Adds basic authentication logic.

+ Users need to be registered in order to send appointments (free registration for now)
+ The tower gives them a number of appointments to work with
+ Non-registered users and users with no enough appoitnemnts slots return the same error (to avoid proving)

- Authentication does not cover get_* requests yet
- No tests
- No docs

teos/gatekeeper.py

@@ -1,8 +1,13 @@
 import re
 
-SUBSCRIPTION_SLOTS = 100
+import teos.errors as errors
 
-# TODO: UNITTEST
+from common.appointment import Appointment
+from common.cryptographer import Cryptographer
+
+SUBSCRIPTION_SLOTS = 1
+
+# TODO: UNITTEST, DOCS
 class Gatekeeper:
     def __init__(self):
         self.registered_users = {}
@@ -30,3 +35,63 @@ class Gatekeeper:
             self.registered_users[user_pk] += SUBSCRIPTION_SLOTS
 
         return self.registered_users[user_pk]
+
+    def fill_subscription_slots(self, user_pk, n):
+        slots = self.registered_users.get(user_pk)
+
+        # FIXME: This looks pretty dangerous. I'm guessing race conditions can happen here.
+        if slots == n:
+            self.registered_users.pop(user_pk)
+        else:
+            self.registered_users[user_pk] -= n
+
+    def identify_user(self, appointment_data, signature):
+        """
+        Checks if the provided user signature is comes from a registered user with available appointment slots.
+
+        Args:
+            appointment_data (:obj:`dict`): the appointment that was signed by the user.
+            signature (:obj:`str`): the user's signature (hex encoded).
+
+        Returns:
+            :obj:`tuple`: A tuple (return code, message) as follows:
+
+            - ``(0, None)`` if the user can be identified (recovered pk belongs to a registered user) and the user has
+                available slots.
+            - ``!= (0, None)`` otherwise.
+
+            The possible return errors are: ``APPOINTMENT_EMPTY_FIELD`` and ``APPOINTMENT_INVALID_SIGNATURE``.
+        """
+
+        if signature is None:
+            rcode = errors.APPOINTMENT_EMPTY_FIELD
+            message = "empty signature received"
+
+        else:
+            appointment = Appointment.from_dict(appointment_data)
+            rpk = Cryptographer.recover_pk(appointment.serialize(), signature)
+            compressed_user_pk = Cryptographer.get_compressed_pk(rpk)
+
+            if compressed_user_pk and compressed_user_pk in self.registered_users:
+                rcode = 0
+                message = compressed_user_pk
+
+            else:
+                rcode = errors.APPOINTMENT_INVALID_SIGNATURE_OR_INSUFFICIENT_SLOTS
+                message = "invalid signature or the user does not have enough slots available"
+
+        return rcode, message
+
+    def get_slots(self, user_pk):
+        """
+        Returns the number os available slots for a given user.
+
+        Args:
+            user_pk(:mod:`str`): the public key that identifies the user (33-bytes hex str)
+
+        Returns:
+            :obj:`int`: the number of available slots.
+
+        """
+        slots = self.registered_users.get(user_pk)
+        return slots if slots is not None else 0