levy@ttrdc.UUCP (Daniel R. Levy) (05/10/86)
In article <507@ucsfcca.UUCP>, dick@ucsfcca.UUCP (Dick Karpinski) writes: >Why are one time pads impractical in military field communications? >If a CD ROM holds 500 megabytes of key in a drive like the ones that >joggers are happy to wear on their belts, I see no hard problems. > >Dick Karpinski Manager of Unix Services, UCSF Computer Center Question[s]. Given the one-time pad, how does the receiver of the encrypted message know which "sheet" of his pad to use as the decryption key? Is this information (sheet number of pad) sent as "clear" information (or encrypted using another kind of key)?ZZ Also, in a situation where a sent message from a one-time pad might not always succeed in being received, how can it be guaranteed that the pad will truly remain "one-time", that is, someone else will not use the same sheet in their copy of the pad that the unsuccessful sender used? Maybe sheet number to use in the pad is based on the time of transmission? In any case, there is an obvious security problem if a man out in the field carrying one of these pads happens to be captured by the enemy; many copies of the pad would need to be carried, wouldn't they, to make the scheme useful for field communications? (Of course the unit could have an "erase" button on it for emergency data destruction, but that doesn't help much if the soldier is killed in action and the body captured.) -- ------------------------------- Disclaimer: The views contained herein are | dan levy | yvel nad | my own and are not at all those of my em- | an engihacker @ | ployer or the administrator of any computer | at&t computer systems division | upon which I may hack. | skokie, illinois | -------------------------------- Path: ..!{akgua,homxb,ihnp4,ltuxa,mvuxa, vax135}!ttrdc!levy
gwyn@brl-smoke.UUCP (05/16/86)
In article <871@ttrdc.UUCP> levy@ttrdc.UUCP (Daniel R. Levy) writes: >Question[s]. Given the one-time pad, how does the receiver of the encrypted >message know which "sheet" of his pad to use as the decryption key? Is >this information (sheet number of pad) sent as "clear" information (or >encrypted using another kind of key)?ZZ There is no need to encrypt the sheet number. Information such as this is usually contained in a fixed place in either the message header or in one of the first or last few code groups. One-time pads are a special case, since the top remaining sheet should always be the correct one (to deal with lost messages, the first few sheets can be tried until one produces a readable decipherment). Incidentally, the business of determining information from external message information such as routing, traffic volume, key indicators, etc. independently of cryptanalysis is called "traffic analysis". It's amazing how much information one can obtain like this; it also provides help toward actual cryptanalysis of the traffic. >Also, in a situation where a sent message from a one-time pad might not >always succeed in being received, how can it be guaranteed that the pad >will truly remain "one-time", that is, someone else will not use the >same sheet in their copy of the pad that the unsuccessful sender used? >Maybe sheet number to use in the pad is based on the time of transmission? One-time pads are generally used in pairs, one at each end of a single communication link. A sheet is destroyed immediately after use, and no other communication link even has the same sheet. >In any case, there is an obvious security problem if a man out in the field >carrying one of these pads happens to be captured by the enemy; many copies >of the pad would need to be carried, wouldn't they, to make the scheme >useful for field communications? (Of course the unit could have an "erase" >button on it for emergency data destruction, but that doesn't help much if >the soldier is killed in action and the body captured.) Loss of any crypto key is bad news. One-time pads are primarily used by espionage agents, who conceal them carefully. Discovery of a one-time pad in one's possession is a strong indicator that one is a spy..
henry@utzoo.UUCP (Henry Spencer) (05/27/86)
> ...One-time pads are primarily used > by espionage agents, who conceal them carefully... And for diplomatic communications, where secure distribution of substantial volumes of key isn't hard and high-grade communications security is vital. To take the most extreme case, the Washington-Moscow Hotline uses one-time encryption for its data. (Contrary to popular belief, the Hotline is a teletypewriter link, not a voice line.) -- Usenet(n): AT&T scheme to earn revenue from otherwise-unused Henry Spencer @ U of Toronto Zoology late-night phone capacity. {allegra,ihnp4,decvax,pyramid}!utzoo!henry