1400s amateur crypto and human puzzle tendencies

[Mark Knowles]

What do you mean by "simple substitution"?

I refer to the paper of @Nickpelling ( see above ) respectively to the presented ciphers of Simeone de Crema and Michele Steno. As I see it, these are ciphers that essentially simply perform a letter substitution in the alphabet.

Nick's paper is a very good entry point to the subject.

If you want a bit more depth on the early 15th century then you could look at:

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If you want further depth then there are other sources that I can refer you to.

I should say that the Steno cipher key does a bit more than perform a letter substitution in the alphabet as it has substitutions for whole words as well as nulls.

From my research it seems that all the techniques used in diplomatic ciphers, that I know of, over the next century were in place by 1424.