feat(session): support Signer trait through a Session

[RaitoBezarius]

signature is a widely used "standard" trait, offering high level APIs to manipulate signers and verifier.

This implements a Signer trait on a returned SignatureRequest through Session which is a prepared "signature request" with filled mechanism and key.

This makes it possible to wire cryptoki further with other APIs of the Rust ecosystem.

I made it a default feature for now, let me know if you prefer to remove this.

[baloo]

I think that works, but we'll also need to expose ObjectHandle as a Keypair. This is used by the various object builders of the rust-crypto ecosystem (x509 cert, pkcs7, ...) because this is used to pull the public key of the key doing the signing in various places.

[RaitoBezarius]

I think that works, but we'll also need to expose ObjectHandle as a Keypair. This is used by the various object builders of the rust-crypto ecosystem (x509 cert, pkcs7, ...) because this is used to pull the public key of the key doing the signing in various places.

Alas, you need a Session to fetch attributes of an object handle, so I'm not sure Keypair on ObjectHandle is the best idea.

We probably need to do Keypair on the SignatureRequest rather than the ObjectHandle.

[baloo]

and it would require some abstraction as to expose an abstract of ecdsa/rsa keys. They expose different attributes in the PKCS#11 world.

[RaitoBezarius]

and it would require some abstraction as to expose an abstract of ecdsa/rsa keys. They expose different attributes in the PKCS#11 world.

Yeah, right now, I am asking myself a basic question like: given an ObjectHandle of a private key, how can you find the public key's ObjectHandle?

[baloo]

Get the KeyType with get_attributes,

For RSA you then need to fetch PublicExponent and Modulus
For ECDSA, grab the EcPoint and EcParams. Params defines the curve (nist-p256, ...), the point being the "public key".

There is half a ton of things that need to be done correctly, not least of which verifying the EcPoint is on the specified curve.

Note: you can fetch all of those in a single call.

[RaitoBezarius]

Closing in favor of #192.