Fix bug in getBackupSigningKey

This function was missing a function application at the end, meaning that
its return type was function and not string as it should be. This has
now been fixed.

It took me a bit too long to find this bug so included in this commit is
a small change to make the code a bit less compact by declaring the
return value from getBackupSigningKey to an appropriately named local
variable.

Also improves nomenclature in getBackupSigningKey by explictly
distinguishing between regular private keys and extended private keys.

Author: Sword-Smith

app/sign.js

@@ -120,14 +120,14 @@ const promptForConfirmationAndKey = function(recoveryRequest, outputs, skipConfi
 
 /**
  * Gets the backup private key that can be used to sign the transaction.
- * @param key The provided private key.
+ * @param xprv The provided extended private key (BIP32).
  * @param expectedXpub The public key specified with the request. 
  * @returns The private key to sign the transaction.
  */
-const getBackupSigningKey = function(key, expectedXpub) {
-  const backupKeyNode = getHDNodeAndVerify(key, expectedXpub);
+const getBackupSigningKey = function(xprv, expectedXpub) {
+  const backupKeyNode = getHDNodeAndVerify(xprv, expectedXpub);
 
-  return backupKeyNode.keyPair.getPrivateKeyBuffer;
+  return backupKeyNode.keyPair.getPrivateKeyBuffer();
 }
 
 const handleSignUtxo = function(recoveryRequest, key, skipConfirm) {
@@ -237,8 +237,9 @@ const signEthTx = function(recoveryRequest, key, skipConfirm, isToken) {
   }
 
   key = promptForConfirmationAndKey(recoveryRequest, outputs, skipConfirm, key);
+  const signingKey = getBackupSigningKey(key, recoveryRequest.backupKey);
 
-  transaction.sign(getBackupSigningKey(key, recoveryRequest.backupKey));
+  transaction.sign(signingKey);
 
   return transaction.serialize().toString('hex');
 };
@@ -258,8 +259,9 @@ const handleSignTrx = function(recoveryRequest, key, skipConfirm) {
   });
 
   key = promptForConfirmationAndKey(recoveryRequest, outputs, skipConfirm, key);
+  const signingKey = getBackupSigningKey(key, recoveryRequest.backupKey);
 
-  builder.sign({ key: getBackupSigningKey(key, recoveryRequest.backupKey) });
+  builder.sign({ key: signingKey });
   return JSON.stringify(builder.build().toJson());
 };