func (k Key) String() string {
	return hex.EncodeToString(k[:])
}
Stringer interface compliance.
package doubleratchet
import "encoding/hex"
type Crypto interface {
	GenerateDH() (DHPair, error)
	DH(dhPair DHPair, dhPub Key) (Key, error)
	Encrypt(mk Key, plaintext, ad []byte) (authCiphertext []byte, err error)
	Decrypt(mk Key, ciphertext, ad []byte) (plaintext []byte, err error)
	KDFer
}
type DHPair interface {
	PrivateKey() Key
	PublicKey() Key
}
type Key []byte