Added a RandomString class from stackoverflow, to generate random alphanumeric strings (to be used as passwords).

Added some code to VaultDoor, including random password generation and code to encrypt/decrypt under a Vernam-like cypher.
Modified door opening policy to include the possibility of keeping the door open.

Server/src/main/resources/config.properties

@@ -84,3 +84,7 @@ wg_maxCopperCount=2
 # ----------------------------------------------
 # Maximum execution time of user code in ms
 user_timeout=100
+
+
+# ----------------------------------------------
+vault_door_open_time=4