Prime numbers are pretty easy to think about because you don't have to mess with fractions at all. This is true, young Skywalker, but when it comes to prime numbers, dividing by 1 is not allowed, that leads to the Dark Side. Go ahead, try it, I'll wait. The bigger the number gets, the harder it is to tell if it's prime without thinking real hard about it. In Cryptonomicon , Randy's grandfather can pretty much look at any number and know if it's prime, but he's not normal.
Cryptography uses really, really, really big prime numbers.
These are so big that only a computer can possibly deal with them. Stephenson has frequently been compared to Thomas Pynchon, and these comparisons are perhaps more valid to me after reading Cryptonomicon . There are occasional descriptions of events and happenings that have no real place in the overall story, but are wonderful all the same. I am reminded of Randy Waterhouse reminiscing about a past event in his life, which Stephenson uses to take potshots at liberal arts academia. As a graduate of the liberal arts, I can relate to every little poke and jab.