PBKDF2 with cached HMAC key

This gives a 2x speed increase and removes the ability to DoS with large passwords.

Sorry this is my hello world for Python, and I don't even know how to run it. It probably works but you should test it.

django/utils/crypto.py

@@ -133,6 +133,18 @@ def _fast_hmac(key, msg, digest):
     return dig2
 
 
+def _cached_hmac(msg, inner, outer):
+    """
+    A trimmed down version of Python's HMAC implementation.
+
+    This function operates on bytes.
+    """
+    dig1, dig2 = inner.copy(), outer.copy()
+    dig1.update(msg)
+    dig2.update(dig1.digest())
+    return dig2
+
+
 def pbkdf2(password, salt, iterations, dklen=0, digest=None):
     """
     Implements PBKDF2 as defined in RFC 2898, section 5.2
@@ -160,11 +172,18 @@ def pbkdf2(password, salt, iterations, dklen=0, digest=None):
 
     hex_format_string = "%%0%ix" % (hlen * 2)
 
+    inner, outer = digest(), digest()
+    if len(password) > inner.block_size:
+        password = digest(password).digest()
+    password += b'\x00' * (inner.block_size - len(password))
+    inner.update(password.translate(hmac.trans_36))
+    outer.update(password.translate(hmac.trans_5C))
+
     def F(i):
         def U():
             u = salt + struct.pack(b'>I', i)
             for j in xrange(int(iterations)):
-                u = _fast_hmac(password, u, digest).digest()
+                u = _cached_hmac(u, inner, outer).digest()
                 yield _bin_to_long(u)
         return _long_to_bin(reduce(operator.xor, U()), hex_format_string)