or just screw it all, a stream cipher is better, like enigma was, so where's my RC4 decoder ring? 56 buttons of input, plus END-of-message and a "cipher me" button the wasted extra bits can be used to store entropy. 27 lights of output (27th is a button indicating "more", so keep hitting "cipher me" until the more light turns off) or better, two rows of 16 (maybe the sixteen simple morse) 16*16=256. so one char of input= 2 chars of output. a bunch (39?) of cylinder mounted dials indicating the key at the top. it automatically resets itself to zero when "load key" happens.