test: Move modinv to util and add unit test

1 files changed, 31 insertions, 0 deletions

diff --git a/test/functional/test_framework/util.py b/test/functional/test_framework/util.py
index 52306c8c3d..641e18fae7 100644
--- a/test/functional/test_framework/util.py
+++ b/test/functional/test_framework/util.py
@@ -15,6 +15,7 @@ import os
 import random
 import re
 import time
+import unittest
 
 from . import coverage
 from .authproxy import AuthServiceProxy, JSONRPCException
@@ -629,3 +630,33 @@ def find_vout_for_address(node, txid, addr):
         if any([addr == a for a in tx["vout"][i]["scriptPubKey"]["addresses"]]):
             return i
     raise RuntimeError("Vout not found for address: txid=%s, addr=%s" % (txid, addr))
+
+def modinv(a, n):
+    """Compute the modular inverse of a modulo n using the extended Euclidean
+    Algorithm. See https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Modular_integers.
+    """
+    # TODO: Change to pow(a, -1, n) available in Python 3.8
+    t1, t2 = 0, 1
+    r1, r2 = n, a
+    while r2 != 0:
+        q = r1 // r2
+        t1, t2 = t2, t1 - q * t2
+        r1, r2 = r2, r1 - q * r2
+    if r1 > 1:
+        return None
+    if t1 < 0:
+        t1 += n
+    return t1
+
+class TestFrameworkUtil(unittest.TestCase):
+    def test_modinv(self):
+        test_vectors = [
+            [7, 11],
+            [11, 29],
+            [90, 13],
+            [1891, 3797],
+            [6003722857, 77695236973],
+        ]
+
+        for a, n in test_vectors:
+            self.assertEqual(modinv(a, n), pow(a, n-2, n))