PhD in Cryptography after Master's in Pure Mathematics
My advice would be to find a way of getting some research experience associated with cryptography before committing to the idea of doing a Ph.D. in it. Several advantages to this:

a) you'll know that you'll like working on open problems in cryptography, as opposed to just liking learning about the field in the classroom;

b) it'll give you a sense of the types of research you might want to pursue, which would pave the way for a well-developed statement of purpose in your grad school applications;

c) it could help you make additional useful connections. A Ph.D. application supported by letters from cryptography professors attesting to your research potential in the field would make a lot of sense if you do indeed end up wanting to aim to do a Ph.D. in it.