Re: [openpgp] Backwards compatibility

Justus Winter <justus@sequoia-pgp.org> Wed, 25 October 2023 11:35 UTC

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From: Justus Winter <justus@sequoia-pgp.org>
To: Werner Koch <wk@gnupg.org>, Paul Wouters <paul@nohats.ca>
Cc: openpgp@ietf.org
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Date: Wed, 25 Oct 2023 13:35:06 +0200
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Subject: Re: [openpgp] Backwards compatibility
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Werner Koch <wk@gnupg.org> writes:

>> be used by those who want and those who do not want AES_GCM, and the
>> complexity of a code point in addition to EAX and OCB is well, miniscule.
>
> This was the major reason to drop the deployed format and introduce a
> the HKDF.

No.  The key derivation step was introduced to provide key space
separation between different modes.  This is a defense-in-depth
mechanism.  See
https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#section-5.13.2-4:

> The KDF mechanism provides key separation between cipher and AEAD
> algorithms.

OpenPGP has suffered from the lack of key space separation in the past.
In the following attack, a SEIPD packet (type 18) was downgraded to a
SED packet (type 9), effectively stripping the authentication bits,
making the encryption container malleable.  Key space separation would
have prevented this.

https://www.metzdowd.com/pipermail/cryptography/2015-October/026685.html

Best,
Justus