"Now choose a key-word, or preferably, a key-sentence. For simplicity's sake, we'll take the short word: BEAD, and suppose we wish to send in cypher the message: CAB FEED."
"Which is pure nonsense."
"Granted. I merely select two words at random which can be coded on my incomplete square. If I had the whole diagram drawn out the message could be anything you like."
"Go on," commanded Betty, her eyes snapping.
"First you write down your message; then above it you put the key-word, repeated in whole or in part as many times as may be necessary, thus:"
| B | E | A | D | B | E | A | ||
| C | A | B | F | E | E | D |
"Turning to the diagram you find B, the first letter of the key-word, in the top horizontal line; and C, the first letter of the word to be put into code, in the left-hand vertical line. Now look for the letter at the intersection of the vertical column headed by B and the horizontal line which C begins. You will find it to be E. Set this down as the first symbol of your cypher message, and obtain the other letters in a similar manner. Your despatch will then read: E F C J G J E. As an object lesson, place these letters under your original arrangement of key-word and message, thus:"
| B | E | A | D | B | E | A | ||
| C | A | B | F | E | E | D | ||
| E | F | C | J | G | J | E |
"You see at a glance that the substitution is irregular and variant. For example, the symbol E stands for both C and D. Again, the letter E in the word F E E D is at one time represented by G and secondly by J."
"How do you translate the cypher?" asked Betty.