On: French scientists decode 500-year-old letter

I think that is using a brute force method of cracking. Even though much of the code is a substitution, the absence of letters, the mixing of symbols with words, and the diacritics would make it hard for a computer to crack.

I imagine the brute force computer may run into a case where it can make sense of many of the words using substitution and then plateau without completely cracking it.