One of my favorites is to build a DRBG; download one of the NIST DRBG specifications and try to implement it and have your implementation's output match the tes

One of my favorites is to build a DRBG; download one of the NIST DRBG specifications and try to implement it and have your implementation's output match the test vectors. You'll be building cryptography without having to design or build the underlying foundational layers. It's actually a lot harder to compose these things then it first seem. Then as you go, try to understand where every single limit in the DRBG specifications come from. Those will teach you a lot about how security factors are worked out. Plus it's hard; and it gets you into that phase of making big dark numbers match other big dark numbers. It's a very unforgiving form of programming.