Fix #162: Blinding uses slow algorithm

Store blinding factor + its inverse, so that they can be reused & updated
on every blinding operation. This avoids expensive computations.

The reuse of the previous blinding factor is done via squaring (mod n), as
per section 9 of 'A Timing Attack against RSA with the Chinese Remainder
Theorem' by Werner Schindler, https://tls.mbed.org/public/WSchindler-RSA_Timing_Attack.pdf

1 files changed, 13 insertions, 4 deletions

diff --git a/tests/test_key.py b/tests/test_key.py
index 9db30ce..b00e26d 100644
--- a/tests/test_key.py
+++ b/tests/test_key.py
@@ -21,11 +21,20 @@ class BlindingTest(unittest.TestCase):
         message = 12345
         encrypted = rsa.core.encrypt_int(message, pk.e, pk.n)
 
-        blinded = pk.blind(encrypted, 4134431)  # blind before decrypting
-        decrypted = rsa.core.decrypt_int(blinded, pk.d, pk.n)
-        unblinded = pk.unblind(decrypted, 4134431)
+        blinded_1 = pk.blind(encrypted)  # blind before decrypting
+        decrypted = rsa.core.decrypt_int(blinded_1, pk.d, pk.n)
+        unblinded_1 = pk.unblind(decrypted)
 
-        self.assertEqual(unblinded, message)
+        self.assertEqual(unblinded_1, message)
+
+        # Re-blinding should use a different blinding factor.
+        blinded_2 = pk.blind(encrypted)  # blind before decrypting
+        self.assertNotEqual(blinded_1, blinded_2)
+
+        # The unblinding should still work, though.
+        decrypted = rsa.core.decrypt_int(blinded_2, pk.d, pk.n)
+        unblinded_2 = pk.unblind(decrypted)
+        self.assertEqual(unblinded_2, message)
 
 
 class KeyGenTest(unittest.TestCase):