Jarlve wrote:Moonrock, out of all his cycle types considered regional cycling for the 340.
My hypothesis was that the 340 cipher used a combination of regional cycles and semi-regional cycles for the nine most common English plaintext letters and likely more typical methods thereafter due to the less frequent letters not having as many substitutions to work with. This is evidenced by (1) the combined frequency of all ciphertext letters that appear regionally and semi-regionally roughly matching the combined frequency of the nine most common English plaintext letters, and (2) ciphertext letters M and backward L alternating with each other perfectly for the entire cipher and having a combined frequency matching English plaintext letters D and L, which are the 10th and 11th most common English plaintext letters, having a similar frequency and not being close to other letters in frequency, thus acting as a barrier between the high frequency letters and the other letters of the alphabet.
I created multiple test ciphers to test this after someone suggested to do so and found that it was easy to manually produce ciphers that have similar statistical characteristics to the 340 cipher. However, all of these ciphers were easily decipherable. That implies that if my work is correct, then likely a transposition method was used before homophonic substitution and that that is the reason why the 340 cipher hasn't been deciphered.
Something worth mentioning about regional and semi-regional cycles is that you can get a good guess of what plaintext letters a ciphertext letter might be substituting by looking at its frequency in areas where it does occur (in the case of regional cycles) or where its frequency is high (in the case of semi-regional cycles). These are only estimations, but consider ciphertext symbol W, which is limited to two areas of the ciphertext and has a high frequency in both areas. In that case, it isn't a stretch to assume that W is substituting a high frequency letter.
Another way to approach these two cycles is that ciphertext letters that behave inversely may be substituting the same plaintext letter whereas ciphertext letters that behave the same are unlikely to be the same plaintext letter, meaning that W is unlikely to be substituting the same letter as the circle with a horizontal line through it since these two symbols both only occur in the same lines as each other. It is more likely then that W is grouped with ciphertext letters that either don't occur where it occurs or occur at a relatively low frequency where it occurs.