"Cipher!" commented Harleston, looking at it with half-closed eyes....
"The Blocked-Out Square, I imagine. No earthly use in trying to dig it
out without the key-word; and the key-word--" he gave a shrug. "I'll let
Carpenter try his hand on it; it's too much for me."
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.
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