feat: compute signature hash for p2sh

tx.py

@@ -112,14 +112,21 @@ def fee(self):
 
         return input_sum - output_sum
 
-    def sig_hash(self, input_index):
+    def sig_hash(self, input_index: int, redeem_script=None):
         """
-        Compute the signature hash for a transaction
+        Compute the integer representation of the signature hash (of a transaction) that needs 
+        to get signed fir index input_index.
         """
         s = int_to_little_endian(self.version, 4)
+
         s += encode_varint(len(self.tx_ins))
         for i, tx_in in enumerate(self.tx_ins):
             if i == input_index:
+                if redeem_script:
+                    script_sig = redeem_script
+                else:
+                    script_sig = tx_in.script_pubkey(self.testnet)
+
                 s += TxIn(
                     prev_tx=tx_in.prev_tx,
                     prev_index=tx_in.prev_index,
@@ -142,13 +149,21 @@ def sig_hash(self, input_index):
         h256 = hash256(s)
         return int.from_bytes(h256, 'big')
 
-    def verify_input(self, input_index):
+    def verify_input(self, input_index: int):
         """
         Verify a transaction input
         """
         tx_in: TxIn = self.tx_ins[input_index]
         script_pubkey = tx_in.script_pubkey(testnet=self.testnet)
-        z = self.sig_hash(input_index)
+
+        if script_pubkey.is_p2sh_script_pubkey():
+            cmd = tx_in.script_sig.cmds[-1]
+            raw_redeem = encode_varint(len(cmd)) + cmd
+            redeem_script =  Script.parse(BytesIO(raw_redeem))
+        else:
+            redeem_script = None
+
+        z = self.sig_hash(input_index, redeem_script)
         combined: Script = tx_in.script_sig + script_pubkey
 
         return combined.evaluate(z)