« Various are the methods which have been devised for transmitting secret information from one individual to another by means of writing, illegible to any except him for whom it was originally destined ; and the art of thus secretly communicating intelligence has been generally termed cryptography. Many species of secret writing were known to the ancients. Sometimes a slave’s head was shaved and the crown written upon with some indelible colouring fluid ; after which the hair being permitted to grow again, information could be transmitted with little danger that discovery would ensue until the ambulatory epistle safely reached its destination. Cryptography, however pure, properly embraces those modes of writing which are rendered legible only by means of some explanatory key which makes known the real signification of the ciphers employed to its possessor. »
La phrase-clef de cette cryptographie est :
tout autre que le destinataire ; on a généralement appelé cet art de correspondance secrète la cryptographie. Les anciens ont connu plusieurs genres d’écriture secrète. Quelquefois on rasait la tête d’un esclave, et l’on écrivait sur le crâne avec quelque fluide coloré indélébile ; après quoi on laissait pousser la chevelure, et ainsi l’on pouvait transmettre une information sans aucun danger de la voir découverte avant que la dépêche ambulante arrivât à sa destination. La Cryptographique proprement dite embrasse tous les modes d’écriture rendus lisibles au moyen d’une clef explicative qui fait connaître le sens réel du chiffre employé. »
