For all you crypto freaks out there, I’m reading Eric Ambler’s The Intercom Conspiracy in which the following cipher is mentioned:
A French ten-franc note has on its face side four groups of figures. There is the date of issue and a printing number batch. Then there are two serial numbers, one of five figures in the lower left-hand corner and one of ten figures in the centre below the words Banque de France. In all, there are at least twenty-five digits on every note, and no two of the notes are exactly alike. It had been Brand’s idea to use these notes as “one-time” cypher pads. Jost himself had devised the matrix. The method was crude, no doubt, but it worked and was as safe as such things could be: a ten-franc note in one airmail envelope, the encyphered message in another. The limitation was that you could only send short, simple messages.
The message which had brought him there had been short and simple:
URGENT MEET PROPOSE COVER MILAN THEN VISIT GODCHILD TWENTIETH PM EVIAN STEAMER AFTER VEVEY CONFIRM
I have no French ten-franc notes (they were replaced by coins sometime in the ’70’s or ’80’s). But perhaps someone else out there does and can reconstruct the matrix (or at least one of the possibilities) used by the characters Brand and Jost. I offer it up as an intriguing challenge.