Switch to Fernet crypto as per gtank's recommendation

util/security/crypto.py

@@ -0,0 +1,18 @@
+import base64
+
+from cryptography.fernet import Fernet, InvalidToken
+
+def encrypt_string(string, key):
+  """ Encrypts a string with the specified key. The key must be 32 raw bytes. """
+  f = Fernet(base64.urlsafe_b64encode(key))
+  return f.encrypt(string)
+
+def decrypt_string(string, key):
+  """ Decrypts an encrypted string with the specified key. The key must be 32 raw bytes. """
+  f = Fernet(base64.urlsafe_b64encode(key))
+  try:
+    return f.decrypt(string)
+  except InvalidToken:
+    return None
+  except TypeError:
+    return None