Why create such a complex cipher?

[MarcoP]

As I was thinking about this, I was reminded of the Zodiac ciphers from the 1970s. I'm sure most cipher nerds know that the Z-340 was cracked about four years ago now. It was cracked through a computer brute forcing every imaginable way to encode a text

"a computer brute forcing every imaginable way to encode a text" doesn't sound like an accurate description of how Zodiac 340 was solved. From You are not allowed to view links. Register or Login to view.:

Blake [...] responded to Oranchak’s talk with mathematical ideas about how to approach a code that includes both homophonic substitution — in which one letter might be swapped for more than one symbol — and transposition — in which letters are reordered in a systematic way. Oranchak and Blake began corresponding and eventually generated hundreds of thousands of possible ways to read the code.

Other ciphers by the killer had already been solved. You are not allowed to view links. Register or Login to view. looked similar: it was a homophonic substitution, without word spaces. The solvers assumed that Z340 was a more complex variant, in which the characters were not presented in the original order, but had been rearranged somehow. So they tried a number (You are not allowed to view links. Register or Login to view.) of regular transpositions and fed each to a software designed to crack homophonic ciphers. They were then able to pick those solutions that appeared to contain meaningful fragments and they could fix the encoding errors and come to a complete solution. Though 650,000 can sound like a large number, it really is a tiny fraction of the different imaginable ways to encode a text. Software tools were essential to the task, but the solution depends on researchers making the right hypotheses about the cipher.

In the case of Voynichese, we don’t know if it is a cipher, and if it is we have no idea of what kind of cipher it could be: Z340 is similar to Z480, but Voynichese isn’t similar to any late-medieval cipher. Moreover, the underlying language is unknown, which of course adds a further layer of uncertainty for cryptographic approaches.