Sun Shuzhou

The curve parameter a of secp256k1 is 0, while the package `crypto/elliptic` only supports short-form Weierstrass curves with a=-3. We have to use another library, e.g., the [secp256k1](https://godoc.org/github.com/ethereum/go-ethereum/crypto/secp256k1) in go-ethereum....

@Baha-sk Nice to hear about that. If the secp256k1 curve is made available in native go `crypto/elliptic`, supporting ES256K keys in go-jose will be easy. I will track that issue...