Why did Giovanni Fontana write his book in cipher?

[-JKP-]

It's a simple substitution cipher. A mnemonic one. It takes 10 minutes to break it.

I don't think it was created for secrecy. It was used in manuscripts that showcase his engineering ideas. I think the cipher is a way to demonstrate his abilities when combined with the engineering diagrams.

Is it your contention that simple substitution ciphers were never used for secrecy? Looking at 14th century diplomatic ciphers it appears that they used simple substitution ciphers for secrecy then.

If you had actually read my blog, all of it, you would have seen me mention that Fontana WROTE A BOOK on mnemonic strategies and ways to arrange letters (Secretum de thesauro experimentorum ymaginationis hominum). So it is not just the fact that it is a simple substitution cipher, or that it is a mnemonic one (which makes it even easier to break and quite easy to read), but he WROTE about mnemonic systems, using his cipher. He had a specific interest in memory aids that he expressed using his cipher. It was designed to be easy to learn and remember (and thus easier to write an entire book using the cipher, which he did).

I was not implying that substitution ciphers were never used for secrecy. I was pointing out that there are several indicators (not just one), that Fontana had other motivations besides secrecy, such as demonstrating some of his mnemonic theories. His cipher is distinctly different from others in the way that the entire alphabet (except maybe one character) is mnemonic in an overtly systematic way. I have not seen this in any other medieval cipher.