Implementation of bitwise XOR function for bytes object

Author: adamantike

rsa/_compat.py

@@ -99,6 +99,27 @@ def byte(num):
     return pack("B", num)
 
 
+def xor_bytes(b1, b2):
+    """
+    Returns the bitwise XOR result between two bytes objects, b1 ^ b2.
+
+    Bitwise XOR operation is commutative, so order of parameters doesn't
+    generate different results. If parameters have different length, extra
+    length of the largest one is ignored.
+
+    :param b1:
+        First bytes object.
+    :param b2:
+        Second bytes object.
+    :returns:
+        Bytes object, result of XOR operation.
+    """
+    if PY2:
+        return ''.join(byte(ord(x) ^ ord(y)) for x, y in zip(b1, b2))
+
+    return bytes(x ^ y for x, y in zip(b1, b2))
+
+
 def get_word_alignment(num, force_arch=64,
                        _machine_word_size=MACHINE_WORD_SIZE):
     """

tests/test_compat.py

@@ -17,10 +17,12 @@
 import unittest
 import struct
 
-from rsa._compat import byte, is_bytes, range
+from rsa._compat import byte, is_bytes, range, xor_bytes
 
 
 class TestByte(unittest.TestCase):
+    """Tests for single bytes."""
+
     def test_byte(self):
         for i in range(256):
             byt = byte(i)
@@ -33,3 +35,46 @@ def test_raises_StructError_on_overflow(self):
 
     def test_byte_literal(self):
         self.assertIsInstance(b'abc', bytes)
+
+
+class TestBytes(unittest.TestCase):
+    """Tests for bytes objects."""
+
+    def setUp(self):
+        self.b1 = b'\xff\xff\xff\xff'
+        self.b2 = b'\x00\x00\x00\x00'
+        self.b3 = b'\xf0\xf0\xf0\xf0'
+        self.b4 = b'\x4d\x23\xca\xe2'
+        self.b5 = b'\x9b\x61\x3b\xdc'
+        self.b6 = b'\xff\xff'
+
+        self.byte_strings = (self.b1, self.b2, self.b3, self.b4, self.b5, self.b6)
+
+    def test_xor_bytes(self):
+        self.assertEqual(xor_bytes(self.b1, self.b2), b'\xff\xff\xff\xff')
+        self.assertEqual(xor_bytes(self.b1, self.b3), b'\x0f\x0f\x0f\x0f')
+        self.assertEqual(xor_bytes(self.b1, self.b4), b'\xb2\xdc\x35\x1d')
+        self.assertEqual(xor_bytes(self.b1, self.b5), b'\x64\x9e\xc4\x23')
+        self.assertEqual(xor_bytes(self.b2, self.b3), b'\xf0\xf0\xf0\xf0')
+        self.assertEqual(xor_bytes(self.b2, self.b4), b'\x4d\x23\xca\xe2')
+        self.assertEqual(xor_bytes(self.b2, self.b5), b'\x9b\x61\x3b\xdc')
+        self.assertEqual(xor_bytes(self.b3, self.b4), b'\xbd\xd3\x3a\x12')
+        self.assertEqual(xor_bytes(self.b3, self.b5), b'\x6b\x91\xcb\x2c')
+        self.assertEqual(xor_bytes(self.b4, self.b5), b'\xd6\x42\xf1\x3e')
+
+    def test_xor_bytes_length(self):
+        self.assertEqual(xor_bytes(self.b1, self.b6), b'\x00\x00')
+        self.assertEqual(xor_bytes(self.b2, self.b6), b'\xff\xff')
+        self.assertEqual(xor_bytes(self.b3, self.b6), b'\x0f\x0f')
+        self.assertEqual(xor_bytes(self.b4, self.b6), b'\xb2\xdc')
+        self.assertEqual(xor_bytes(self.b5, self.b6), b'\x64\x9e')
+        self.assertEqual(xor_bytes(self.b6, b''), b'')
+
+    def test_xor_bytes_commutative(self):
+        for first in self.byte_strings:
+            for second in self.byte_strings:
+                min_length = min(len(first), len(second))
+                result = xor_bytes(first, second)
+
+                self.assertEqual(result, xor_bytes(second, first))
+                self.assertEqual(len(result), min_length)