TH EX EN EM YM OV ES AT DA WN and enciphered: HW AU AL AK XP TE LU VR MR HL The message is then broken up into groups of five letters for transmission. To decipher such a cryptogram, (knowing the key word), the receiver divides it into pairs, and from his table finds the equivalent of these pairs, taking the letter immediately above each, when they are in the same vertical line; those immediately on the left, when in the same horizontal line; and those at opposite angles of the rectangle when this is formed. It is evident, from the foregoing description, that any letter of the plain text may be represented in cipher by one of five letters, viz: The one next below it and the other four letters in the same horizontal line with it in the square. Take, for example, the letter D of the plain text, in combination with each of the other letters of the alphabet. We have, using the key LEAVENWORTH: DA DB DC DE DF DG DH DI DK DL DM DN DO DP DQ DR DS DT DU DV DW DX DY DZ MR FC FD CA FG FB GR BM CM BA MX GA CR FM GM MD BX FR CX FA BR MA FX GX This gives D represented by B C F G M 4 4 8 4 4 times, and, connected with these five letters representing D, we have A R D M X B C G 5 5 2 4 5 1 1 1 times. Note that these letters are those of the vertical column containing D plus the letters B, C and G, of the horizontal line containing D. Lieut. Frank Moorman, U. S. Army, has developed a method for determining the letters which make up the key word in a Playfair cipher. In the first place, a key word necessarily contains vowels in the approximate proportion of two vowels to three consonants and it is also likely that a key word will contain other common letters. This key word is placed in the first row or rows. Now if a table is made, showing what letters in the cipher occur with every letter, it will be found that the letters having the greatest number of other letters in combination with them are very likely to be letters of the key word, or in other words, letters occurring in the first or second lines. An example will make this clear: Message DB FN EX TZ MF TO VB QB QT OB XA OF PR TZ EQ RH QK QV DX OK AB PR QI EL TV KE EX XS FS BP WD BO BY BF RO EA BO RH QK QV TX GU EL AB TH TR XN ON EA AY XH BO HN EX BS HR QB ZM SE XP HF GZ UG KC BD PO EA AY XH BO XP HF KR QI AB PR QI EL BX FZ BI SE FX PB RA PR QI WC BR XD YG TB QT EA AY XH BO HN EX BS HR QB PR QI EL BX BT HB QB NF SI SE BX NU XP BU RB XB QR OX BA TB RH BP WD RP RO GU GX QR SE ZY OX BA EL AX CW BY BA SX RK RO PR HB OP BD PI CN OX EM RP KR XT EL AX CW EQ FZ SX EL RH RO PR HB UX DA SE XN ZN GU EL BX FS DG DB TB ZL VE RH BO RQ. From this message, we make up the following table, considering the letters of each pair: First Letters of Pairs