He knew from experience the futility of attempting the solution of a cipher by any but an expert; and even with an expert it was rarely successful.
As a general rule, the key to a secret cipher is discovered only by accident or by betrayal. There are hundreds of secret ciphers—any person can devise one—in everyday use by the various departments of the various governments; but, in the main, they are amplifications or variations of some half-dozen that have become generally accepted as susceptible of the quickest and simplest translation with the key, and the most puzzling without the key. Of these, the Blocked-Out Square, first used by Blaise de Vigenerie in 1589, is probably still the most generally employed, and, because of its very simplicity, the most impossible of solution. Change the key-word and one has a new cipher. Any word will do; nor does it matter how often a letter is repeated; neither is one held to one word: it may be two or three or any reasonable number. Simply apply it to the alphabetic Blocked-Out Square and the message is evident; no books whatever are required. A slip of paper and a pencil are all that are necessary; any one can write the square; there is not any secret as to it. The secret is the key-word.
Harleston took a sheet of paper and wrote the square:
Assume that the message to be transmitted is: “To-morrow sure,” and that the key-word is: “In the inn.” Write the key-word and under it the message:
Then trace downward the I column of the top line of the square, and horizontally the T column at the side of the square until the two lines coincide in the letter B: the first letter of the cipher message. The N and the O yield B; the T and the M yield F; the H and the O yield V, and so on, until the completed message is:
The translator of the cipher message simply reverses this proceeding. He knows the key-word, and he writes it above the cipher message: