Voynich Authorship

[Mark Knowles]

- a top cipher developer would know very well that it is a very bad idea to encrypt a very long text with a single method/key

Why would it be a very bad idea?

One reason is that generally in cryptanalysis, the more source material you have to work with, the better your chances are at cracking the cypher.  The more sentences there are, the better your chances of finding repetitions or other patterns that lead to the breakthrough.

This is the response I expected.

The Voynich definitely violates this general principle! 

And this is the response that immediately occurred to me. However there is a more thoughtful response that I could provide. Diplomatic ciphers were not used in a professional capacity to encipher long manuscripts or documents, but rather letters, which by their very nature are very short. (Although it was not uncommon for two or three enciphered letters with the same key to be intercepted). So I don't think the kinds of issues that you suggest were ones that they seriously considered.

I suspect that their thought process was not:

1) We are writing a scientific document
2) It is vital that it is kept secret
3) We must devise a cipher to keep it secret.
4) We must cater for the scenario that someone intercepts the manuscript and tries to break its cipher.

But rather

1) We are writing a scientific document
2) This would be a great opportunity to implement or exhibit, if only to ourselves, the cutting edge ciphers we have developed.

Maybe they really were concerned about others stealing the manuscript and discovering its secrets, but I think that if they weren't cipher enthusiasts they wouldn't have tried to create a cipher with which to write the manuscript. So in one sense their first thought was not about how they can prevent someone deciphering the manuscript. However again this is human psychology and so pure speculation.

If the mind(s) behind the Voynich knew enough about cyphers to design/adapt and use one but lacked practical experience or knowledge of how cyphers are attacked, that is one explanation of why this general principle wasn't followed.

I am sure they were in the habit of trying to crack the ciphers of those letters the Duchy of Milan had intercepted of their political rivals. However they weren't intercepting 200+ page manuscripts.

Alternatively, they may only have wanted to protect the text from cursory snooping and never expected it to be subject to lengthy cryptanalysis.

I think that most likely.