G70 CRYPTOGRAPHY but Trithemius did not at first intend to publish it, on the ground that it would be injurious to public interests. The next treatises of importance were those of John Baptist Porta, a Neapolitan mathematician, who wrote De furtivis literarum notis, 1563 ; and of Blaise de Vigenere, whose Traite des chiffres appeared in Paris, 1587. Lord Verulam proposed an ingenious system of cryptography on the plan of what is called the double cipher ; but while thus lending to the art the influence of his great name, he gave an intima tion as to the general opinion formed of it and as to the classes of men who used it. For when prosecuting the earl of Somerset in the matter of the poisoning of Overbury, he urged it as an aggravation of the crime that the earl and Overbury " had cyphers and jargons for the king and queen and all the great men, things seldom used but either by princes and their ambassadors and ministers, or by such as work or practise against or, at least, upon princes." Other eminent Englishmen were afterwards con nected with the art. John Wilkins, subsequently bishop of Chester, published in 1641 an anonymous treatise entitled Mercury, or The Secret and Swift Messenger, a small but comprehensive work on the subject, and a timely gift to the diplomatists and leaders of the civil war. The deciphering of many of the royalist papers of that period, such as the letters that fell into the hands of the Parliament at the battle of Naseby, has by Henry Stubbe been charged on the celebrated mathematican Dr John Wallis (Athen. OJCOH., iii. 1072), whose connection with the subject of cipher- writing is referred to by himself in the Oxford edition of his mathematical works., 1689, page 659 ; as also by John Davys. Dr Wallis elsewhere states that this art, formerly scarcely known t3 any bat the secretaries of princes, <fec., laai grown very convnsu and familiar during the civil com motions, " so that now there is scarce a person of quality but is more or less acquainted with it, and doth, as there is occasion, make use of it." Subsequent writers on the subject ara John Falconer (Cryptomenysis Patefactd), 1635 ; John Davys (An Essay on the Art of Deciphering : in which is inserted a Discourse of Dr Wallis), 1737 ; Philip Thicknessa (A Treatise on. the Art of Decyphering and of Writing in Cypher), 1772 ; William Blair (the writer of the comprehensive article " Cipher " in Eees s Cyclopaedia], 1819 ; and G. von Marten (Cours Diplomatique), 1801 (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), who was drawn into the investigation by inclination and official circumstances. In this work the different methods of cryptography are classified. Amongst others of lesser merit who have treated on this art, may be named Gustavus Selenus (i.e., Augustus, duke of Brunswick), 1624; Cospi, translated by Niceron in 1641 ;the marquis of Worcester, 1659 ; Kircher, 1663 ; Schott, 1665 ; Killer, 1682 ; Comiers, 1690 ; Baring, 1737 ; Conrad, 1739, &c. Schemes of cryptography are endless in their variety. Bacon lays down the f jllowing as the " virtues " to be looked for in them : " that they be not laborious to write and read ; that they be impossible to decipher ; and, in some cases, that they be without suspicion." These principles are more or less disregarded by all the modes that have bean advanced, including that of Bacon himself, which has been unduly extolled by his admirers as " one of the most ingenious methods of writing in cypher, and the most difficult to be decyphered, of any yet contrived " (Thicknesse, p. 13). The simplest and commonest of all the ciphers is that in which the writer selects in place of the proper letters certain other letters in regular advance. This method of transposition was used by Julius Caesar. He " per quartam elaruentorum literam," 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 writers. An illustration of it occurs in Jeremiah (xxv. 26), where the prophet, to conceal the meaning of his prediction from all but the initiated, writes Sheshach instead of Babel (Babylon), the place meant; i.e., in place of using the second and twelfth letters of the Hebrew alphabet (b, b, I) from the beginning, he wrote the second and twelfth (sh, sh, ch) from the end. To this kind of cipher-writing Buxtorf gives the name Athbash (from a, the first letter of the Hebrew alphabet, and th the last ; b the second from the beginning, and h the second 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. In its adaptation to English this method of transposition, of which there are many modifications, is comparatively easy to decipher. A rough key may be derived from an examination of the respective quantities of letter? in a type founder 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 occura oftenest is e ; and the next in order of frequency is t. The following groups come after these, separated from each other by degrees of decreasing recurrence : a, o, n, i; r, s, h; d, I; c, iv, u, m ; f, y, g, p, b ; v, Jc ; x, q, j, z. _ All the single letters must be a, I, or 0. Letters occurring together are ee, oo, ff, U, ss, &c. The commonest words of two letters are (roughly arranged in the order of their frequency) of, to, in, it, is, be, he, by, or, as, at, an, so, &c. The commonest words of three letters are the and and (in great excess), for, are, but, all, not, &c. ; and of four letters that, with, from, have, this, they, &c. Familiarity with the composition of the language will suggest numerous other points that are of value to the decipherer. He may obtain other hints from Poe s tale called The Gold Bug. As to messages in the Continental languages constructed upon this system of transposition, rules for deciphering may be derived from Breithaupt s Ars decifratoria, 1737, and other treatises. 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. In recent years such codes have been adapted by merchants and others to communications by telegraph, and have served the purpose not only of keeping business affairs private, but also of reducing the excessive cost of telegraphic messages to distant markets. Obviously this class of ciphers presents greater difficulties to the skill of the decipherer. Figures and other characters have been also used as letters ; and with them ranges of numerals have been com bined 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, who were adepts in the use of ciphers, are of the same description. One of that monarch s letters, a document of considerable interest, consisting entirely of numerals purposely complicated, was in IP 58 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 Manuscripts, 1870. In the second and subsequent reports of the same commission, several keys to ciphers have been catalogued, which seem
to refer themselves to the methods of cryptography under