Cryptography. The art of secret writing, also called Writing in Cipher, Hieroglyphic Writing, Secret Writing, Steganography, Polygraphy, has been in use from an early date in correspondence between diplomatists and others engaged in important affairs requiring secrecy. Every government used to employ its staff of deciphers, who availed themselves of extraordinary means for interpreting despatches which (fairly or unfairly) came into their possession. The ciphers and the deciphers waged a constant struggle to outwit each other; the one by constructing new difficulties; the other by conquering the difficulties as soon as constructed. How often we hear of a courier being murdered and his despatches carried off! And without the key to decipher letters so written, to what purpose would they be intercepted by such a deed? In these modern times, however, there has been so great an improvement in the morals of governments that the custom of killing foreign-office messengers for the sake of their despatch-bags is entirely obsolete in diplomacy, and statesmen have ceased to pillage post-offices or rifle portmanteaus for cryptographic messages.
Most of the odd knacks, contrivances, decoys, blinds, now employed by cryptographers were to some extent known to and employed by the ancients. Substituting points for vowels; arranging threads, knots, or ink-spots at determinate distances; substituting one letter for another; inventing new arbitrary characters for whole words or even sentences—now made use of extensively in telegraph codes; abbreviating words in their prefixes and affixes; writing a long sentence of nonsense, with a clue to find the words which gave the proper sense—all were brought into requisition. Perhaps the most amusing of all cryptographs was the one mentioned by Herodotus. Histiaens, a Greek at the Persian court, being desirous of sending a secret message to Aristagoras at Miletus, selected a slave who was afflicted with bad eyes, and shaved his head, pretending that it was necessary for his recovery. In performing this, Histiaens imprinted his secret intention in legible characters on the man's head, and kept him in close confinement till his hair grew again, when he sent him to Aristagoras for a perfect cure. Aristagoras repeated the shaving, read the writing underneath, and thus obtained the desired information by means of the unconscious messenger.
One of the simplest methods of cryptography is to use instead of each letter of the alphabet a certain other letter at a regular interval in advance. Such was a mode of secret writing adopted by Julius Cæsar. As a variety of this plan, the alphabet is used invertedly, z for a, y for b, x for c, and so on; or, while the first seven letters are represented by the second seven, the next six may be represented by the last six. And many other variations may be adopted. But the decipherment of such messages is naturally not difficult, and with a little consideration of the peculiarities of the English language, all the ups and downs of many an interesting love story related in cipher in the columns of the Times can be followed from start to finish with comparative ease. It is known that e is the most frequent letter; that the is the commonest word; that ea and ou are the double vowels which most frequently occur; that the consonants most common at the ends of words are r, s, and t, &c. We also know that a word of a single letter must be either the pronoun I, or the vowels a or O; that an, at, on, to, of, and in are the most common words of two letters; that the and and are the most frequent words in three letters; that the most usual doubled letters are ee, oo, ll, ss, ff; that double vowels are mostly followed by l, m, n, r; that the letter a begins three two-letter words in very extensive use—an, as, at; that the letter o begins or ends eight two-letter words in very common use—do, go, no, so, to, of, on, or; that more words in a sentence of average English begin with t than with any other letter; that in about three-fourths of all the words in a sentence, either the first or the second letter is a vowel; that among consonants, d and h are most largely used, after which come n, s, r, t; that the letter q is always followed by u; that no English word of two letters or more ends with i. All these considerations will guide us to the solution of any simple cipher, enabling a skilful decipherer to read almost any ordinary piece of cryptographic writing in a very short time.
In British history, cryptography has at no time been in greater requisition than during the Civil War. Charles I.'s celebrated letter to the Earl of Glamorgan (afterwards Marquis of Worcester), in which he made some compromising concessions to the Catholics of Ireland, was composed in an alphabet (sometimes supposed to be Charles's own, but more probably Worcester's invention) of twenty-four short strokes variously situated upon a line (see OGHAMS). Other letters by the same monarch are to appearance a mere series of numbers of two or three figures divided by semicolons. In such cases it was necessary that the two parties engaging in the correspondence should have previously concerted what words each number was to represent.
In the reign of William III. the Jacobites invented many curious ciphers to enable them to communicate with their exiled king. All the Jacobite clubs had distinct methods of their own—their great aim being to write in such a manner that the very ciphers themselves should pass through their enemies' hands without suspicion. This they accomplished by means of sympathetic inks. A favourite Jacobite cipher was the use of parables, conveying, by means of ordinary language, a double meaning, which only the person acquainted with the writer's views would think of. The use of cryptography for purposes of state in England ended, it may be said, with the Peace of 1815. During the Peninsular war the government attached a cryptographer to the office of the Minister for Foreign Affairs to read and write the ciphers received and despatched. It is said that on more than one occasion the minister was unable to comprehend his own cipher.
The earliest elaborate treatise on writing in cipher is the Steganographia (Frankf. 1606) of the abbot John Trithemius, a MS. copy of which was bought for a thousand crowns at Antwerp by Dr Dee in 1563. Lord Bacon, who esteemed cryptography one of the most useful arts of his time, framed what he believed a not easily penetrable cipher—in which he employed only a and b, arranging each of these letters in groups of five, in such collocations as to represent all the twenty-four letters. Thus aabab, ababa, babba conveyed the word fly. In his De Augustis he styled this an omnia per omnia cipher, believing that in this case preconcertment would be necessary; but in reality any clever modern decipherer could have read any letter composed in such a manner if it were of any length.
Mr Donnelly, in his work The Great Cryptogram, endeavours to prove that Bacon inserted a cipher in the Shakespearean plays—which he claims is the work of the great philosopher—but the cipher is of so elaborate a kind that nobody but Mr Donnelly has been able to follow its intricacies. The unfortunate Earl of Argyll used a mode of secret writing which consisted in setting down the words at certain intervals, which he afterwards filled up with other words, making of the whole something intelligible, but of no use to any one else reading the message. The Marquis of Worcester invented a cipher composed of dots and lines variously ordered within a geometrical figure; while Dr Blair made one of three dots, placed over, under, or on the line, by which he could represent no fewer than eighty-one letters, figures, or words. The Doctor, in his able article in Rees's Cyclopædia, declares this cipher to be as nearly as possible undecipherable by strangers; but two years afterwards, Mr Gage, of Norwich, published a pamphlet on purpose to solve Dr Blair's riddle. As he devoted fourteen closely printed octavo pages to the explanation, any description of the cipher is beyond the limits of this article. Mr Thickness, a well-known expert of the 18th century, also devised a plan of conveying information in the disguise of music, the notes, rests, expression-marks, &c., standing for letters.
All the methods, however, of cryptography may thus be summarised: (1) By invisible ink; (2) by superfluous words; (3) by misplaced words; (4) by vertical and diagonal reading; (5) by artificial word-grouping; (6) by stencil-plates cut out so as to show certain words beneath; (7) by using two letters (Lord Bacon's cipher); (8) by transposing the letters; (9) by substitution of letters; (10) by counterpart tabulations; (11) by mixed symbols; (12) by a printed key and code-book, used chiefly in telegrams; (13) by the employment of numerals.
The present century has seen the decline of cryptography for all practical purposes, and the art is now only regarded as a curious study, closely connected with the history of all nations.