Cryptography (New Intl. Encyclopaedia, 2nd ed., 1914 article)



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 CRYPTOGRAPHY (Gk. κρυπτός, kryptos, secret + γράϕειν, graphein , to write). The art of writing messages and documents in cipher, intended to be read only by those possessing the key. The use of secret methods of correspondence in important matters of state is of considerable antiquity. Plutarch and Gellius tell of a method employed by Spartan ephors in communicating with their generals abroad, which has received the name of scytale, from the staff used in deciphering it. A narrow strip of parchment was first wound spirally upon the staff, its edges just meeting, and the message was then written along the line of jointure. When it was unwound, the broken letters could afterward be read only by rolling the parchment upon a duplicate staff in possession of the general to whom it was sent. This is but one of a large number of mechanical devices for reading secret dispatches, such as papers pierced with holes, to he laid over the document, revealing only such words or letters as compose the secret message.

Cryptography, in its stricter sense of a cipher alphabet formed either by changing the value of the different letters or by substituting for them groups of letters, numbers, or arbitrary symbols, if not of Semitic origin, was at least already known to and used by the sacred writers, in its simplest form of using the alphabet in its inverted order. By the Jews this form is known by the name of Atbos, a word formed from A, the first letter of the Hebrew alphabet; T, the last letter; B, the second; and S, the last but one. An instance of its use occurs in Jeremiah xiv, where the prophet, wishing to veil his meaning from all but the initiated, writes "Shehach" instead of "Babel," using the second and twelfth letters from the end of the alphabet instead of from the beginning. Julius Caesar's "quarta elementorum littera," in which D takes the place of A and E of B, is only a variant of this simplest form of cipher. Suetonius states that a similar method was employed by Augustus. In mediaeval and modern times many scholars have turned their attention to cryptography. Among them are John Trithemius, Abbot of Sponheim, in his Poligraphia (1500); Anastasius Kircher, and his pupil, Kaspar Schott, whose work, De Magia Universali (W&uuml;rzburg, 1676), contains a cryptographic table that lies at the foundation of the modern cipher telegraph systems. It consists simply of the alphabet repeated 24 times in horizontal rows, each successive row dropping off one or more letters from the beginning and adding it at the end. Thus, in the second row B stands under A, C under B, etc. In the third C represents A, in the fourth D = A, etc. Thus correspondents have a choice of 24 alphabets, it being necessary only to agree between themselves upon the first or key letter. For diplomatic purposes this form is much too simple, since by the simple mechanical task of making at most 24 versions, any one could decipher dispatches made in this way. Accordingly various methods of complicating the cipher have been tried, one simple and effective way being that which is known in France as the method of Saint-Cyr. It consists in using alternately two or more of these 24 alphabets, the order in which they are to be used being determined by a key word previously agreed upon. Thus, if the key word is "Army," four alphabets are to be used, viz., those in the rows beginning respectively with A, R, M, and Y. Various other elaborations have been sometimes employed, such as arbitrarily changing the sequence of the letters of the alphabet, inserting at regular intervals letters or symbols that have no meaning ("nulls and insignificants," Bacon called them), or using groups of letters to represent separate letters. Of the last-named variety is the famous biliteral cipher of Bacon himself, consisting of various combinations of A and B, arranged in groups of five. Thus, aabab, ababa , babba = fly. So used, such a cipher would be but little more difficult to detect than any ordinary set of single symbols. But Bacon went a step further; for the a 's and the b 's of his groups he substituted two fonts of type, differing so slightly as to present little distinction to the untrained eye. These, called respectively the a font and the b font, can be used for setting up any ordinary page of printed matter when, by the proper admixture of the two fonts, each successive group of five letters on the page may be made to stand for a single letter in Bacon's biliteral system. The fact that Bacon took a deep interest in cryptograms is probably the origin of Ignatius Donnelly's theory that the Shakespearean plays contain a cipher which, if interpreted, would prove that Bacon wrote them. And recently a still bolder attempt has been made by a certain Mrs. Gallup to apply the biliteral cipher to tinearly Shakespeare folios, in which, as is generally known, more than one variety of type was used. The general principle involved in Bacon's method &mdash; that of representing the whole alphabet with groups and combinations of two symbols only &mdash; lies at the basis of many modern methods of signaling.

It is hardly too much to say that ingenuity and perseverance will solve any code based upon a regular mathematical principle. If the language of the document is known in advance, the relative frequency with which the letters of the alphabet normally occur in that language forms an important initial clue. Thus e is the letter of most frequent occurrence, not only in our own language, but in French and German as well. In English the next in order of frequency are t, a , o , n , i , r , s , h , d , l , c , w , u , m , etc. Single letters must be either a , i , or o. Words of two letters most likely to occur are of, to , in , it , is , be , he , by , or , etc. Double letters are most apt to be ee , oo , ff , ll , or ss. If there is doubt whether the cipher is in Latin, English, French, or German, the lack of double letters at the end of words suggests that it is Latin; if but faw words end with double letters, it is probably French; if double letters are very numerous, it is German. Those who make a science of interpreting cipher documents receive no small assistance from a knowledge of tiie frequency with which certain symmetrical combinations of letters occur in the vocabulary of a language. Thus, the combination which may be represented for convenience by the formula abab, is comparatively rare in English: one may cite papa , dodo ; in French, t&ecirc;te , b&eacute;b&eacute;. The form abeba is found in level; French, r&ecirc;ver. In German, the formula abba gives only Anna, Ebbe , Egge , Esse , Otto ; in French, the formula abcdabc only two words, cherche and quelque. An interesting example of the relative frequency of letters being used to solve a cryptogram will be found in Poe's tale, The Gold Bug.

Consult: John Baptist Porta, De Furtivia Literarum Notis (1563); Blaise de Vigen&egrave;re, Trait&eacute; des chiffres (1587); Thicknesse, A treatise on the Art of Deciphering and of Writing in Cipher (1772); and among more modern writers, J. L. Kl&uuml;ber, Kryptographik (T&uuml;bingen, 1800); Romani, La cryptographic devoilee (1875); Fleissner, Handbuch der Kryptographik (Vienna, 1881): A. J. Butler's "Elizabethan Cipher-Books," in Transactions of Bibliographical Society (London, 1901).

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Bibliography of Cryptography: a Catalog of Books Pertaining to the Science of Codes And Cyphers. (Cincinnati, 1938)