linux-kernel - Re: [PATCH 16/18] crypto: dh - calculate Q from P for the full public key verification

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Date:   Sun, 05 Dec 2021 07:07:11 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Nicolai Stange <nstange@...e.de>
Cc:     Hannes Reinecke <hare@...e.de>, Torsten Duwe <duwe@...e.de>,
        Zaibo Xu <xuzaibo@...wei.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        qat-linux@...el.com, keyrings@...r.kernel.org,
        Nicolai Stange <nstange@...e.de>
Subject: Re: [PATCH 16/18] crypto: dh - calculate Q from P for the full public key verification

Am Mittwoch, 1. Dezember 2021, 01:48:56 CET schrieb Nicolai Stange:

Hi Nicolai,

> As the ->q in struct dh_ctx gets never set anywhere, the code
> in dh_is_pubkey_valid() for doing the full public key validation in
> accordance to SP800-56Arev3 is effectively dead.
> 
> However, for safe-prime groups, Q = (P - 1)/2 by definition and this
> enables dh_is_pubkey_valid() to calculate Q on the fly for these groups.
> Implement this.
> 
> With this change, the last code accessing struct dh_ctx's ->q is now gone.
> Remove this member from struct dh_ctx.

Isn't it expensive to always calculate Q for a-priori known values? Why not 
add Q to the safe-prime definitions and do not do this operation here?

If you need Q for all of those safe-primes, you may get them from [1] and 
following lines.

[1] https://github.com/smuellerDD/acvpparser/blob/master/parser/
safeprimes.h#L346

Ciao
Stephan