RSA Implement Keypair trait for the RSA keys

This is rather intrusive implementation. It reworks the way RsaPrivateKey is organized. Unfortunately it also meant that default Serialize an Deserilialize derivations provide a list of tokens that are not backwards-compatible. Thus I had to implement those traits manually.

Sep 16 '22 19:09 lumag

@lumag I'm confused why you need to change RsaPublicKey/RsaPrivateKey at all

Sep 16 '22 23:09 tarcieri

@lumag I'm confused why you need to change RsaPublicKey/RsaPrivateKey at all

@tarcieri because I have to embed a VerifyingKey as a field into the SigningKey. Otherwise as_ref() will fail with the returning a reference for temp variable error. And putting VerifyingKey next to RsaPrivateKey results in information duplication. So...I ended up with RsaPrivateKeyParts,

Sep 18 '22 16:09 lumag

@lumag I opened a tracking issue for problems with Keypair and suggested a possible alternative: https://github.com/RustCrypto/traits/issues/1124

Sep 19 '22 21:09 tarcieri

@tarcieri as a side note: While this PR is intrusive, it removes the required usage of RsaPrivateKey, making all SigningKey implementations first class citizens. Also note, that ECDSA signing keys also incorporate VerifyingKey as a field.

Sep 24 '22 15:09 lumag

I have rebased this PR on top of RSA 0.7.0. Note, I had to disable AsRef<RsaPrivateKey> for SigningKey<D> implementations. Since RsaPrivateKey is not a field of SigningKey, I don't see a way to implement AsRef

Oct 22 '22 23:10 lumag

@lumag PTAL at the Keypair changes to the signature crate here:

https://github.com/RustCrypto/traits/pull/1141/files#diff-1eb80c5452df3ad684ce36b4bfb4590a600978c10f0135bcba713ca18617d9bc

It makes the AsRef support an optional optimization, and changes the default Keypair trait to return the public key by value.

This is a proposed breaking change which I think would help simplify this use case.

Oct 24 '22 19:10 tarcieri