Schemes of cryptography are endless in their variety. Bacon lays down the follow ing 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." The principles are more or less disregarded by all the modes that have been advanced, including that of Bacon himself, which has been unduly extolled by his admirers as "one of the most ingenious methods of writing in cipher, and the most difficult to be deciphered, of any yet contrived." The simplest and commonest of all 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 Cmsar. He, "per quartam elementorum 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. Jeremiah xxv. 26, where the prophet, to conceal the meaning of his prediction from all but the initiated, writes Shell/telt 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), counting from the beginning, he wrote the second and twelfth (sh, sh, ch), counting 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 It, 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 com paratively easy to decipher. A rough key may be derived from an examination of the respective quantities of letters in a type-founder's bill, or a printer's "case." The de cipherer's first business is to classify the letters of the secret message in the order of their frequency. The letter that occurs oftenest is e; and the next in order of frequency is t. The following groups come after these, separated from each other lly degrees of decreasing recurrence: a, o, r, s, h; d, I; c, w, u, m; f, y, g, p, b; v, k, x, g, j, z. All the single letters must be a, I, or 0. Letters occurring together are ee, oo, II. 88, etc. 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, etc. The commonest words of three letters are the and and (in great excess), for, are, but, not, etc.; and of four letters: that, lcith, from, lucre, this, they, etc. Familiarity with the composition of the language will suggest numerous other points of value to the decipherer. He may obtain other hints from Poe's tale called The Gold Bug. As to messages in the continental languag,es 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 with 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 reduc ing the excessive cost of telegraphic messages to distant markets. Obviously this class of ciphers present 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 combined as the representatives of syllables, parts of words, words themselves, and com plete phrases. Under this head must be placed the dispatches 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, con sisting entirely of numerals purposely complicated, was in 1858 deciphered by prof. Wheatstone, the inventor of the ingenious crypto-machine, and printed by the Philo biblon society. Other letters of like character have been published in the Pint Report of the Royal COmmission on•IIistorical Manuscripts. In the second and subsequent reports of the same commission, several keys to ciphers have been catalogued, which seem lo refer themselves to the methods of cryptography under notice. In this connection also should be mentioned the "characters" which the diarist Pepys drew up when clerk to sir George Downing and secretary to the earl of Sandwich and to the admiralty, and which are frequently mentioned in his journal. Pepys described one of them as "a
great large character," over which lie spent much time, but which was at length finished, 25th April, 1660; " it being, says lie, " very handsomely done and a very good one in itself, but that not truly alphabetical." Shorthand marks and other arbitrary characters have also been largely imported into cryptographic systems to represent both letters and words—commonly the latter. This plan is said to have been first put into use by the old Roman poet Ennius. It forms the basis of the method of Cicero's freedman, Tiro, who seems to have systematized the labors of his predecessors. A large quantity of these characters have been engraved in Gruter's Inscriptiones. The correspondence of Charlemagne was in part made upon marks of this nature. In Recs's C,yeloptedia, specimens were engraved of the cipher used by cardinal Woltoy at the court of Vienna, in 1524, of that used by sir Thomas Smith at Paris jji 1563, and of that of sir Edward Stafford at Madrid in 1586; in all of which arbitrary !harks are introduced. The first English system of shorthand—Bright's Uharacterie, 1588—almost belongs in the same category of ciphers. A favorite system of Charles I., used by him during the year 1646, was made up of an alphabet of twenty four letters, which were represented by four simple strokes varied in length, slope, and position. This alphabet is engraved in Clive's Linear System of Shorthand, 1830, having. been found amongst the royal manuscripts in the British museum. An interest attached to this cipher from the fact that it was employed in the well-known letter addressed by the king to the earl of Glamorgan, in which the former made concessions to the Roman Catholics of Ireland.
Complications have been introduced into ciphers by the employment of " dummy" letters—" nulls and insignificants," as Bacon terms them. Other devices have been introduced to perplex the decipherer, such as spelling words backwards, making false divisions between words, etc. The greatest security against the decipherer has been found in the use of elaborate tables of letters arranged in the form of the mul tiplication-table, the message being constructed by the aid of preconcerted key-words. Details of the working of these ciphers may be found in the treatises named in this article. The deciphering of them is one of the most difficult tasks. A method of this kind is. explained iu the Latin and English lives of Dr. John Barwick, whose cor respondence with Hyde, afterwards earl of Clarendon, was carried on in cryptog raphy. In a letter dated 20th Feb., 1659-60, Hyde, alluding to the skill of his political opponents in deciphering, says that " nobody needs to fear them" if they write carefully in good ciphers." In his next he allays his correspondent's apprehen ness as to the deciphering of their letter: "I confess to you, as I am sure no copy could be gotten of any of my cyphers from hence, so I did not think it probable that they could lie got on your side of the water. But I was as confident, till you tell me you believe it, that the devil himself cannot decipher a letter that is well written, or find that 100 stands for sir II. Vane. I have heard of many of the pretenders to that skill, and have spoken with some of them, but have found them all to be mountebanks; nor did I ever hear that more of the king's letters that were found at Naseby, than those which they found deciphered, or found the ciphers in which they were writ were deciphered. And I very well remember that in the volume they published there was 'much left in cipher which could not be understood, and which I believe they would have explained if it had been in their power." An excellent modification of the key-word principle was constructed by the late admiral sir Francis Beaufort; it has been recently published in view of its adaptation to tele grams and post-cards. Ciphers have been constructed on the principle of altering the places of the letters without changing their powers. The message is first written Chinese-wise upward and and the letters are then combined in given rows from left to right. In the celebrated cipher used by the earl of Argyle when plotting against James II., he altered the position of the words. , Sentences of an indifferent nature were constructed, but the real meaning of the message was to lie gathered from words placed at certain intervals. This method, which is connected with the name of Caidan, is sometimes called the trellis or card-hoard cipher. The wheel-cipher, which is an Italian invention, the string-cipher, the circle-cipher, and many others, are fully explained, with the necessary diarouns, in the authorities named above—more particu larly by Kluber in his Kryptographik,. [The substance of the above is from Enyclopadia Britannica, ninth edition.]