crypto: add ECDH interface and implementation

With this commit we create a new interface that abstracts away the
ECDH implementation. We also create one implementation that satisfies
the interface that can be used if the private key is known directly.
Later we will change the generateSharedSecret function to use this
interface instead of doing the ECDH operation directly.

Author: guggero

crypto.go

@@ -22,6 +22,55 @@ const (
 // the output of a SHA256 hash.
 type Hash256 [sha256.Size]byte
 
+// SingleKeyECDH is an abstraction interface that hides the implementation of an
+// ECDH operation against a specific private key. We use this abstraction for
+// the long term keys which we eventually want to be able to keep in a hardware
+// wallet or HSM.
+type SingleKeyECDH interface {
+	// PubKey returns the public key of the private key that is abstracted
+	// away by the interface.
+	PubKey() *btcec.PublicKey
+
+	// ECDH performs a scalar multiplication (ECDH-like operation) between
+	// the abstracted private key and a remote public key. The output
+	// returned will be the sha256 of the resulting shared point serialized
+	// in compressed format.
+	ECDH(pubKey *btcec.PublicKey) ([32]byte, error)
+}
+
+// PrivKeyECDH is an implementation of the SingleKeyECDH in which we do have the
+// full private key. This can be used to wrap a temporary key to conform to the
+// SingleKeyECDH interface.
+type PrivKeyECDH struct {
+	// PrivKey is the private key that is used for the ECDH operation.
+	PrivKey *btcec.PrivateKey
+}
+
+// PubKey returns the public key of the private key that is abstracted away by
+// the interface.
+//
+// NOTE: This is part of the SingleKeyECDH interface.
+func (p *PrivKeyECDH) PubKey() *btcec.PublicKey {
+	return p.PrivKey.PubKey()
+}
+
+// ECDH performs a scalar multiplication (ECDH-like operation) between the
+// abstracted private key and a remote public key. The output returned will be
+// the sha256 of the resulting shared point serialized in compressed format. If
+// k is our private key, and P is the public key, we perform the following
+// operation:
+//
+//  sx := k*P
+//  s := sha256(sx.SerializeCompressed())
+//
+// NOTE: This is part of the SingleKeyECDH interface.
+func (p *PrivKeyECDH) ECDH(pub *btcec.PublicKey) ([32]byte, error) {
+	s := &btcec.PublicKey{}
+	s.X, s.Y = btcec.S256().ScalarMult(pub.X, pub.Y, p.PrivKey.D.Bytes())
+
+	return sha256.Sum256(s.SerializeCompressed()), nil
+}
+
 // DecryptedError contains the decrypted error message and its sender.
 type DecryptedError struct {
 	// Sender is the node that sent the error. Note that a node may occur in