This paper analyses the encryption of the correspondence of the imperial resident ambassador in Constantinople, Alexander von Greiffenklau zu Vollrads (1643–48). For early modern diplomats, information security was a vital necessity because intercepted letters could negatively affect the course of negotiations. For encryption purposes, Habsburg diplomats used a method where numbers were substituted for letters, vowels or words. Greiffenklau’s encryption key stands out as comparatively simple, and would have hardly withstood a serious decryption attempt. He only partially encrypted his letters, but two types of information were encrypted particularly often: reports concerning the peace between the Ottomans and the Habsburgs and accounts regarding the Ottoman Empire’s internal affairs.