Re: [openpgp] AEAD Chunk Size

"Derek Atkins" <derek@ihtfp.com> Thu, 14 March 2019 14:17 UTC

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Date: Thu, 14 Mar 2019 10:17:41 -0400
From: "Derek Atkins" <derek@ihtfp.com>
To: "Neal H. Walfield" <neal@walfield.org>
Cc: "Derek Atkins" <derek@ihtfp.com>, "Werner Koch" <wk@gnupg.org>, openpgp@ietf.org, "Vincent Breitmoser" <look@my.amazin.horse>
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Subject: Re: [openpgp] AEAD Chunk Size
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On Thu, March 14, 2019 9:47 am, Neal H. Walfield wrote:
>
> AEAD catches not only these errors, but also providers ciphertext
> integrity.
>
> Are you arguing like Werner that catching transmission errors is
> enough and that we shouldn't bother with ciphertext integrity?

I don't see how these two are mutually exclusive.

Each chunk can provide protection against both a transmission error and
ciphertext integrity (per chunk).  A simple counter in the chunk header
can protect against splicing attacks, so an attacker could not remove a
middle chunk or otherwise swap chunk orders.  So the only issue is
truncation, where an attacker prevents transmission at the end.

Obviously the receiver/verifier needs to know how to handle the case of a
failure at a chunk or truncation level.  What that means, of course, is up
to the application.

But I don't see how using AEAD per chunk can do anything but help.

-derek

-- 
       Derek Atkins                 617-623-3745
       derek@ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant