crypto/ecies: make authenticated shared data work

The s2 parameter was not actually written to the MAC.

crypto/ecies/ecies.go

@@ -192,11 +192,9 @@ func concatKDF(hash hash.Hash, z, s1 []byte, kdLen int) (k []byte, err error) {
 // messageTag computes the MAC of a message (called the tag) as per
 // SEC 1, 3.5.
 func messageTag(hash func() hash.Hash, km, msg, shared []byte) []byte {
-	if shared == nil {
-		shared = make([]byte, 0)
-	}
 	mac := hmac.New(hash, km)
 	mac.Write(msg)
+	mac.Write(shared)
 	tag := mac.Sum(nil)
 	return tag
 }
@@ -243,9 +241,11 @@ func symDecrypt(rand io.Reader, params *ECIESParams, key, ct []byte) (m []byte,
 	return
 }
 
-// Encrypt encrypts a message using ECIES as specified in SEC 1, 5.1. If
-// the shared information parameters aren't being used, they should be
-// nil.
+// Encrypt encrypts a message using ECIES as specified in SEC 1, 5.1.
+//
+// s1 and s2 contain shared information that is not part of the resulting
+// ciphertext. s1 is fed into key derivation, s2 is fed into the MAC. If the
+// shared information parameters aren't being used, they should be nil.
 func Encrypt(rand io.Reader, pub *PublicKey, m, s1, s2 []byte) (ct []byte, err error) {
 	params := pub.Params
 	if params == nil {

crypto/ecies/ecies_test.go

@@ -408,6 +408,36 @@ func TestEncryptDecrypt(t *testing.T) {
 	}
 }
 
+func TestDecryptShared2(t *testing.T) {
+	prv, err := GenerateKey(rand.Reader, DefaultCurve, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	message := []byte("Hello, world.")
+	shared2 := []byte("shared data 2")
+	ct, err := Encrypt(rand.Reader, &prv.PublicKey, message, nil, shared2)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check that decrypting with correct shared data works.
+	pt, err := prv.Decrypt(rand.Reader, ct, nil, shared2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(pt, message) {
+		t.Fatal("ecies: plaintext doesn't match message")
+	}
+
+	// Decrypting without shared data or incorrect shared data fails.
+	if _, err = prv.Decrypt(rand.Reader, ct, nil, nil); err == nil {
+		t.Fatal("ecies: decrypting without shared data didn't fail")
+	}
+	if _, err = prv.Decrypt(rand.Reader, ct, nil, []byte("garbage")); err == nil {
+		t.Fatal("ecies: decrypting with incorrect shared data didn't fail")
+	}
+}
+
 // TestMarshalEncryption validates the encode/decode produces a valid
 // ECIES encryption key.
 func TestMarshalEncryption(t *testing.T) {