"I know the principle on which it operates," the chief replied, "but I can't say that I've ever come into contact with it."
"So far as we know," went on the State Department official, "it is the most efficient cipher system in the world—simple, easy to operate, almost impossible to decode without the key, and susceptible of being changed every day, or every hour if necessary, without impairing its value. However, in common with every other code, it has this weakness—once the key is located the entire system is practically valueless.
"When did you discover the disappearance of the code secret?" asked Taylor, examining his cigarette with an exaggerated display of interest.
"How did you know it was lost?" demanded the Under Secretary.
"I didn't—but the fact that your chief sent for mine and then you launch into a dissertation on the subject of the code itself is open to but one construction—some one has lifted the key to the cipher."
"Yes, some one has. At least, it was in this safe last night"—here a wave of his hand indicated a small and rather old-fashioned strong box in the corner—"and it wasn't there when I arrived this morning. I reported the matter to the Secretary and he asked me to give you the details."
"You are certain that the cipher was there last evening?" asked the chief.
"Not the cipher itself—at least not a code-book as the term is generally understood," explained the Under Secretary. "That's one of the beauties of the Carruthers system. You don't have to lug a bulky book around with you all the time. A single slip of paper—a cigarette paper would answer excellently—will contain the data covering a man's individual code. The loss or theft of one of these would be inconvenient, but not fatal. The loss of the master key, which was in that safe, is irreparable. If it once gets out of the country it means that the decoding of our official messages is merely a question of time, no matter how often we switch the individual ciphers."
"What was the size of the master key, as you call it?"
"Merely a slip of government bond, about six inches long by some two inches deep."