“Not in the cryptographic sense. It is the first letter of the text repeated after the line had been completed without recurring letters. There is a special reason for its use. The old man has worked on what is called the keyword cipher, which is the most difficult of all ciphers to discover. This system consists of various arrangements, more or less elaborate, of tables of letters, set down in the form of the multiplication table, and from the table agreed upon messages are constructed whose solution depends on the use of some preconcerted keyword. The most scientific adaptation of this principle was constructed by Admiral Sir Francis Beaufort. In his system the letters of the alphabet are set down one under another from A to Z, then A is added to the line. The next line starts with B and runs to another B at the bottom. You continue till you have the whole alphabet set down in this fashion. From this table and an agreed keyword, which may consist of a proper name or a sentence of several words, you construct a cipher message.”
“How?” asked Marsland, in a tone of keen interest.
“That is what I now propose to demonstrate to you, if, as I think, the old man constructed his cryptogram in accordance with this principle. I have come to the conclusion that he modified and adapted this system to his own ends, using the letters of the text from the Bible to conceal it better, and then made it more difficult still by turning the letters into figures after the manner I have described. He has also made a slight but not uncommon variation from the Beaufort principle by striking out the ‘G’ in ‘gather,’ which would follow the ‘O’ if every letter in the text was used once, and substituting the final S, instead of placing the ‘S’ after ‘G.’ But the clue that suggested to my mind that he had worked on this principle are the two figures 6 coming together at the top of the circle. In the substituted letters they form two S’s. Now, why does he have two S’s when he carefully avoids recurring letters in the rest of the table? And why did he insert the first S again, as represented by the figure 6, instead of taking the next S in this table?
“In pondering over these points I discovered, as I believe, the system of cryptogram he used to construct his secret. He wanted to make the cryptogram difficult of solution, but at the same time he wanted to give some indication of the form of cryptogram he was using when his heirs came to search for the money. The recurring S indicates that he was working on a modification of the system I have explained, in which you add the first letter of your first column to the bottom, and continue on that system throughout the table. It is not much of a hint, because we have got to find the keyword before we can use the table, but by its help we will start with the assumption that the old man worked on the following table:
S R E L Y V M N W A K T H I U D Q P C O S
R E L Y V M N W A K T H I U D Q P C O S R
E L Y V M N W A K T H I U D Q P C O S R E
L Y V M N W A K T H I U D Q P C O S R E L
Y V M N W A K T H I U D Q P C O S R E L Y
V M N W A K T H I U D Q P C O S R E L Y V
M N W A K T H I U D Q P C O S R E L Y V M
N W A K T H I U D Q P C O S R E L Y V M N
W A K T H I U D Q P C O S R E L Y V M N W
A K T H I U D Q P C O S R E L Y V M N W A
K T H I U D Q P C O S R E L Y V M N W A K
T H I U D Q P C O S R E L Y V M N W A K T
H I U D Q P C O S R E L Y V M N W A K T H
I U D Q P C O S R E L Y V M N W A K T H I
U D Q P C O S R E L Y V M N W A K T H I U
D Q P C O S R E L Y V M N W A K T H I U D
Q P C O S R E L Y V M N W A K T H I U D Q
P C O S R E L Y V M N W A K T H I U D Q P
C O S R E L Y V M N W A K T H I U D Q P C
O S R E L Y V M N W A K T H I U D Q P C O
S R E L Y V M N W A K T H I U D Q P C O S
“It is from this table, unless I am very much mistaken, that he constructed the cipher at the top of the sheet,” said Crewe.
Marsland examined the curious table of letters, with close scrutiny, from various points of view, finally reversing it and examining it upside-down. He returned it to Crewe with a disappointed shake of his head.
“I can make nothing of it,” he said.
“It is necessary for us to discover the keyword he worked on before we can make use of it,” said Crewe. “Once we get the keyword, we will have no trouble in deciphering the mysterious message. The keyword is the real difficulty in ciphers of this kind. It is like the keyword of a combination lock. Without it, you cannot unlock the cipher. It is absolutely insoluble. Suppose, for example, he had picked a word at random out of the dictionary, and died without divulging it to anybody, we should have to go through the dictionary word for word, working the table on each word, till we came to the right one.”
“But that would take years,” exclaimed Marsland blankly.