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 Vigenèrie 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:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
BCDEFGHIJKLMNOPQRSTUVWXYZA
CDEFGHIJKLMNOPQRSTUVWXYZAB
DEFGHIJKLMNOPQRSTUVWXYZABC
EFGHIJKLMNOPQRSTUVWXYZABCD
FGHIJKLMNOPQRSTUVWXYZABCDE
GHIJKLMNOPQRSTUVWXYZABCDEF
HIJKLMNOPQRSTUVWXYZABCDEFG
IJKLMNOPQRSTUVWXYZABCDEFGH
JKLMNOPQRSTUVWXYZABCDEFGHI