Change HMAC comparison which binds AK-EK to constant-time operation

Signed-off-by: Jean Snyman <git@jsnyman.com>

keylime/models/registrar/registrar_agent.py

@@ -1,4 +1,5 @@
 import base64
+import hmac
 
 import cryptography.x509
 from cryptography.hazmat.primitives.asymmetric import ec, rsa
@@ -315,7 +316,7 @@ def produce_ak_challenge(self):
     def verify_ak_response(self, response):
         expected_response = crypto.do_hmac(self.key.encode(), self.agent_id)
 
-        result = response == expected_response
+        result = hmac.compare_digest(response, expected_response)
 
         self.change("active", result)
         return result