transaction.py: sign_txin. allow override for get_preimage_script.

lib/transaction.py

@@ -820,6 +820,10 @@ class Transaction:
 
     @classmethod
     def get_preimage_script(self, txin):
+        preimage_script = txin.get('preimage_script', None)
+        if preimage_script is not None:
+            return preimage_script
+
         pubkeys, x_pubkeys = self.get_sorted_pubkeys(txin)
         if txin['type'] == 'p2pkh':
             return bitcoin.address_to_script(txin['address'])
@@ -1041,8 +1045,17 @@ class Transaction:
                     sec, compressed = keypairs.get(x_pubkey)
                     pubkey = public_key_from_private_key(sec, compressed)
                     # add signature
-                    pre_hash = Hash(bfh(self.serialize_preimage(i)))
-                    pkey = regenerate_key(sec)
+                    sig = self.sign_txin(i, sec)
+                    self.add_signature_to_txin(txin, j, sig)
+                    #txin['x_pubkeys'][j] = pubkey
+                    txin['pubkeys'][j] = pubkey # needed for fd keys
+                    self._inputs[i] = txin
+        print_error("is_complete", self.is_complete())
+        self.raw = self.serialize()
+
+    def sign_txin(self, txin_index, privkey_bytes):
+        pre_hash = Hash(bfh(self.serialize_preimage(txin_index)))
+        pkey = regenerate_key(privkey_bytes)
         secexp = pkey.secret
         private_key = bitcoin.MySigningKey.from_secret_exponent(secexp, curve=SECP256k1)
         public_key = private_key.get_verifying_key()
@@ -1050,12 +1063,7 @@ class Transaction:
         if not public_key.verify_digest(sig, pre_hash, sigdecode=ecdsa.util.sigdecode_der):
             raise Exception('Sanity check verifying our own signature failed.')
         sig = bh2u(sig) + '01'
-                    self.add_signature_to_txin(txin, j, sig)
-                    #txin['x_pubkeys'][j] = pubkey
-                    txin['pubkeys'][j] = pubkey # needed for fd keys
-                    self._inputs[i] = txin
-        print_error("is_complete", self.is_complete())
-        self.raw = self.serialize()
+        return sig
 
     def get_outputs(self):
         """convert pubkeys to addresses"""