They're not on the roof for that scene; they're in his office, using the computer room's air conditioning as a sound mask (and using a Cray Y-MP as a bench). While the movie takes some very early-90s liberties with technology (especially the scene where they use the chip), it's remarkably respectful of hacking -- there's a stronger emphasis on social engineering then the purely technical, and my "head canon" is that the chip itself is a quantum chip capable of breaking any then-known encryption.