Adding new public key KEM API

[Falko Strenzke]

There is another point to consider for the design of a generic KEM API: 
the use of the public in the key derivation, which makes it necessary to 
pass the public key to the decapsulation function if one doesn't want to 
run the computation of the public key from the private key in the 
decapsulation function.

We are using using this now for instance in our OpenPGP PQC draft 
(https://github.com/openpgp-pqc/draft-openpgp-pqc/pull/66 corrects the 
ECC-KEM function signature regarding that matter).

- Falko

Am 24.10.23 um 08:25 schrieb NIIBE Yutaka:
> Werner Koch<wk at gnupg.org>  wrote:
>> On Thu, 19 Oct 2023 16:37, NIIBE Yutaka said:
>>
>>> gcry_error_t gcry_kem_decap (int algo,
>>>                               const void *seckey,
>>>                               const void *ciphertext,
>>>                               void *shared_secret);
>> I still don't feel comfortable without a size argument.
> Assumption here (for lower level API) is:
>
> 	It's caller side (user of libgcrypt) which does static
> 	compile-time check against ALGO and the length of each
> 	byte-array.
>
>          If not static, caller side can do run-time check, if needed,
>          before the call.
>
> Having a size argument would mean,
>
> 	libgcrypt does run-time check of the length (for each call)
>
> I wonder if this kind of run-time check in libgcrypt is useful in lower
> level API.
>
> I could imagine having an API offering static compile-time check.  In
> this case, it would provide a macro something like gcry_kem_decap_check
> which has length arguments.  The ABI is gcry_kem_decap.
-- 

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