Correct ECIES shared key length check

* Ensure the ECIES shared key is padded with zero bytes if it's
  smaller than the requested key length.
* Split the ECIES shared key error into two;
  one for when the generated key is too big for the params
  and one for when it's nil (point of infinity returned by the
  curve scalar multiplication).

ecies.go

@@ -17,7 +17,8 @@ var (
 	ErrInvalidCurve               = fmt.Errorf("ecies: invalid elliptic curve")
 	ErrInvalidParams              = fmt.Errorf("ecies: invalid ECIES parameters")
 	ErrInvalidPublicKey           = fmt.Errorf("ecies: invalid public key")
-	ErrSharedKeyTooBig  = fmt.Errorf("ecies: shared key is too big")
+	ErrSharedKeyIsPointAtInfinity = fmt.Errorf("ecies: shared key is point at infinity")
+	ErrSharedKeyTooBig            = fmt.Errorf("ecies: shared key params are too big")
 )
 
 // PublicKey is a representation of an elliptic curve public key.
@@ -90,16 +91,20 @@ func MaxSharedKeyLength(pub *PublicKey) int {
 // ECDH key agreement method used to establish secret keys for encryption.
 func (prv *PrivateKey) GenerateShared(pub *PublicKey, skLen, macLen int) (sk []byte, err error) {
 	if prv.PublicKey.Curve != pub.Curve {
-		err = ErrInvalidCurve
-		return
+		return nil, ErrInvalidCurve
+	}
+	if skLen+macLen > MaxSharedKeyLength(pub) {
+		return nil, ErrSharedKeyTooBig
 	}
 	x, _ := pub.Curve.ScalarMult(pub.X, pub.Y, prv.D.Bytes())
-	if x == nil || (x.BitLen()+7)/8 < (skLen+macLen) {
-		err = ErrSharedKeyTooBig
-		return
+	if x == nil {
+		return nil, ErrSharedKeyIsPointAtInfinity
 	}
-	sk = x.Bytes()[:skLen+macLen]
-	return
+
+	sk = make([]byte, skLen+macLen)
+	skBytes := x.Bytes()
+	copy(sk[len(sk)-len(skBytes):], skBytes)
+	return sk, nil
 }
 
 var (