Add ECDSAPrivateKey.ECDH which takes a *ecdh.PublicKey

[AGWA]

The current ECDSAPrivateKey.SharedKey method takes a *ecdsa.PublicKey. However, using crypto/ecdsa and crypto/elliptic for ECDH has been deprecated in the standard library in favor of crypto/ecdh.

This commit adds a new ECDH method to ECDSAPrivateKey which takes a *ecdh.PublicKey. This method has the same signature as ecdh.PrivateKey.ECDH, meaning the following interface can be be used to do ECDH with both standard library private keys and piv-go keys, providing the same flexibility as crypto.Signer and crypto.Decrypter:

interface {
	ECDH(*ecdh.PublicKey) ([]byte, error)
}

ECDSAPrivateKey.SharedKey has been re-implemented as a small wrapper around ECDSAPrivateKey.ECDH. It has not been marked as deprecated, but it could be.

[ericchiang]

thanks!

[maraino]

It's quite easy to convert on both directions crypto/ecdh key to crypto/elliptic. See, for example, this, https://github.com/smallstep/crypto/blob/47febcd8eeae3b35809606fe1f148babe38dc4e0/kms/mackms/mackms.go#L1184-L1208

We can use a method like this to implement ECDH(*ecdh.PublicKey), converting the public key to ecdsa.PublicKey and use SharedKey to perform ECDH.

Broader support in PrivateKey and SetPrivateKeyInsecure is possible too, for example, if we pass as ecdh.P256 key to PrivateKey we can return a type that implements ECDH and returns the ecdh.PublicKey in the Public() crypto.PublicKey method. We can also implement Curve() ecdh.Curve or PublicKey() *ecdh.PublicKey but methods like Equal(x crypto.PrivateKey) bool will be harder to support, we can compare the public keys and assume there are no collisions :)