Why did Giovanni Fontana write his book in cipher?

[Mark Knowles]

I always thought that John Friedman's discussion of medieval scientific ciphers was well founded in his paper talking about John de Foxton's use of cipher in his Liber Cosmographae. 

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He says, after a paragraph summarizing aspects about the cipher, about Fontana's motivations:

Although I do get the logic, I am not convinced the VM and Fontana's work share similar motivations.  Certainly the outcome has been quite different for the two manuscripts -- one got Fontana a job (if I remember correctly), the other, it still can't be read.

It is true that there were much more sophisticated ciphers used at the time than Fontana's, however the knowledge of ciphers was probably quite restricted to a small number of people. It is the case that his cipher skills could impress those unfamiliar with ciphers, though do we have any evidence that his cipher skills helped him gain employment? I know he did deliver a diplomatic message for the Doge of Venice, which was quite probably enciphered, and I wonder if this is how he became acquainted with ciphers.

It has long been my perspective that the difference in the sophistication of the Voynich and Fontana ciphers was a function in the difference in knowledge of ciphers of the two authors not their motivations. To me that means Fontana didn't know a lot about ciphers whilst the author of the Voynich had cutting edge knowledge of ciphers, which most likely went out of usage. (I believe the sophistication of the 1425 diplomatic ciphers in the Florence archive makes the relative simplicity of Fontana's cipher apparent.)