book discloses the whole secret. Do you understand?" - In Jeopardy

Betty thought she did, and seemed so interested that I was emboldened to

assume my best lecture manner.

"Under the second head we may consider the mechanical device known as

the grid, grille, or screen.

"The instrument in question consists of a plate, usually made of metal,

pierced by a number of holes of different sizes and irregularly spaced.

When the writer sets out to prepare his message he lays the grid on the

paper, and marks in the letters making up the words of his despatch

through the apertures. Then the screen is removed, and the blank spaces

are filled up with writing which has nothing to do with the real subject

matter, the process being repeated until the entire message has been

coded. The recipient is provided with a precisely similar grid. By

applying it to the communication he is then able to read, through the

holes, the text of the secret message. The ancient Romans used a

variation of this method, somewhat as follows. A long strip of paper was

wound spirally about a cylinder or cone; the writing was then done

parallel with the axis of the metal form. When unrolled, the

communication seemed to be made up of arbitrary signs really parts of

letters which were entirely unintelligible. The recipient, however, by

rewinding the strip on a precisely similar form, would be able to read

the message.

"Of course we may rule out the mechanical device. In this case we have a

long communication of several hundred words, and the grille would be

impracticable--too wasteful of space."

"That disposes of No. 2," said Betty hopefully. "What next?"

"In class 3 the coded message consists of numbers, or even of pure

symbols--stars and daggers or what not. The latter variation is

generally pure substitution, and may be called kindergarten cryptology.

No one but a rank amateur would employ such a system.

"In the numeral code each correspondent is supplied with a dictionary,

the same edition of course. Each word of the original message is

represented by a group of five numbers, two designating the location of

the required word on the page, and the remaining three denoting the

number of the page itself. The process, both of coding and of uncoding,

is very laborious, and hardly pays for the trouble involved. Another way

to use the two dictionaries is to interpret the words of the code

message by substituting other words removed a certain definite distance

up or down the column. Suppose it is agreed that 'fifteen down' shall be

the key, and that the despatch, as received, reads: _Bull Collier_. The

recipient takes his copy of the dictionary, looks up the word _Bull_,

and counts down fifteen, getting the word _Buy_. Similarly, _Collier_

gives him _Copper_, and the decoded message will mean: 'Buy copper.'

Finally, we may use a predetermined series of numbers as a key formula.

We then divide the message to be coded into the same number of letter

groups, and work out an intricate transposition, reversing the process

in order to decode."

"Rather makes your head ache," remarked Betty plaintively. "Besides,

this cypher doesn't use numbers at all."

"Right you are," I acquiesced, "and we are undoubtedly dealing with a

system of the fourth order in which the letters are transposed according

to a constant prearranged formula.

"Let us first consider the simple form; the regular substitution of one

letter of the alphabet for another. For example, X always takes the

place of E, while B invariably means T, and so on. Such cyphers are

easily read by the expert, who works on the principle that all the

letters of the English alphabet may be ranked on a numerical scale of

average frequency in use. The letter E heads the list; consequently, if

any particular symbol predominates in the message it must correspond to

that hard-worked vowel. Again, as _the_ is the commonest word group in

the language we are quickly able to identify what stands for T and H.

But this is quite too transparent a code for serious use."

"Then don't waste time over it," said my practical-minded wife. "Old Mr.

Thaneford was not a foolish person."

I took a long look at the incomprehensible jumble of letters.

"There are any number of formulae," I went on, "by means of which we may

effect a transposition of letters, the substitution being variable or

irregular. For instance, the 'Checkerboard,' invented by the Russian

nihilists, and similar devices, most of which depend for secrecy upon

single or double key-words. Perhaps the cleverest system in this group

is the cypher called by the French, 'Le Chiffre Indechiffrable.'"

"'The Undecypherable Cypher,'" commented Betty. "Sounds rather

hopeless."

"Well, you can decide for yourself if there is any reasonable

possibility of unravelling it, unless you are lucky enough to stumble on

the key-word."

"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 space 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 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.

"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 space 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

-----------------------------------------------------

| |A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|

-----------------------------------------------------

|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|

-----------------------------------------------------

|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|

-----------------------------------------------------

|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|

-----------------------------------------------------

|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|

-----------------------------------------------------

|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|

-----------------------------------------------------

|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|

-----------------------------------------------------

|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|

-----------------------------------------------------

|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|

-----------------------------------------------------

|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|

-----------------------------------------------------

|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|

-----------------------------------------------------

|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|

-----------------------------------------------------

|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|

-----------------------------------------------------

|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|

-----------------------------------------------------

|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|

-----------------------------------------------------

|O|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|

-----------------------------------------------------

|P|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|

-----------------------------------------------------

|Q|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|

-----------------------------------------------------

|R|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|

-----------------------------------------------------

|S|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|

-----------------------------------------------------

|T|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|

-----------------------------------------------------

|U|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|

-----------------------------------------------------

|V|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|

-----------------------------------------------------

|W|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|

-----------------------------------------------------

|X|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|

-----------------------------------------------------

|Y|Z|A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|

-----------------------------------------------------

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.