On Wed, 20 Jan 2021 at 20:00, Alessandrelli, Daniele <daniele.alessandre...@intel.com> wrote: > > 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? >
You should set the CRYPTO_ALG_NEED_FALLBACK flag on both implementations, to ensure that neither of them are considered as fallbacks themselves.