Several websites have carried the story of the decoding of an encrypted letter from the Emperor Charles V to his ambassador at the French court early in 1547. The letter itself is in the archives at Nancy.
The first report I saw about this research was on the phys.org website at Emperor Charles V's secret code cracked after five centuries
The Guardian also had a report about the letter which can be seen at Emperor Charles V's secret code cracked after five centuries
There is a longer and fuller account on the BBC News site which places the letter in the immediate context of when it was written, just after the death of King Henry VIII, and also into the rather wider picture of the shifting fortunes of the Habsburg and Valois monarchies as they jockeyed for position and advantage amidst the political and confessional uncertainties of the time. It also shows that part of the letter was written en clair so that were it to fall into opponant’s hands it could still be useful in shaping diplomacy. This report can be seen at Charles V: French scientists decode 500-year-old letter