Ubi could make authentication a fun part of the game.
Enigma
Registered users would download once per month a list of that month's Enigma settings.
Prior to every patrol, the player would access either an ingame or independent Enigma simulator (of which there are several online)
Using the codes downloaded, the player sends Ubi a password or some other piece of information.
Once the message has been acknowledged, The player/Kaluen is authorized by Ubi/BDU to start the patrol. The player will then exercise the appropriate "radio silence" i.e. disconnect from the server.
Ubi gets authentication that would be hard to hack
Players get immersion into the coding and decoding that was part of the U-boat experience.
If we have to have authentication, make it fun and part of the game experience.