Support for signatures without v value

[jannikluhn]

What was wrong?

The v4 identity scheme defined in the ENR spec uses signatures consisting of only r and s, not v. eth-keys currently does not allow verification of signatures without v.

How can it be fixed?

Suggested approach:

• Implement ecdsa_verify in all backends without falling back to ecdsa_recover
• Allow instantiation of Signature with an (r, s) tuple and/or a 64 byte signature. All recover methods should raise some kind of exception (ValidationError?) if v is not set.

[jannikluhn]

Oh, and we probably need a method to create such a signature. Maybe PrivateKey.sign_non_recoverable?

[pipermerriam]

At the highest level I'm 👎 on this.

Wondering if we could segregate this into two classes so that rather than having APIs that might raise an error, you instead decide what type of signature you are working with and don't have to do any value checking.

Signature -> RecoverableSignature

We can figure out how to deal with names and the existing signature class if we like this approach.

[jannikluhn]

Agreed, two classes sound like the safer approach (probably with a shared base class).