keychain: add single key wrapper implementations

keychain/ecdh.go

@@ -0,0 +1,82 @@
+package keychain
+
+import (
+	"crypto/sha256"
+
+	"github.com/btcsuite/btcd/btcec"
+)
+
+// NewPubKeyECDH wraps the given key of the key ring so it adheres to the
+// SingleKeyECDH interface.
+func NewPubKeyECDH(keyDesc KeyDescriptor, ecdh ECDHRing) *PubKeyECDH {
+	return &PubKeyECDH{
+		keyDesc: keyDesc,
+		ecdh:    ecdh,
+	}
+}
+
+// PubKeyECDH is an implementation of the SingleKeyECDH interface. It wraps an
+// ECDH key ring so it can perform ECDH shared key generation against a single
+// abstracted away private key.
+type PubKeyECDH struct {
+	keyDesc KeyDescriptor
+	ecdh    ECDHRing
+}
+
+// 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 *PubKeyECDH) PubKey() *btcec.PublicKey {
+	return p.keyDesc.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 *PubKeyECDH) ECDH(pubKey *btcec.PublicKey) ([32]byte, error) {
+	return p.ecdh.ECDH(p.keyDesc, pubKey)
+}
+
+// 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
+}
+
+var _ SingleKeyECDH = (*PubKeyECDH)(nil)
+var _ SingleKeyECDH = (*PrivKeyECDH)(nil)