Add ECDSA public key format

[tomaka]

libp2p/specs#100

[twittner]

Is there a specific need for this? In the spec ECDSA is optional and does not seem particularly attractive on its own.

[tomaka]

We don't have any need for this, but let's have an issue open.

[davxy]

Hi @tomaka , I've started working on this issue.

I already have a working implementation compatible with the go-libp2p one but, before PR submission, I would like to ask some opinions wrt multiple curves support.

Preliminary:

• I've used RustCrypto p256 crate. I also made an attempt using ring but I would like to have support for wasm and I prefer a plain Rust crate.
• The feature is optional (as rsa and secp256k1)
• Currently the implementation (as the golang one) supports only p256 curve with sha256.

I was wondering if we would like to keep a door open for more curves that may be inserted in the future.

For example: my current implementation has introduced the following function in identity.rs:

+    #[cfg(feature = "ecdsa")]
+    pub fn generate_ecdsa() -> Keypair {
+        Keypair::Ecdsa(ecdsa::Keypair::generate())
+    }

Maybe could be good to define a CurveId enum in ecdsa module:

    pub enum CurveId {
        Secp256r1
    }

And then define ecdsa::Keypair::generate to take a CurveId parameter (maybe optional with default to P256?).

As I said, for the moment there is only one possibility (i.e. p256), but maybe is a good idea to keep a door open for future extensions.

Thank you

[mxinden]

I've used RustCrypto p256 crate. I also made an attempt using ring but I would like to have support for wasm and I prefer a plain Rust crate.

@burdges and @dignifiedquire, as experts on rust crypto, do you have any opinions on this?

As I said, for the moment there is only one possibility (i.e. p256), but maybe is a good idea to keep a door open for future extensions.

I am not too worried making breaking changes at the in-process API level in the future in case we do decide to support other curves. (Quite the opposite for network level breaking changes.)