Re: [openpgp] AEAD Chunk Size

Bill Frantz <frantz@pwpconsult.com> Fri, 29 March 2019 00:10 UTC

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Date: Thu, 28 Mar 2019 17:10:23 -0700
From: Bill Frantz <frantz@pwpconsult.com>
To: Bart Butler <bartbutler@protonmail.com>
cc: "Neal H. Walfield" <neal@walfield.org>, openpgp@ietf.org, Justus Winter <justuswinter@gmail.com>, Jon Callas <joncallas@icloud.com>, Jon Callas <joncallas=40icloud.com@dmarc.ietf.org>
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Subject: Re: [openpgp] AEAD Chunk Size
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On 3/28/19 at 4:47 PM, 
bartbutler=40protonmail.com@dmarc.ietf.org (Bart Butler) wrote:

>Maybe a reasonable compromise is, as we discussed a while ago, 
>simply an upper bound on the chunk size rather than fixing it 
>to a single value. What's a reasonable upper bound for 
>constrained systems? 4MB?

The Arduino Uno, which the web site says is the most popular 
Arduino in the line 
<https://store.arduino.cc/usa/arduino-uno-rev3>, has:

  Flash Memory 32 KB (ATmega328P) of which 0.5 KB used by bootloader
  SRAM 2 KB (ATmega328P)
  EEPROM 1 KB (ATmega328P)

So it might be able to use a chunk up to 1KB without having to 
do the kind of pipelining that leads to security bugs and messy code.

YMMV!


In general, when asked about the smallest target for crypto 
algorithms, I think it was in the cryptography mailing list, the 
consensus was an 8 bit microprocessor with 64K of addressing 
would always be the target. People will always find a use for 
cheaper and smaller processors and 8 bits with 64K addresses 
seems to be where we have settled.

Cheers - Bill

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