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.

Part 1

Part 2

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.

There are a lot of papers out there but two that are relevant to CIA activities (the context of Kryptos cryptoanalysis) and worth the read are a paper by Christopher Felix and one by Charles J. Micoleau.

–Kryptos Fan