Use constant time string comparisons

This uses constant time string comparisons everywhere that looked plausibly like it
might matter:

 - Plaintext password comparison
 - Digest nonce comparison
 - Digest hash comparison

Fixes miguelgrinberg#82

flask_httpauth.py

@@ -10,6 +10,7 @@
 
 from functools import wraps
 from hashlib import md5
+from hmac import compare_digest
 from random import Random, SystemRandom
 from flask import request, make_response, session
 from werkzeug.datastructures import Authorization
@@ -143,7 +144,8 @@ def authenticate(self, auth, stored_password):
                 client_password = self.hash_password_callback(username,
                                                               client_password)
         return client_password is not None and \
-            client_password == stored_password
+            stored_password is not None and \
+            compare_digest(client_password, stored_password)
 
 
 class HTTPDigestAuth(HTTPAuth):
@@ -169,7 +171,10 @@ def default_generate_nonce():
             return session["auth_nonce"]
 
         def default_verify_nonce(nonce):
-            return nonce == session.get("auth_nonce")
+            session_nonce = session.get("auth_nonce")
+            if nonce is None or session_nonce is None:
+                return False
+            return compare_digest(nonce, session_nonce)
 
         def default_generate_opaque():
             session["auth_opaque"] = _generate_random()
@@ -235,7 +240,7 @@ def authenticate(self, auth, stored_password_or_ha1):
         ha2 = md5(a2.encode('utf-8')).hexdigest()
         a3 = ha1 + ":" + auth.nonce + ":" + ha2
         response = md5(a3.encode('utf-8')).hexdigest()
-        return response == auth.response
+        return compare_digest(response, auth.response)
 
 
 class HTTPTokenAuth(HTTPAuth):