I'm not worried about security here: the connection could have fifty million passwords and other safety checks that the audience and the user need not be aware of at all; the computer would already know that Spock is an authorized user and the Enterprise an authorized workstation for him. The number string is just the identifier for this particular starship, nothing more.
What does worry me is that the number string cannot have repeating numbers. That is, each digit is chosen by flipping a switch, which then obviously cannot be flipped again. So, instead of 99999 possible ships, the system can only identify, what, 10*9*8*7*6 = 30240 distinct ships. Probably sufficient, but a bit odd - why use the flipping switches to begin with? I mean, are they dedicated to the use of the prefix code? If not, why not use a generic keyboard that would give the full 99999 distinct numbers?
Timo Saloniemi