The Cab of the Sleeping Horse

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
KLMNOPQRSTUVWXYZABCDEFGHIJ
LMNOPQRSTUVWXYZABCDEFGHIJK
MNOPQRSTUVWXYZABCDEFGHIJKL
NOPQRSTUVWXYZABCDEFGHIJKLM
OPQRSTUVWXYZABCDEFGHIJKLM