Re: [openpgp] Disadvantages of Salted Signatures

"Neal H. Walfield" <neal@walfield.org> Tue, 12 December 2023 10:25 UTC

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Date: Tue, 12 Dec 2023 11:25:18 +0100
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From: "Neal H. Walfield" <neal@walfield.org>
To: Nickolay Olshevsky <o.nickolay@gmail.com>
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Subject: Re: [openpgp] Disadvantages of Salted Signatures
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Hi Nickolay,

On Mon, 11 Dec 2023 17:56:31 +0100,
Nickolay Olshevsky wrote:
> As far as I  remember there were proposal of adding some 'salt' signature subpacket, which would serve exactly same purpose.
> 
> This would work in both cases: if implementation needs salt, it would add it as subpacket, or do not add otherwise.

There's an important, but subtle difference between using the salt as
specified in the crypto-refresh, and putting the salt in a subpacket.
In the crypto refresh, the salt is placed at the very start:

  When creating or verifying a v6 signature, the salt is fed into the
  hash context before any other data.

  https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#section-5.2.4-2

Putting the salt in a subpacket means that there is still some data
that an attacker can predict and potentially control, which means it
doesn't preclude a chosen prefix attack.

> From the implementors point of view this would also be more interoperable with older implementations / RFC 4880.

Even more interoperable would be to use a notation, which is what we
are doing in Sequoia for v4 signatures.

Neal