Cryptography, or secret writing

from the Greek cryptos , a secret, and graphein , to write—has been largely employed in state despatches, commercial correspondence, love epistles, and riddles. The telegraphic codes employed in the transmission of news by electric wire, partakes somewhat of the cryptographic character, the writer employing certain words or figures, the key to which is in the possession of his correspondent. The single-word despatch sent by Napier to the Government of India, was a sort of cryptographic conundrum—Peccavi , I have sinned (Scinde); and in the agony column of the Times  there commonly appear paragraphs which look puzzling enough until we discover the key-letter or figure. Various and singular have been the devices adopted—as, for instance, the writing in the perforations of a card especially prepared, so as only to allow the real words of the message to be separated from the mass of writing by means of a duplicate card with similar perforations; the old Greek mode of writing on the edges of a strip of paper wound round a stick in a certain direction, and the substitution of figures or signs for letters or words. Where one letter is always made to stand for another, the secret of a cryptograph is soon discovered, but when, as in the following example, the same letter does not invariably correspond to the letter for which it is a substitute, the difficulty of deciphering the cryptograph is manifestly increased:
Ohs ya h sych, oayarsa rr loucys syms
Osrh srore rrhmu h smsmsmah emshyr snms.
The translation of this can be made only by the possessor of the key.
a b c d e f g h i j k l m n o p q r s t u v w x y z
h u s h m o n e y b y c h a r l e s h r o s s e s q
"Hush Money, by Charles H. Ross, Esq."—twenty-six letters which, when applied to the cryptograph, will give a couplet from Parnell's "Hermit":
"Far in a wild, unknown to public view,
From youth to age a reverend hermit grew."
The employment of figures and signs for letters is the most usual form of the cryptograph. From the following jumble we get a portion of Hamlet's address to the Ghost:

cryptography example

it is easy to write and not very hard to read the entire speech. The whole theory of the cryptogram is that each correspondent possesses the key to the secret. To confound an outside inquirer the key is often varied. A good plan is to take a line from any ordinary book and substitute the first twenty-six of its letters for those of the alphabet. In your next cryptogram you take the letters from another page or another book. It is not necessary to give an example. Enough will be seen from what we have written to instruct an intelligent inquirer.