initialize NewOnionErrorEncrypter with shared secret directly

Allow for more flexible usage of the error encrypter. This is useful when upgrading an
existing legacy error encrypter to fat errors in lnd.

obfuscation.go

@@ -15,17 +15,10 @@ type OnionErrorEncrypter struct {
 // NewOnionErrorEncrypter creates new instance of the onion encrypter backed by
 // the passed router, with encryption to be doing using the passed
 // ephemeralKey.
-func NewOnionErrorEncrypter(router *Router,
-	ephemeralKey *btcec.PublicKey) (*OnionErrorEncrypter, error) {
-
-	sharedSecret, err := router.generateSharedSecret(ephemeralKey)
-	if err != nil {
-		return nil, err
-	}
-
+func NewOnionErrorEncrypter(sharedSecret Hash256) *OnionErrorEncrypter {
 	return &OnionErrorEncrypter{
 		sharedSecret: sharedSecret,
-	}, nil
+	}
 }
 
 // Encode writes the encrypter's shared secret to the provided io.Writer.

obfuscation_test.go

@@ -34,9 +34,7 @@ func TestOnionFailure(t *testing.T) {
 	}
 
 	// Emulate creation of the obfuscator on node where error have occurred.
-	obfuscator := &OnionErrorEncrypter{
-		sharedSecret: sharedSecrets[len(errorPath)-1],
-	}
+	obfuscator := NewOnionErrorEncrypter(sharedSecrets[len(errorPath)-1])
 
 	// Emulate the situation when last hop creates the onion failure
 	// message and send it back.
@@ -46,9 +44,7 @@ func TestOnionFailure(t *testing.T) {
 	for i := len(errorPath) - 2; i >= 0; i-- {
 		// Emulate creation of the obfuscator on forwarding node which
 		// propagates the onion failure.
-		obfuscator = &OnionErrorEncrypter{
-			sharedSecret: sharedSecrets[i],
-		}
+		obfuscator = NewOnionErrorEncrypter(sharedSecrets[i])
 		obfuscatedData = obfuscator.EncryptError(false, obfuscatedData)
 	}
 
@@ -207,16 +203,16 @@ func TestOnionFailureSpecVector(t *testing.T) {
 			t.Fatalf("unable to decode spec shared secret: %v",
 				err)
 		}
-		obfuscator := &OnionErrorEncrypter{
-			sharedSecret: sharedSecrets[len(sharedSecrets)-1-i],
-		}
+		obfuscator := NewOnionErrorEncrypter(
+			sharedSecrets[len(sharedSecrets)-1-i],
+		)
 
 		var b bytes.Buffer
 		if err := obfuscator.Encode(&b); err != nil {
 			t.Fatalf("unable to encode obfuscator: %v", err)
 		}
 
-		obfuscator2 := &OnionErrorEncrypter{}
+		obfuscator2 := NewOnionErrorEncrypter(Hash256{})
 		obfuscatorReader := bytes.NewReader(b.Bytes())
 		if err := obfuscator2.Decode(obfuscatorReader); err != nil {
 			t.Fatalf("unable to decode obfuscator: %v", err)