ORDENOQUE

Then, in the A-B cipher, A equals

ADEROVIVA

The key is found; VIVA_ADERO and a trial of M in the blank space shows correct results. This checks with our theory that a ten letter key word was used and deciphering the message we have:

PARA EL ATAQUE CONTRA TORREON ORDENO QUE SUS TROPAS MARCHEN ESTE NOCHE X.

The reason for breaking camp is now evident.

This method may be used, with some labor, on short words like THE, AND, etc. Parts of the key will appear whenever an assumed word is found in the message and the whole key may be assembled if enough of the parts are available. Even if only part of the key may be so recovered, it will always lead to the ultimate solution of the cipher by trial of the partially recovered key on the message letter by letter.

As an example of recovery of a key by use of short common words, let us refer to the message of Case [7-a]. There are twenty-four groups of three letters each in this message and we will try them against THE, ARE and YOU, assuming that the Vigenere cipher is used.

123456789101112
IfOSBVOIGSWCYYZSZBVJXLDOSYUVDYJLSQAHSI
equalsTHETHETHETHETHETHETHETHETHETHETHETHE
orAREAREAREAREAREAREAREAREAREAREAREARE
orYOUYOUYOUYOUYOUYOUYOUYOUYOUYOUYOUYOU
then A equalsVLXCHENLSJRUGLVIOFEEZVLUBOZFCHZJWOLE
orOBXVXEGBSCHUZBVBEFXUZOBUUEZYSHSZWHBE
orQEHXAOIECEKEBEFDHPZXJQEEWHJAVRUCGJEO
131415161718192021222324
IfBJVSFXDQBAIIOHZIVXJBFESFJSCNLUCTWCSM
equalsTHETHETHETHETHETHETHETHETHETHETHETHE
orAREAREAREAREAREAREAREAREAREAREAREARE
orYOUYOUYOUYOUYOUYOUYOUYOUYOUYOUYOUYOU
then A equalsICRZYTKJXHBEVAVPOTQUBLLBQLYUEQJMSJLI
orBSRSOTDZXAREOQVIETJKBEBBJBYNUQCCSCBI
orDVBURDFCHYUOQTFKHDLNLGELLEIPXAEFCEES