CRYPTOGRAPHY, or writing in cipher (from Gr. Kpvlrros hidden, and 'ypackEiv, to write), called also steganography (from Gr. Or€'y6xn, a covering), the art of writing in such a way as to be incomprehensible except to those who possess the key to the system employed. The unravelling of the writing is called de ciphering. Secret modes of communication have been in use from the earliest times. The Lacedemonians had a method called the scytale, from the staff (QKVraXrj) employed in constructing and deciphering the message. Polybius has enumerated other methods of cryptography. The art was in use also amongst the Romans.
John Trithemius (d. 1516) , the abbot of Spanheim, was the first important writer on cryptography. His Polygraphia, pub lished in 1518, has supplied the basis upon which subsequent writers have worked. A Steganographia published at Lyons (? 15 51) and later at Frankfort (16o6) is also attributed to him. The next treatises of importance were those of Giovanni Battista della Porta, the Neapolitan mathematician, who wrote De furtivis litterarum notis, 1563; and of Blaise de Vigenere, whose Traite des cliifires appeared in Paris, 1586. Cryptography having become a distinct art, Bacon classed it (under the name ciphers) as a part of grammar. He proposed an ingenious system on the plan of what is called the double cipher. John Wilkins, subsequently bishop of Chester, published in 1641 an anonymous treatise entitled Mercury, or The Secret and Swift Messenger. The de ciphering of many of the royalist papers of that period has by Henry Stubbe been charged to the celebrated mathematician, Dr. John Wallis (Atlien. Oxon. iii. 1,o72), whose connection with the subject of cipher-writing is referred to by himself in the Oxford edition of his mathematical works, 1689, p. 659. Subsequent writers on the subject are John Falconer (Cryptomenysis pate f acta), 1685; John Davys (An Essay on the Art of De cyphering: in which is inserted a Discourse of Dr. Wallis), Philip Thicknesse (A Treatise on the Art of Decyphering and of Writing in Cypher), 17 7 2 ; William Blair (the writer of the com prehensive article "Cipher" in Rees's Cyclopaedia), 1819; and G. von Marten (Coors diplomatique), 18o1 (a fourth edition of which appeared in 1851). Perhaps the best modern work on this subject is the Kryptographik of J. L. Kliiber (Tubingen, 1809), in which the different methods of cryptography are classified. Amongst others of lesser merit who have treated of this art may be named Gustavus Selenus (i.e., Augustus, duke of Brunswick), 1624; Cospi, translated by Niceron in 1641; the marquess of Worcester, 1659; Kircher, 1663; Schott, 1665 ; Ludwig Heinrich Hiller, 1682 ; Comiers, 169o; Baring, 1737 ; Conrad, 1739, etc. See also a paper on Elizabethan Cipher-books by A. J. Butler in the Bibliographical Society's Transactions, London, 19oI.
The simplest and commonest of all the ciphers is that used by Julius Caesar, in which the writer selects in place of the proper letters certain other letters in regular advance. Caesar wrote d for a, e for b, and so on. There are instances of this arrangement in the Jewish rabbis, and even in the sacred An illustra tion of it occurs in Jeremiah (xxv. 26), where the prophet writes Sheshak instead of Babel (Babylon), i.e., in place of using the second and twelfth letters of the Hebrew alphabet (b, b, l) from the beginning he wrote the second and twelfth (sh, sh, h) from the end. Another Jewish cabalism of like nature was called Albam; of which an example is in Isaiah vii. 6, where Tabeal is written for Remaliah. A rough key to this method of transposi tion, in its adaptation to English, may be derived from an exami nation of the respective quantities of letters in a typefounder's bill, or a printer's "case." The decipherer's first business is to classify the letters of the secret message in the order of their frequency. The letter that occurs of tenest is e ; and the next in order of frequency is t. Similarly the commonest words of two, three and four, etc., letters can be ascertained. The decipherer may obtain other hints from Poe's tale called The Gold Bug. Rules for deciphering messages in the Continental languages con structed upon this system may be derived from Breithaupt's Ars deci f ratoria (i 7 3 7) .
Bacon remarks that though ciphers were commonly in letters and alphabets yet they might be in words. Upon this basis codes have been constructed, classified words taken from dictionaries being made to represent complete ideas. Figures and other char acters have been also used as letters; and with them ranges of numerals have been combined as the representatives of syllables, parts of words, words themselves, and complete phrases. Under this head must be placed the despatches of Giovanni Michael, the Venetian ambassador to England in the reign of Queen Mary, documents which have only of late years been deciphered. Many of the private letters and papers from the pen of Charles I. and his queen are of the same description. One of that monarch's letters, consisting entirely of numerals purposely complicated, was in 1858 deciphered by Professor Wheatstone, the inventor of the ingenious crypto-machine, and printed by the Philobiblon Society. Other letters of the like character have been published in the First Report of the Royal Commission on Historical Man uscripts (1870). In the second and subsequent reports of the same commission several keys to ciphers have been catalogued. In this connection also should be mentioned the "characters," which the diarist Pepys drew up and which are frequently men tioned in his journal.
The plan of importing shorthand marks and other arbitrary characters into cryptographic systems to represent both letters and words is said to have been first put into use by the old Roman poet Ennius. A large quantity of these characters has been engraved in Gruter's Inscriptiones. The correspondence of Charlemagne was in part made up of marks of this nature. In Rees's Cyclopaedia specimens were engraved of the cipher used by Cardinal Wolsey at the court of Vienna in 1524, of that used by Sir Thomas Smith at Paris in 1563, and of that of Sir Edward Stafford in 1586; in all of which arbitrary marks are introduced. The first English system of shorthand—Bright's Characterie, 5588—almost belongs to the same category of ciphers. A favour ite system of Charles I., made up of an alphabet of twenty-four letters, represented by four simple strokes varied in length, slope and position, is engraved in Clive's Linear System of Shorthand (183o).
Complications have been introduced into ciphers by the employ ment of "dummy" letters, by spelling words backwards, making false divisions between words, and especially by the use of elaborate tables of letters, arranged in the form of the multiplica tion table, the message being constructed by the aid of precon certed key-words. A method of this kind is explained in the Latin and English lives of Dr. John Barwick.
An excellent modification of the key-word principle was con structed by Admiral Sir Francis Beaufort. Ciphers have been constructed on the principle of altering the places of the letters without changing their powers. In the celebrated cipher used by the earl of Argyll when plotting against James II., sentences of an indifferent nature were constructed, but the real meaning of the message was to be gathered from words placed at certain intervals. This method, which is connected with the name of Cardan, is sometimes called the trellis or card-board cipher.
The wheel-cipher, which is an Italian invention, the string cipher, the circle-cipher and many others are fully explained, with the necessary diagrams, in the authorities named above—more particularly by Kluber in his Kryptographik.
For modern developments see CODES and CIPHERS.
See H. G. Fiske, Studies in the Biliteral Cipher of Francis Bacon (1913) ; A. Langie, Cryptography (1922) ; A. Figl, Systeme des Chi$rierens (1926) .