Merge pull request #2600 from alex/builtin-constant-time-compare

Use the stdlib's compare_digest for constant time comparisons when available

django/utils/crypto.py

@@ -77,27 +77,31 @@ def get_random_string(length=12,
     return ''.join(random.choice(allowed_chars) for i in range(length))
 
 
-def constant_time_compare(val1, val2):
-    """
-    Returns True if the two strings are equal, False otherwise.
+if hasattr(hmac, "compare_digest"):
+    # Prefer the stdlib implementation, when available.
+    constant_time_compare = hmac.compare_digest
+else:
+    def constant_time_compare(val1, val2):
+        """
+        Returns True if the two strings are equal, False otherwise.
 
-    The time taken is independent of the number of characters that match.
+        The time taken is independent of the number of characters that match.
 
-    For the sake of simplicity, this function executes in constant time only
-    when the two strings have the same length. It short-circuits when they
-    have different lengths. Since Django only uses it to compare hashes of
-    known expected length, this is acceptable.
-    """
-    if len(val1) != len(val2):
-        return False
-    result = 0
-    if six.PY3 and isinstance(val1, bytes) and isinstance(val2, bytes):
-        for x, y in zip(val1, val2):
-            result |= x ^ y
-    else:
-        for x, y in zip(val1, val2):
-            result |= ord(x) ^ ord(y)
-    return result == 0
+        For the sake of simplicity, this function executes in constant time only
+        when the two strings have the same length. It short-circuits when they
+        have different lengths. Since Django only uses it to compare hashes of
+        known expected length, this is acceptable.
+        """
+        if len(val1) != len(val2):
+            return False
+        result = 0
+        if six.PY3 and isinstance(val1, bytes) and isinstance(val2, bytes):
+            for x, y in zip(val1, val2):
+                result |= x ^ y
+        else:
+            for x, y in zip(val1, val2):
+                result |= ord(x) ^ ord(y)
+        return result == 0
 
 
 def _bin_to_long(x):