[Grant] ECDH

[gregdhill]

ECDH Grants

Establish a shared secret using nacl box: https://godoc.org/golang.org/x/crypto/nacl/box#GenerateKey which uses Elliptic Curve Diffie Hellman to get shared secret.

Uses Curve25519, so if we want to use underlying ed25519 keys we need some key derivation. Also we may want to generate ephemeral keys to do sharing for perfect forward secrecy: https://en.wikipedia.org/wiki/Station-to-Station_protocol