If you already know the mechanics of the cipher (for example, the Naibbe cipher) and have some hypothesis about the plaintext language, then you can perform some form of statistical analysis (for example, simulated annealing) using the known bigram and trigram statistics of the plaintext language. This is not particularly complicated and setting it up won't take much time, the latest versions of ChatGPT/Claude/DeepSeek/Gemini should be able to create the code and you can run it with some test encoding scheme to ensure it works. Finding good sources for the statistics of the plaintext languages can be problematic, but in principle solvable.
The hard part is identifying the details of the encoding and the plaintext language. Since there are no strong arguments that point exclusively towards any particular plaintext language, it makes sense to create a multilingual pipeline that could cover many medieval languages at the same time.
I tried to collect the list of plausible plaintext languages for this task in the following thread: You are not allowed to view links. Register or Login to view.
The hard part is identifying the details of the encoding and the plaintext language. Since there are no strong arguments that point exclusively towards any particular plaintext language, it makes sense to create a multilingual pipeline that could cover many medieval languages at the same time.
I tried to collect the list of plausible plaintext languages for this task in the following thread: You are not allowed to view links. Register or Login to view.