"Try me," she challenged.
"To begin with, you write down the twenty-six letters of the English alphabet in a horizontal line, indenting it the s.p.a.ce of a single letter."
"Indenting?"
"You"ll understand when you see the diagram I"m preparing."
"Oh, you"re making a magic square!"
"Yes. Now you repeat the process twenty-five times, the only difference being that all these other lines begin at the left-hand margin, each with a different letter in their strict alphabetical order. Your diagram will then look like this. For the present I am putting it in skeleton form:"
A B C D E F G.............W X Y A B C D E F G H.............X Y Z B C D E F G H I.............Y Z A C D E F G H I J.............Z A B D E F G H I J K.............A B C E F G H I J K L.............B C D F G H I J K L M.............C D E .................................
W X Y Z A B C D.............T U V X Y Z A B C D E.............U V W Y Z A B C D E F.............V W X
"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 subst.i.tution 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.
"Merely reverse the process. You write down the cypher message, and above it as many letters of your key-word as may be needed, thus:"
B E A D B E A E F C J G J E
"Now follow down the vertical column headed by B until you reach the symbol letter E; then move your pointer over left to the end of that horizontal line which will give you C, the first letter of the original message. Understand?"
Betty tried her hand, and quickly caught the trick; really it was very easy.
"One more point; it is better not to divide the cypher message into word groups as the continuous string of letters looks more mystifying. There is no difficulty in picking out the sense when decoding."
"Finally, you notice that the upper left-hand s.p.a.ce in the diagram is vacant; consequently you must not use the letter Z in either the key-word or in the message to be coded. But this restriction is not of any practical disadvantage, Z being a letter that is seldom used. It will often appear, of course, in the cypher itself."
"Certainly it is all very simple," remarked Betty.
"But without the key-word where would you get off?"
"I don"t see how anybody could possibly work it out; why the complications are absolutely overwhelming."
"And you can make them still more intricate by merely using a longer key-word, or indeed a whole sentence. For example: "I love Betty Hildebrand.""
"Everybody knows that," retorted Betty. "Still I don"t mind an occasional restatement of the established fact. Please, Hugh! I spent any amount of time in getting those ruffles starched just so."
Betty took the diagram and carefully tucked it away in a drawer of her secretary. "Of course we can"t be sure that old Mr. Thaneford really used "Le Chiffre Indechiffrable,"" she said thoughtfully.
"Only a possibility," I agreed.
"And without the key-word or key-sentence we shall never be any wiser than we are."
"Granted again."
"So there you are. Just the same, Hugh, I wish you would make me a complete diagram; I"d like to experiment with it."
"I"ll do it for you to-night. Here"s your precious diary."
Betty kissed me and went upstairs. It took me the best part of an hour to draw out the diagram in full; then I had to mount it on cardboard so as to keep it in good condition for constant handling. For the benefit of the curious-minded I reproduce it below:
LE CHIFFRE INDeCHIFFRABLE
-----------------------------------------------------ABCDEFGHIJKLMNOPQRSTUVWXY-----------------------------------------------------ABCDEFGHIJKLMNOPQRSTUVWXYZ-----------------------------------------------------BCDEFGHIJKLMNOPQRSTUVWXYZA-----------------------------------------------------CDEFGHIJKLMNOPQRSTUVWXYZAB-----------------------------------------------------DEFGHIJKLMNOPQRSTUVWXYZABC-----------------------------------------------------EFGHIJKLMNOPQRSTUVWXYZABCD-----------------------------------------------------FGHIJKLMNOPQRSTUVWXYZABCDE-----------------------------------------------------GHIJKLMNOPQRSTUVWXYZABCDEF-----------------------------------------------------HIJKLMNOPQRSTUVWXYZABCDEFG-----------------------------------------------------IJKLMNOPQRSTUVWXYZABCDEFGH-----------------------------------------------------JKLMNOPQRSTUVWXYZABCDEFGHI-----------------------------------------------------KLMNOPQRSTUVWXYZABCDEFGHIJ-----------------------------------------------------LMNOPQRSTUVWXYZABCDEFGHIJK-----------------------------------------------------MNOPQRSTUVWXYZABCDEFGHIJKL-----------------------------------------------------NOPQRSTUVWXYZABCDEFGHIJKLM-----------------------------------------------------OPQRSTUVWXYZABCDEFGHIJKLMN-----------------------------------------------------PQRSTUVWXYZABCDEFGHIJKLMNO-----------------------------------------------------QRSTUVWXYZABCDEFGHIJKLMNOP-----------------------------------------------------RSTUVWXYZABCDEFGHIJKLMNOPQ-----------------------------------------------------STUVWXYZABCDEFGHIJKLMNOPQR-----------------------------------------------------TUVWXYZABCDEFGHIJKLMNOPQRS-----------------------------------------------------UVWXYZABCDEFGHIJKLMNOPQRST-----------------------------------------------------VWXYZABCDEFGHIJKLMNOPQRSTU-----------------------------------------------------WXYZABCDEFGHIJKLMNOPQRSTUV-----------------------------------------------------XYZABCDEFGHIJKLMNOPQRSTUVW-----------------------------------------------------YZABCDEFGHIJKLMNOPQRSTUVWX-----------------------------------------------------
Note that while the diagram is a necessary piece of machinery in using this particular cypher system, it has no value in itself; the whole secret depends upon the possession of the key-word or key-sentence. As this may easily be memorized by the two correspondents there is no risk of discovery through the accident of loss or theft.
Chapter XIV
_Another Break in the Circle_
It was the first of June and the loveliest time of the year at the "Hundred." Why had I never realized before that, in spite of my urban upbringing, I was a born countryman? Can there be a greater pleasure in life than living on one"s own land, and honestly plying the oldest and most important of human industries--the tilling of the soil! Provided, of course, that one possesses a reasonable amount of capital; the hand-to-mouth struggle of the poor farmer is deadening to both soul and body; as one of my less fortunate neighbors once put it: "It isn"t living; it"s just staying on."
Certainly I had no cause for complaint. The "Hundred" was easily the best farm anywhere about. I could command sufficient ready money to be independent of the banks, and I was beginning to learn my trade. What more could the heart of man desire? And finally, there was Betty--but how could one inventory that immeasurable a.s.set! Enough that our happiness was as complete as anything mundane could be, and I had only to bear in mind the old Greek admonition: "Tread softly lest the high G.o.ds overhear and be moved to celestial ire against a mortal so felicitous!"
Eunice Trevor was still living at the "Hundred," and the question of that other arrangement had been suffered to remain in abeyance. I did not fancy the ungracious task of turning her out of the house, and by temperament I am something of an opportunist; time is the great resolver of our difficulties; moreover, to do the woman justice, she seemed desirous of effacing herself in every possible way; for days on end I would hardly see her except at dinner, our one formal function. And then one day something occurred to set me thinking, an incident small in itself and yet curiously disquieting.
Miss Trevor was in the habit of driving over alone to Calverton two or three times a week. Still she was never absent more than a couple of hours, and it was none of my business how she employed her leisure.
Betty commented upon these journeys once or twice, but neither of us cared to press the direct inquiry; there were plenty of horses available, and the girl"s time was her own; what did it matter.
On this particular morning I chanced to be in the house at the moment of her return from town. She pa.s.sed me in the hall, nodded briefly, and went up to her room. As I walked through the front door I noticed a letter lying on the threshold. I picked it up and saw that it was addressed to Miss Eunice Trevor, Lockbox 31, Calverton, Maryland. The handwriting was that of John Thaneford, a square, bold script with which I was perfectly familiar. The post-mark was that of a small town in Florida.
So Eunice and Thaneford were engaged in correspondence, and a secret one at that. It didn"t look well, and I felt the blood reddening my temples.