henry@utzoo.UUCP (Henry Spencer) (07/14/85)
> ... In _The Codebreakers_ somewhere > David Kahn tells about a code system that was giving trouble ... the > cryptanalysts produced a memo that included some words that were in doubt, > leaked it to the target agents, and then read the encryption as it got > sent verbatim to home base. ... If the issues you are trying to resolve are simple ones, it doesn't even have to be verbatim. Kahn cited another case, where WW2 US cryptanalysts were not sure whether a particular geographic-location code number in use by the Japanese meant "Midway Island". So they arranged for Midway to report, in clear, that its water-purification plant had broken down. Presto, an encrypted Japanese intelligence report that location ___ was short of water. It's also possible sometimes to arrange for the same message to be sent in two different encryption systems (one of which you have broken), or to obtain the plaintext of old, no-longer-important messages sent with an encryption system that is still in use. Sending the same message two ways, or saving message plaintext verbatim, are no-nos if you are really concerned about encryption security. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry