Merge #315: Check unaltered sequence, locktime (payjoin)

f539b02 Check sequence and locktime are conforming to payjoin v0 in transaction from receiver to sender. Also add note about implicit defence against attack mentioned in BIP79; we only ever sign the predetermined set of inputs. (AdamISZ)

jmclient/jmclient/taker.py

@@ -1054,6 +1054,18 @@ class P2EPTaker(Taker):
             ins['outpoint']['index']) for ins in tx['ins'])
         if not tx_utxo_set.issuperset(set(self.utxos[None])):
             return (False, "my utxos are not contained")
+        # Check that the sequence numbers of all inputs are unaltered
+        # from the intended 0xffffffff - 1, and that the locktime
+        # is not zero (could go further and check exact block).
+        # Note that this is hacky and is most elegantly addressed by
+        # use of PSBT (although any object encapsulation of tx input
+        # would serve the same purpose).
+        if tx["locktime"] == 0:
+            return (False, "Invalid PayJoin v0 transaction: locktime 0")
+        for i in tx["ins"]:
+            if i["sequence"] != 0xffffffff - 1:
+                return (False, "Invalid PayJoin v0 transaction: "+\
+                        "sequence is not 0xffffffff -1")
 
         # Before even starting fee calculations, reject  > 5
         # inputs from counterparty as an abuse (accidental or
@@ -1180,6 +1192,9 @@ class P2EPTaker(Taker):
 
         # All checks have passed; we sign and broadcast
         self.latest_tx = tx
+        # Note that self.self_sign will only sign the self.input_utxos specified at
+        # the start of the processing, which guards against the "unwittingly sign
+        # extra inputs" attack mentioned in BIP79.
         self.self_sign_and_push()
         # returning False here is not an error condition, only stops processing.
         return (False, "OK")