HistoCrypt 2019, an annual conference on historical cryptology, convenes this week. The proceedings have been published and include many fascinating papers:

http://www.ep.liu.se/ecp/158/ecp19158.pdf

Those here who like to tinker with solvers and the Zodiac ciphers need to read these:

Nils Kopal, “Cryptanalysis of Homophonic Substitution Ciphers Using Simulated Annealing with Fixed Temperature” (This paper in particular is the first to formally acknowledge and accurately describe the long-available solvers such as ZKDecrypto and AZDecrypt)

Tom S Juzek, “Using the Entropy of N-Grams to Evaluate the Authenticity of Substitution Ciphers and Z340 in Particular” Notable conclusion:

We applied those two SVM models to the the Zodiac Killer’s two major ciphers, z408 and z340. z408, which has been solved, is correctly predicted to be a real substitution cipher. z340, which remains unsolved, is predicted to not be a substitution cipher. We think that it is likely that z340 is either another type of cipher, e.g. a transpose cipher or a Vigenere cipher, or that it might not be a bona fide substitution cipher after all.

Also, while this one is about PlayFair ciphers, the algorithmic techniques described in the paper would well apply to homophonic solvers:

George Lasry, “Solving a 40-Letter Playfair Challenge with CrypTool 2”

The proceedings are definitely worth a read if you have any technical interest in cryptology in general and the Zodiac ciphers in particular!