Hi Ard, Thank you very much for your valuable feedback.
On Mon, 2021-01-18 at 13:09 +0100, Ard Biesheuvel wrote: > This is rather unusual compared with how the crypto API is typically > used, but if this is really what you want to implement, you can do so > by: > - having a "ecdh" implementation that implements the entire range, and > uses a fallback for curves that it does not implement > - export the same implementation again as "ecdh" and with a known > driver name "ecdh-keembay-ocs", but with a slightly lower priority, > and in this case, return an error when the unimplemented curve is > requested. > > That way, you fully adhere to the API, by providing implementations of > all curves by default. And if a user requests "ecdh-keembay-ocs" > explicitly, it will not be able to use the P192 curve inadvertently. I tried to implement this, but it looks like the driver name is mandatory, so I specified one also for the first implementation. Basically I defined two 'struct kpp_alg' variables; both with cra_name = "ecdh", but with different 'cra_driver_name' (one with cra_driver_name = "ecdh-keembay-ocs-fallback" and the other one with cra_driver_name = "ecdh-keembay-ocs"). Is this what you were suggesting? Anyway, that works (i.e., 'ecdh-keembay-ocs' returns an error when the unimplemented curve is requested; while 'ecdh-keembay-ocs' and 'ecdh' work fine with any curve), but I have to set the priority of 'ecdh- keembay-ocs' to something lower than the 'ecdh_generic' priority. Otherwise the implementation with fallback ends up using my "ecdh- keembay-ocs" as fallback (so it ends up using a fallback that still does not support the P-192 curve). Also, the implementation without fallback is still failing crypto self- tests (as expected I guess). Therefore, I tried with a slightly different solution. Still two implementations, but with different cra_names (one with cra_name = "ecdh" and the other one with cra_name = "ecdh-keembay"). This solution seems to be working, since, the "ecdh-keembay" is not tested by the self tests and is not picked up as fallback for "ecdh" (since, if I understand it correctly, it's like if I'm defining a new kind of kpp algorithm), but it's still picked when calling crypto_alloc_kpp("ecdh- keembay"). Does this second solution looks okay to you? Or does it have some pitfall that I'm missing? Regards, Daniele