htlcswitch/test: more realistic mock encryption

This mock is used in the switch test TestUpdateFailMalformedHTLCErrorConversion.
But because the mock isn't very realistic, it doesn't detect problems
in the handling of malformed failures in the link.

htlcswitch/mock.go

@@ -419,12 +419,16 @@ func (o *mockObfuscator) Reextract(
 	return nil
 }
 
+var fakeHmac = []byte("hmachmachmachmachmachmachmachmac")
+
 func (o *mockObfuscator) EncryptFirstHop(failure lnwire.FailureMessage) (
 	lnwire.OpaqueReason, error) {
 
 	o.failure = failure
 
 	var b bytes.Buffer
+	b.Write(fakeHmac)
+
 	if err := lnwire.EncodeFailure(&b, failure, 0); err != nil {
 		return nil, err
 	}
@@ -436,7 +440,12 @@ func (o *mockObfuscator) IntermediateEncrypt(reason lnwire.OpaqueReason) lnwire.
 }
 
 func (o *mockObfuscator) EncryptMalformedError(reason lnwire.OpaqueReason) lnwire.OpaqueReason {
-	return reason
+	var b bytes.Buffer
+	b.Write(fakeHmac)
+
+	b.Write(reason)
+
+	return b.Bytes()
 }
 
 // mockDeobfuscator mock implementation of the failure deobfuscator which
@@ -447,7 +456,13 @@ func newMockDeobfuscator() ErrorDecrypter {
 	return &mockDeobfuscator{}
 }
 
-func (o *mockDeobfuscator) DecryptError(reason lnwire.OpaqueReason) (*ForwardingError, error) {
+func (o *mockDeobfuscator) DecryptError(reason lnwire.OpaqueReason) (
+	*ForwardingError, error) {
+
+	if !bytes.Equal(reason[:32], fakeHmac) {
+		return nil, errors.New("fake decryption error")
+	}
+	reason = reason[32:]
 
 	r := bytes.NewReader(reason)
 	failure, err := lnwire.DecodeFailure(r, 0)