Cryptanalysis
The Army manual is a pdf so I had to do this as screenshots but this is a very clear description of what we are attempting to do with Kryptos.
You can also read some more about the semiotic definitions of code in an excerpt from the section on Code in the Handbook of Semiotics by Winfried Noth.
I discuss Kerckhoff’ Principle and Kryptos.
I describe two properties of some cipher systems: fractionation and diffusion.
Here is a short bit about key choice.
Read here about symmetric key algorithms and here about asymmetric key algorithms.
I also blather on about the chance a code (not cipher) system was used in Kryptos.
I’m always working on re-learning probability as well.
Heuristics, cognitive bias, and cognitive dissonance are important topics if only to serve as a warning to any potential cryptographer.
I described block ciphers and stream ciphers.
I found a programming reference that sounds similar to K4: obfuscated code.
Continuing on the topic of modern ciphering, read about pseudorandom number generators.
This stuff is a little over my head