Cryptography

Cryptography, as the name denotes, signifies secret writing. At all times since the introduction of writing there have been occasions when men, for one reason or another, have desired to make communications which should be unintelligible if they fell into hostile or curious hands. The Spartan ephors wound a piece of paper, or its equivalent, spirally round a staff and wrote along the line of the staff. When unrolled this strip carried no connected intelligence, but became legible when rolled round a similar stick in the possession of the recipient. Bacon set great store by cipher, and Charles I. and his queen largely employed it in their correspondence. Pepys's diary is in cipher. Many ingenious systems of cipher have been invented, from the simple substitution of one letter of the alphabet for another, well known to every schoolboy, to the employment of an elaborately changing series of symbols, which present greater difficulties, but not insuperable ones, to a would-be decipherer, provided the writing is of some length. Many people use a secret writing of their own in making entries in their diary. If the same symbol always represents the same letter, deciphering is comparatively easy. How to set about reading a cipher is interestingly set forth in Edgar Allan Poe's tale, The Gold Beetle. A cipher which presents much difficulty is that where two persons who have each a copy of a certain edition of a book, refer to such a number of letters on such a line, on such a page; but the writing and reading are both tedious processes. The modern telegraph codes used by commercial houses are the most usual kind of secret-writing to be met with nowadays.