Signer module - kairos-crypto

[koxu1996]

This PR moves cryptography from kairos-cli - introduced temporarily in Base CLI parsing #12 - into a separate package named kairos-crypto.

There are few major improvements, mainly:

1. Introduced CryptoSigner trait to provide implementation-agnostic interface.
2. Casper specific implementation is available via feature flag crypto-casper (enabled by default).
3. Introduced sign() and verify() methods - it will be used for signature checking, once Deterministic payload format: ASN.1 schema with DER encoding #30 is merged.
4. Serialization error was split into distinct CryptoError::Serialization and CryptoError::Deserialization.

[koxu1996]

Rebased and ready for review.

[Avi-D-coder]

I'd prefer AsRef<[u8]> instead of &[u8].

[koxu1996]

Fixed in 9792ee3.

[Avi-D-coder]

This is fine.
I don't love traits with a single impl, but since we are planing on adding more it's good to merge.

[xcthulhu]

Mentioned elsewhere, casper_types::crypto::sign has an unused public_key.

Here is the crypto signing code that litmus uses (which has been unit tested and exhaustively tested against casper_types for compatibility).

https://github.com/cspr-rad/litmus/blob/673f8182560f7f1ff6fee04d775070a6b3e380c3/src/crypto.rs#L24-L40

// Signs the given message using the given key.
pub fn sign<T: AsRef<[u8]>>(secret_key: &SecretKey, message: T) -> Signature {
    match secret_key {
        SecretKey::System => Signature::System,
        SecretKey::Ed25519(secret_key) => {
            let signature = secret_key.sign(message.as_ref());
            Signature::Ed25519(signature)
        }
        SecretKey::Secp256k1(secret_key) => {
            let signature = secret_key
                .try_sign(message.as_ref())
                .expect("should create signature");
            Signature::Secp256k1(signature)
        }
        _ => panic!("SecretKey is marked as non-exhaustive, but this should never happen"),
    }
}

[koxu1996]

I am aware that public key is not used internally, but I see no point in duplicating the Casper implementation just to get rid of it:

Additionally, public key is always available in Signer, so there is no extra code necessary.

[Rom3dius]

Looks good to go