arguments in favor:

* It's what is done now.
* As noted in Bart and Neal's comments, there isn't going to be a huge performance difference.
* It works in small implementations, as Derek says.

Arguments against -- 
* I have no idea. Someone who isn't me should say something even like "I want it."

Anyone else?

	Jon

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