<p>I have a friend doing his PhD at Harvard in Cryptography. Their computer science department is small but to be honest, have elite faculties. I think they only have like 3 people in the crytopgraphy part and one of them is a turing award winner. The other one is possible to win turing as well. So if you apply then these people will be reading your appilcation and might be kinda tough. They have one guy working on Information Security and he is a joint professor with Harvard Law school and currently the director at some internationally funded internet security institute. So considering the quality of their people in these areas, it might be tough. You can apply to a different area and still end up doing cryptography. Oh, one thing, at Harvard computer science, I think most of the faculty is only theory. All them faculties in Cryptography did their PhD in pure math. You can also apply to UIUC computer science. It’s easier and it is also a great department.</p>