ecc: allow tests to disable ecdsa R-value grinding

see https://github.com/spesmilo/electrum/pull/7453#issuecomment-912594926

electrum/ecc.py

@@ -41,6 +41,11 @@ from .ecc_fast import _libsecp256k1, SECP256K1_EC_UNCOMPRESSED
 _logger = get_logger(__name__)
 
 
+# Some unit tests need to create ECDSA sigs without grinding the R value (and just use RFC6979).
+# see https://github.com/bitcoin/bitcoin/pull/13666
+ENABLE_ECDSA_R_VALUE_GRINDING = True
+
+
 def string_to_number(b: bytes) -> int:
     return int.from_bytes(b, byteorder='big', signed=False)
 
@@ -463,11 +468,12 @@ class ECPrivkey(ECPubkey):
             return r, s
 
         r, s = sign_with_extra_entropy(extra_entropy=None)
-        counter = 0
-        while r >= 2**255:  # grind for low R value https://github.com/bitcoin/bitcoin/pull/13666
-            counter += 1
-            extra_entropy = counter.to_bytes(32, byteorder="little")
-            r, s = sign_with_extra_entropy(extra_entropy=extra_entropy)
+        if ENABLE_ECDSA_R_VALUE_GRINDING:
+            counter = 0
+            while r >= 2**255:  # grind for low R value https://github.com/bitcoin/bitcoin/pull/13666
+                counter += 1
+                extra_entropy = counter.to_bytes(32, byteorder="little")
+                r, s = sign_with_extra_entropy(extra_entropy=extra_entropy)
 
         sig_string = sig_string_from_r_and_s(r, s)
         self.verify_message_hash(sig_string, msg_hash)

electrum/tests/test_bitcoin.py

@@ -86,6 +86,24 @@ def needs_test_with_all_chacha20_implementations(func):
     return run_test
 
 
+def disable_ecdsa_r_value_grinding(func):
+    """Function decorator to run a unit test with ecdsa R-value grinding disabled.
+    This is used when we want to pass test vectors that were created without R-value grinding.
+    (see https://github.com/bitcoin/bitcoin/pull/13666 )
+
+    NOTE: this is inherently sequential;
+    tests running in parallel would break things
+    """
+    def run_test(*args, **kwargs):
+        is_grinding = ecc.ENABLE_ECDSA_R_VALUE_GRINDING
+        try:
+            ecc.ENABLE_ECDSA_R_VALUE_GRINDING = False
+            func(*args, **kwargs)
+        finally:
+            ecc.ENABLE_ECDSA_R_VALUE_GRINDING = is_grinding
+    return run_test
+
+
 class Test_bitcoin(ElectrumTestCase):
 
     def test_libsecp256k1_is_available(self):
@@ -221,6 +239,12 @@ class Test_bitcoin(ElectrumTestCase):
         sig2 = eckey2.sign_transaction(bfh('642a2e66332f507c92bda910158dfe46fc10afbf72218764899d3af99a043fac'))
         self.assertEqual('30440220618513f4cfc87dde798ce5febae7634c23e7b9254a1eabf486be820f6a7c2c4702204fef459393a2b931f949e63ced06888f35e286e446dc46feb24b5b5f81c6ed52', sig2.hex())
 
+    @disable_ecdsa_r_value_grinding
+    def test_sign_transaction_without_ecdsa_r_value_grinding(self):
+        eckey1 = ecc.ECPrivkey(bfh('7e1255fddb52db1729fc3ceb21a46f95b8d9fe94cc83425e936a6c5223bb679d'))
+        sig1 = eckey1.sign_transaction(bfh('5a548b12369a53faaa7e51b5081829474ebdd9c924b3a8230b69aa0be254cd94'))
+        self.assertEqual('3045022100902a288b98392254cd23c0e9a49ac6d7920f171b8249a48e484b998f1874a2010220723d844826828f092cf400cb210c4fa0b8cd1b9d1a7f21590e78e022ff6476b9', sig1.hex())
+
     @needs_test_with_all_aes_implementations
     def test_aes_homomorphic(self):
         """Make sure AES is homomorphic."""