vendor: pull in missing go-echd library

vendor/github.com/wsddn/go-ecdh/elliptic.go

@@ -0,0 +1,87 @@
+package ecdh
+
+import (
+	"crypto"
+	"crypto/elliptic"
+	"io"
+	"math/big"
+)
+
+type ellipticECDH struct {
+	ECDH
+	curve elliptic.Curve
+}
+
+type ellipticPublicKey struct {
+	elliptic.Curve
+	X, Y *big.Int
+}
+
+type ellipticPrivateKey struct {
+	D []byte
+}
+
+// NewEllipticECDH creates a new instance of ECDH with the given elliptic.Curve curve
+// to use as the elliptical curve for elliptical curve diffie-hellman.
+func NewEllipticECDH(curve elliptic.Curve) ECDH {
+	return &ellipticECDH{
+		curve: curve,
+	}
+}
+
+func (e *ellipticECDH) GenerateKey(rand io.Reader) (crypto.PrivateKey, crypto.PublicKey, error) {
+	var d []byte
+	var x, y *big.Int
+	var priv *ellipticPrivateKey
+	var pub *ellipticPublicKey
+	var err error
+
+	d, x, y, err = elliptic.GenerateKey(e.curve, rand)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	priv = &ellipticPrivateKey{
+		D: d,
+	}
+	pub = &ellipticPublicKey{
+		Curve: e.curve,
+		X: x,
+		Y: y,
+	}
+	
+	return priv, pub, nil
+}
+
+func (e *ellipticECDH) Marshal(p crypto.PublicKey) []byte {
+	pub := p.(*ellipticPublicKey)
+	return elliptic.Marshal(e.curve, pub.X, pub.Y)
+}
+
+func (e *ellipticECDH) Unmarshal(data []byte) (crypto.PublicKey, bool) {
+	var key *ellipticPublicKey
+	var x, y *big.Int
+
+	x, y = elliptic.Unmarshal(e.curve, data)
+	if x == nil || y == nil {
+		return key, false
+	}
+	key = &ellipticPublicKey{
+		Curve: e.curve,
+		X:     x,
+		Y:     y,
+	}
+	return key, true
+}
+
+// GenerateSharedSecret takes in a public key and a private key
+// and generates a shared secret.
+//
+// RFC5903 Section 9 states we should only return x.
+func (e *ellipticECDH) GenerateSharedSecret(privKey crypto.PrivateKey, pubKey crypto.PublicKey) ([]byte, error) {
+	priv := privKey.(*ellipticPrivateKey)
+	pub := pubKey.(*ellipticPublicKey)
+
+	x, _ := e.curve.ScalarMult(pub.X, pub.Y, priv.D)
+	return x.Bytes(), nil
+}