But this is problematic. For one, it is logistically cumbersome. And a submarine that's going on a long-duration mission has to pack lots and lots of keys, which could fall into enemy hands if the sub is overtaken. Moreover, even with plenty of secure keys, the communications between sub and base are extremely slow. In order to penetrate water, transmitters have to use very low frequency radio waves. These only allow for a few characters to be transmitted each second. While the rest of the world (and the battle space) communicates at high speed, subs are stuck with dial-up. In order to send or receive information in bulk or at speed subs have to surface, and that leaves them visible and vulnerable.