It is awkward to instantiate a SigningKey from its value.

Use case:

We have a signing keypair that has been generated by an other program in a different language, and we need to sign and verify messages with them.

The naive way does not work:

message = b'this is a message'
raw_signing_key = b'....' # a bunch of bytes of length 64
signing_key = SigningKey(raw_signing_key, encoder=RawEncoder)
signing_key.sign(message)

That's because the SigningKey constructor expects a seed, not the value:

# in src/nacl/signing.py
class SigningKey(...):

    def __init__(self, seed, encoder=encoding.RawEncoder):
        ...
        public_key, secret_key = nacl.bindings.crypto_sign_seed_keypair(seed)

We have a workaround, though:

We assing the _signing_key variable directly:

message = b'this is a message'
raw_signing_key = b'....' # a bunch of bytes of length 64
fake_seed = b"\0" * 32
signing_key = nacl.signing.SigningKey(fake_seed)
signing_key._signing_key = raw_signing_key
signing_key.sign(message)

And then we can verify the signature by instantiating a VerifyKey directly:

raw_verify_key = b'....' # bunch of bytes of length 32
verify_key = nacl.signing.VerifyKey(public_key, encoder=RawEncoder)

Of course, this is a bad idea because if you use signing_key.verify_key you get the wrong key ...

Proposed fix

Maybe we could have something like this instead ?

class SigningKey:
    @classmethod
    def from_value(signing_key):
           res = cls()
           res._signing_key = signing_key
           res.verify_key = VerifyKey(bindings.crypto_sign_ed25519_sk_to_pk(signing_key))

And then:

signing_key = SigningKey.from_value(raw_signing_key)

Thoughts ?

Start in src/nacl/signing.py with SigningKey.init and its verify_key behavior. Review the proposed raw-key construction and crypto_sign_ed25519_sk_to_pk entry point, then define the API and validation needed so a 64-byte signing value produces the matching public key; done means signing and verification use the imported keypair correctly.