CRYPTOGRAPHY, the art of secret writing, more commonly called the art of writing in cipher (from Arabic sift, void), has been in use from an early date in correspondence between diplomatists and others engaged in important affairs requiring secrecy. In modern times, it has been the subject of learned care to lord Bacon, the ingenious mar quis of Worcester, Dr. Wallis, bishop Wilkins, Thicknesse, Falconer, Blair, etc. In our own history, it has at no time been in greater requsition than during the civil war, and among the politicians of the 17th century. And even now, when there is happily less need for mystery among our statesmen, the need for a perfectly undecipherable mode of secret communication has again had to be looked for, in order that information may pass by the electric telegraph without being understood by the officials in connection with the apparatus.
One of the most simple methods of C. is to use, instead of each letter of the alphabet, a certain other letter at a regular interval in advance of it in that series. Such was a mode of secret writing used by Julius Caesar. As a variety upon 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 for all modes like these, there are modes of decipherment far from difficult. It is only necessary, in general, to bear in mind certain peculiarities of the language presumed to be used. Say it is the English. We readily remember that e is the most frequent letter; that ca and on are the double vowels which most frequently occur; that the consonants most common at the ends of words arc r, a, and t; etc. We also know how a single letter must be either the pro noun /or the article a; how an, at, and on are the most common words in two letters; how the and and are the most frequent words in three letters, etc. By taking advantage of these few obvious principles, a tolerably skilled decipherer will read almost any such piece of cryptographic writing in five minutes. The Times newspaper often gives, in its advertising : columns correspondence on delicate subjects, even assignations for elope merits. written in this manner, the writers of which are of course little aware how open
their secrets thus become to society.
Politicians and important personages conducting affairs of difficulty became long ago sensible of the necessity of using ciphers of greater abstruseness. The celebrated letter of Charles I. to the earl of Glamorgan, is which he made some condemning con cessions (elsewhere denied) to the Catholics of Ireland, was composed in an alphabet of 24 short strokes variously situated upon a line. Other letters by the same monarch are to appearance a mere series of numbers of two and three figures, divided by semicolons. In such cases, it was necessary that the two parties in the correspondence should have previously concerted what words each number was to represent. Bacon devised what lie thought a not easily penetrable cipher, in which he employed only a and b, arrang ing each of these in groups of 5, in such collocations as to represent all the 24 letters. Thus, aabab ababa, babba conveyed the word _fly. The great philosopher thought that preconcertment would here be necessary; but in reality any clever modern decipherer would have found no difficulty in reading any long letter composed in such a manner. The unfortunate earl of Argyle, when preparing his expedition against the tyrannical government of James II., used a mode of secret writing which consisted in setting down the words at certain intervals, which lie afterwards filled up with other words, making on the whole something intelligible, but indifferent. In our day, such a mode would not have been found.proof against the ingenuity of those who have studied the means of decipherment. There are many other modes of secret writing, which it does not seem necessary to detail, as the art has become little more than a matter of curiosity. One of the ablest and amplest treatments of the subject is an article by Dr. William Blair in Rees's Cyclopedia. See also Chambers's Journal, No. 506 (Second series), under "Secrets Exposed," and Nos. 87 and 115, under "Cryptographs."