Issue #15061: Don't oversell the capabilities of the new non-shortcircuiting comparison function in hmac
diff --git a/Lib/hmac.py b/Lib/hmac.py
index 13ffdbe..e47965b 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -13,24 +13,24 @@
digest_size = None
-def secure_compare(a, b):
- """Returns the equivalent of 'a == b', but using a time-independent
- comparison method to prevent timing attacks."""
- if not ((isinstance(a, str) and isinstance(b, str)) or
- (isinstance(a, bytes) and isinstance(b, bytes))):
- raise TypeError("inputs must be strings or bytes")
+def compare_digest(a, b):
+ """Returns the equivalent of 'a == b', but avoids content based short
+ circuiting to reduce the vulnerability to timing attacks."""
+ # Consistent timing matters more here than data type flexibility
+ if not (isinstance(a, bytes) and isinstance(b, bytes)):
+ raise TypeError("inputs must be bytes instances")
+ # We assume the length of the expected digest is public knowledge,
+ # thus this early return isn't leaking anything an attacker wouldn't
+ # already know
if len(a) != len(b):
return False
+ # We assume that integers in the bytes range are all cached,
+ # thus timing shouldn't vary much due to integer object creation
result = 0
- if isinstance(a, bytes):
- for x, y in zip(a, b):
- result |= x ^ y
- else:
- for x, y in zip(a, b):
- result |= ord(x) ^ ord(y)
-
+ for x, y in zip(a, b):
+ result |= x ^ y
return result == 0
diff --git a/Lib/test/test_hmac.py b/Lib/test/test_hmac.py
index 042bc5d..4e5961d 100644
--- a/Lib/test/test_hmac.py
+++ b/Lib/test/test_hmac.py
@@ -302,40 +302,42 @@
self.assertEqual(h1.hexdigest(), h2.hexdigest(),
"Hexdigest of copy doesn't match original hexdigest.")
-class SecureCompareTestCase(unittest.TestCase):
+class CompareDigestTestCase(unittest.TestCase):
def test_compare(self):
# Testing input type exception handling
a, b = 100, 200
- self.assertRaises(TypeError, hmac.secure_compare, a, b)
- a, b = 100, "foobar"
- self.assertRaises(TypeError, hmac.secure_compare, a, b)
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
+ a, b = 100, b"foobar"
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
+ a, b = b"foobar", 200
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
a, b = "foobar", b"foobar"
- self.assertRaises(TypeError, hmac.secure_compare, a, b)
-
- # Testing str/bytes of different lengths
- a, b = "foobar", "foo"
- self.assertFalse(hmac.secure_compare(a, b))
- a, b = b"foobar", b"foo"
- self.assertFalse(hmac.secure_compare(a, b))
- a, b = b"\xde\xad\xbe\xef", b"\xde\xad"
- self.assertFalse(hmac.secure_compare(a, b))
-
- # Testing str/bytes of same lengths, different values
- a, b = "foobar", "foobaz"
- self.assertFalse(hmac.secure_compare(a, b))
- a, b = b"foobar", b"foobaz"
- self.assertFalse(hmac.secure_compare(a, b))
- a, b = b"\xde\xad\xbe\xef", b"\xab\xad\x1d\xea"
- self.assertFalse(hmac.secure_compare(a, b))
-
- # Testing str/bytes of same lengths, same values
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
+ a, b = b"foobar", "foobar"
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
a, b = "foobar", "foobar"
- self.assertTrue(hmac.secure_compare(a, b))
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
+ a, b = bytearray(b"foobar"), bytearray(b"foobar")
+ self.assertRaises(TypeError, hmac.compare_digest, a, b)
+
+ # Testing bytes of different lengths
+ a, b = b"foobar", b"foo"
+ self.assertFalse(hmac.compare_digest(a, b))
+ a, b = b"\xde\xad\xbe\xef", b"\xde\xad"
+ self.assertFalse(hmac.compare_digest(a, b))
+
+ # Testing bytes of same lengths, different values
+ a, b = b"foobar", b"foobaz"
+ self.assertFalse(hmac.compare_digest(a, b))
+ a, b = b"\xde\xad\xbe\xef", b"\xab\xad\x1d\xea"
+ self.assertFalse(hmac.compare_digest(a, b))
+
+ # Testing bytes of same lengths, same values
a, b = b"foobar", b"foobar"
- self.assertTrue(hmac.secure_compare(a, b))
+ self.assertTrue(hmac.compare_digest(a, b))
a, b = b"\xde\xad\xbe\xef", b"\xde\xad\xbe\xef"
- self.assertTrue(hmac.secure_compare(a, b))
+ self.assertTrue(hmac.compare_digest(a, b))
def test_main():
support.run_unittest(
@@ -343,7 +345,7 @@
ConstructorTestCase,
SanityTestCase,
CopyTestCase,
- SecureCompareTestCase
+ CompareDigestTestCase
)
if __name__ == "__main__":