Support passing ssl library key handles to algorithms

[sandro97git]

I have a use case where I already have the private key represented as an openssl EVP_PKEY instace. Currently an alogrithm constructor for passing such an object directly is missing.

[Thalhammer]

Adding support for this is fine from my side. We might wanna consider adding a few things though:

• Like you already said, a RSA variant is missing
• We should probably support passing a public key as well (pkey can store both public and private keys) to allow using it when only verification is required.

[sandro97git]

I added commit e37e259 for supporting public keys as well.

If this is fine I can try to add it for RSA as well.

[Thalhammer]

Looks fine so far to me.

[sandro97git]

@Thalhammer Do you know why the coverage check is failing?

[prince-chrismc]

@Thalhammer Do you know why the coverage check is failing?

Seems to be having an issue with the gtest code gen from the macro 🤔

[prince-chrismc]

Think out load... I am curious if this is more error prone? you can pass any key with any algorithm and name.

Bonus points, new PR with a negative test for the MD* being wrong to the size that would be interesting to see how it fails 🏗️

otherwise this matchs our exsting ctor with a little duplication 👍

[sandro97git]

I agree this is indeed a bit error prone. However this is already possible using the existing ecdsa or rsa constructor.
I'm not an expert on this jwt algorithms but I wonder if it wouldn't be possible to select the alorithm solely based on the key length. Something similar to this code:

const auto bits = EVP_PKEY_bits(private_key.GetRaw());
const EVP_MD* (*md)();
std::size_t siglen;
std::string name;
switch (bits) {
  case 384:
    md = EVP_sha384;
    siglen = 96;
    name = "ES384";
    break;
  case 521: {
    md = EVP_sha512;
    siglen = 132;
    name = "ES512";
    break;
  }
  default:
    throw std::invalid_argument{"unsupported key size: " + std::to_string(bits)};
}

Would this be possible or are keys of the same lenght used for multiple algorithms?