An isolated but encrypted letter sent in February 1547 by Charles V to his ambassador to the King of France, Jean de Saint-Mauris, is preserved at the Bibliothèque municipale de Nancy (France). As no other encrypted letters were preserved alongside it, a cryptanalysis was performed and research was carried out into other copies of the letter and other encrypted letters. These studies have enabled the letter to be placed in its broader epistolary and cryptographic context. The edition of the decrypted text provides full access to the letter’s contents, reconnects the encrypted letter to its draft and offers insights into imperial cryptographic practices in the mid-16th century.