btc: grind for low-r value signatures to match core

basicswap/interface/btc.py

@@ -53,6 +53,7 @@ from coincurve.keys import (
     PrivateKey,
     PublicKey,
 )
+from coincurve.types import ffi
 from coincurve.ecdsaotves import (
     ecdsaotves_enc_sign,
     ecdsaotves_enc_verify,
@@ -1357,7 +1358,17 @@ class BTCInterface(Secp256k1Interface):
         )
 
         eck = PrivateKey(key_bytes)
-        return eck.sign(sig_hash, hasher=None) + bytes((SIGHASH_ALL,))
+        for i in range(10000):
+            # Grind for low-R value
+            if i == 0:
+                nonce = (ffi.NULL, ffi.NULL)
+            else:
+                extra_entropy = i.to_bytes(4, "little") + (b"\0" * 28)
+                nonce = (ffi.NULL, ffi.new("unsigned char [32]", extra_entropy))
+            sig = eck.sign(sig_hash, hasher=None, custom_nonce=nonce)
+            if len(sig) < 71:
+                return sig + bytes((SIGHASH_ALL,))
+        raise RuntimeError("sign failed.")
 
     def signTxOtVES(
         self,

tests/basicswap/test_xmr.py

@@ -1249,6 +1249,10 @@ class Test(BaseTest):
             ci.signTx(b, lock_spend_tx, 0, lock_tx_script, amount),
             lock_tx_script,
         ]
+        assert (
+            len(witness_stack[1]) <= 71
+        )  # Test for low-r, sig size is <= 70 + sighash_type
+        assert len(witness_stack[2]) <= 71
         lock_spend_tx = ci.setTxSignature(lock_spend_tx, witness_stack)
         tx_decoded = ci.rpc("decoderawtransaction", [lock_spend_tx.hex()])
         vsize_actual: int = tx_decoded["vsize"]