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

[Nicolai Stange <nstange@suse.de>]

Herbert Xu <herbert@gondor.apana.org.au> writes:

> 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?

The only "real" user, NVME in-band auth, will indeed only use ephemeral
keys AFAICT, but the known-answer selftests install a static key each.
So those will have to invoke ->set_secret() with a non-NULL parameter.

Thanks,

Nicolai

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