I am always happy to meet with motivated graduate students who are interested in researching the theoretical aspects of cryptography. To see what types of problems I work on, look at my research interests or read some of my papers.
In general, I will only work with students who have already taken a course in cryptography. At the very least, this must include my course Introduction to Cryptography (89-656), or an equivalent. It is also highly recommended to take my graduate course in the Foundations of Cryptography (89-856), or an equivalent.