Neat game. It really teaches some solid general cryptography principles.
I think maybe a Caesar Cipher could be employed to serve as a tutorial/example puzzle sorts as at first before going into the DVORAK and hex ciphers. It was a little confusing what I was supposed to be doing at first. I feel it would be even more difficult for someone new to those concepts (but maybe that's just not the intended audience!).
Nice ASCII art btw. I think the terminal look suits code breaking super well.