Re: [openpgp] OpenPGP SEIP downgrade attack

Peter Gutmann <pgut001@cs.auckland.ac.nz> Wed, 07 October 2015 13:50 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Werner Koch <wk@gnupg.org>
Thread-Topic: [openpgp] OpenPGP SEIP downgrade attack
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Date: Wed, 7 Oct 2015 13:50:52 +0000
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Cc: "cfrg@mail.ietf.org" <cfrg@mail.ietf.org>, Jonas Magazinius <jonas.magazinius@assured.se>, "cryptography@metzdowd.com" <cryptography@metzdowd.com>, "openpgp@ietf.org" <openpgp@ietf.org>
Subject: Re: [openpgp] OpenPGP SEIP downgrade attack
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Werner Koch <wk@gnupg.org>; writes:

>And wait another 15 years until it has been taken up by all implementations?
>What is wrong with the planned AE mode?

Which has just as little support as a planned EtM mode?  

The reason why I prefer EtM is that it can be pretty trivially retrofitted to
existing crypto (just add a SHA-256 MAC somewhere) and is compatible with any
existing cipher, while whatever AEAD mechanism is chosen (I'm guessing AES-
GCM, which seems to be fashionable) is purely for AES, there's no Twofish or
CAST or whatever AEAD mode defined.

Peter.