The New Murrey and Blue blog

The Emperor Charles V was the grandson of Maximilian I (Richard III’s friend and step-nephew-in-law), a nephew of Catherine of Aragon (and thus by marriage to Henry VIII) and father-in-law of Mary I. In February 1547, he feared death at the hands of a Italian mercenary (Pierre Strozzi) and wrote a fiendishly coded letter to the Imperial ambassador (Jean de Saint-Mauris) in France about this and other matters, such as a revolt in Prague cauasing the flight of his nephew Ferdinand. His main aim was to prevent England (newly under the Protector Somerset) from combining with France (under Francis I who died soon afterwards) to aid the Schmalkaldic League (Protestant rebels in Germany).

As you can see, the letter (right) was decrypted by the French, but not until 2022, by scientists at the Loria research laboratory under circumstances that could have easily defeated a computer. It had resided at the Stanislas library in Nancy.