That math looks indecipherable. Recommended prerequisites? I'm mostly self taught with minimal academic background
1) Christof Paar's book, mentioned already by EFruit. It's currently available as a free download, but that will close up soon, so go download it now. https://link.springer.com/book/10.1007/978-3-642-04101-3 I bought the hardcopy ~8 years ago and still refer to it. (Note: also as mentioned, Prof. Paar has something like 20 one-hour lectures -- in English -- on Youtube.)
2) Jean-Philippe Aumasson's excellent book (2018) "Serious Cryptography". Very practical, very readable. JPA is the author of the Blake cryptographic hash functions.
3) David Wong's upcoming book "Real World Cryptography" (Manning, 2020). I've read parts of it via Manning's early access program, the book isn't released yet, but good stuff. David blogs regularly too.
That should give you enough to chew on for a while!
* What you don't understand;
* What you've looked up;
* What your thoughts are about it.
It depends on what you're asking, and there's a good chance I won't actually be able to help, but I might be able to suggest some self-study to get you started on specific issues.
Plus the set notation threw me a little, e.g.: the cardinality of the set of all ciphertexts for key k in K is ... a number Nc. oookkaaayy.... pause ... ...60 minutes later .... OOOHHH got it!!! ... Same goes for the additive theorem, where suddenly the bit strings M and C become numbers that are added with k mod n --> First it's a message and now ... SHAZAM ... its big integer!
I know as an embedded analyst that RSA and ECC crypto spends a lot of time in bigint routines, but it is becoming clear WHY that is the case.
I realize this is all 101 stuff for people who know crypto, but I really want to learn it for real-reals, so I'm creeping through this book a little bit every day until my brain shuts down. It took me 30 minutes to convince myself of Example 2.6 wasn't perfect crypto: I know intuitively why, but applying the definition mathematically made me feel all of the rust in my brain from 30 years out of college.
I"m sure I'll hit more confusing set theory math, fortunately I have the internet.
Such a fun book!