Phd after a master's in pure mathematics I am currently finishing my master's degree in theoretical maths (mostly courses around algebra, geometric algebra, a bit of Riemann geometry, K-theory etc) and I don't know which area to go into for a Phd. 
I was interested in cryptography but have never taken any advance classes in coding theory. Is knowledge in "pure math" enough to apply for a PhD in this area? I wanted to do something which uses my theoretical math but into a more applied subject ... any ideas would be very welcome !
A Math Master's is a good start for any Math Ph.D. program. You may need to spend a minute or two convincing an advisor that you know enough about the field to be interested in it, but that won't take specific coursework most of the time.