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From: gwyn@brl-smoke.UUCP
Newsgroups: net.crypt
Subject: Re: One-time pads, military field communication
Message-ID: <904@brl-smoke.ARPA>
Date: Thu, 15-May-86 20:04:01 EDT
Article-I.D.: brl-smok.904
Posted: Thu May 15 20:04:01 1986
Date-Received: Sun, 25-May-86 16:35:36 EDT
References: <3264@reed.UUCP> <6650@utzoo.UUCP> <507@ucsfcca.UUCP> <871@ttrdc.UUCP>
Reply-To: gwyn@brl.ARPA
Organization: Ballistic Research Lab (BRL)
Lines: 39

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..