If you're interested in this sort of thing, I cannot recommend the cryptopals crypto challenges enough. They are a series of project that take you from XOR up through breaking AES and beyond:
The writing of Cryptopals is unclear. I've made it about half-way through the challenges twice over the past decade. I get that it's prompting me to learn on my own, as if I was a real cryptanalyst encountering a system for the first time, but I literally do not understand what is being asked or the constraints of the problems sometimes. It's like author is trying to be clever and curt and gate-keeper-y on purpose. "You should be able to do this with a shitty, hastily-written problem description, right?"
We ran these challenges at Matasano with the public, under a system where you could only get the next 8 challenges after demonstrating that you'd solved the previous 8, after I wrote an internal guide for our consultants on the cryptographic vulnerabilities they should be capable of addressing on engagements. What you're reading there is basically an internal README. They got very popular (many, many people have solved all of them without additional prompting), but we didn't really invest any extra time in cleaning them up or refining their pedagogy.
I'm blown away by the response. In Sean and Alex's defense (especially Sean's), the writing got better in set 7 and, especially, set 8, which Sean was really careful about and which is clearly the crown jewel of the whole series.
https://cryptopals.com/