linux-kernel - Re: [PATCH v3 11/15] crypto: dh - allow for passing NULL to the ffdheXYZ(dh)s' ->set

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Date:   Fri, 11 Feb 2022 20:09:15 +1100
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Nicolai Stange <nstange@...e.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Stephan Müller <smueller@...onox.de>,
        Hannes Reinecke <hare@...e.de>, Torsten Duwe <duwe@...e.de>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org
Subject: Re: [PATCH v3 11/15] crypto: dh - allow for passing NULL to the
 ffdheXYZ(dh)s' ->set_secret()

On Wed, Feb 02, 2022 at 11:40:08AM +0100, Nicolai Stange wrote:
> Ephemeral key generation can be requested from any of the ffdheXYZ(dh)
> variants' common ->set_secret() by passing it an (encoded) struct dh
> with the key parameter being unset, i.e. with ->key_size == 0. As the
> whole purpose of the ffdheXYZ(dh) templates is to fill in the group
> parameters as appropriate, they expect ->p and ->g to be unset in any
> input struct dh as well. This means that a user would have to encode an
> all-zeroes struct dh instance via crypto_dh_encode_key() when requesting
> ephemeral key generation from a ffdheXYZ(dh) instance, which is kind of
> pointless.
> 
> Make dh_safe_prime_set_secret() to decode a struct dh from the supplied
> buffer only if the latter is non-NULL and initialize it with all zeroes
> otherwise.
> 
> That is, it is now possible to call
> 
>   crypto_kpp_set_secret(tfm, NULL, 0);
> 
> on any ffdheXYZ(dh) tfm for requesting ephemeral key generation.

Why do we need to support the non-NULL case? IOW what in the kernel
will be using these new templates with a non-NULL parameter?

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt