Boxing Clever

Yesterday I talked about the Enigma machine. Until computers came — and breaking the Enigma code was an important step in the development of computers — it was about the best way of encrypting text. But suppose one wanted to send an actual physical object securely? Let us suppose one is an NSA agent working in London, and wants to send something — a folder of photographs of all the people who have been caught dancing on Margaret Thatcher’s grave, a crate of Glen Grant single malt, an inflatable boy scout complete in every anatomical detail — to one’s boss in Washington, and, because of the universal suspicion generated by yourself and your colleagues, you don’t trust the courier not to take a peek. No problem. You put whatever it is in a box, and fit a stout padlock to the box. Then you put the one and only key in your back pocket and send the locked box. When it arrives, the recipient doesn’t ring you up and say ‘Well you prat you didn’t send the key.’ He does something that seems quite lunatic, something that, did we not know it to be out of the question, might suggest NSA agents have a sense of humour: he gets another stout padlock and adds it to the one already there, and puts the key of the new padlock firmly in his back pocket, and sends the doubly locked box back to the original sender. On its arrival back, the original sender gets his own key out of his back pocket, removes the first padlock, and sends the box over to America again. Then, when it arrives the second time, the recipient gets his key out of his back pocket, and removes his padlock, and lo and behold…

Neat, Eh? I only heard of this trick a few years ago. Translated into electronic terms it is, or is supposed to be, how your credit card number is sent securely when you buy things on the internet.