Has anyone ever "deciphered" a paragraph?

[geoffreycaveney]

It would be of great interest to have a historical example of this.

I saw two answers to this.

Morse code is of course not a cipher, and modern (not historical in the sense of our problem).

I had a quick look at what Mark sent me by E-mail, but what I saw is not at all evidence of reducing entropy - quite the contrary.

The following idea is not a historical example, but it is a rather simple "joke cipher" to illustrate the concept of an entropy-reducing cipher (which in the following case will be 100% unsolvable). It seems mathematically simple enough that such a joke could have been conceived in the early 15th century:

Assign each letter its numerical value: A=1, B=2, C=3,...,Z=26.
For each letter of the original plaintext message, add 13. Then multiply this value by 2. Then subtract 2 times the original letter's numerical value. Then divide by 2, and this number is the numerical value of the ciphertext letter.
Of course, this is the equivalent of a simple kids' math puzzle book joke: 
The resulting ciphertext will always be MMMMMMMMMMMMM........, no matter what the original plaintext was. Now this is obvious to any of us who have any experience or skill with ciphers and math puzzles, but the result of the "cipher rules" could easily surprise or fool people at first if they are not mathematically inclined.

Ciphers that are based on word games, word play, puns, homonyms, etc., could possibly reduce the entropy of the resulting ciphertext as well, albeit in a less extreme manner than the 100% unsolvable "cipher" above. The question is at what point they would or would not become completely unsolvable.