# The Problem of Zendia

"The Zendian Problem, presenting an operational communication intelligence situation in miniature, affords opportunity to apply the techniques of traffic analysis and cryptanalysis, to derive intelligence, and to prepare reports. This problem deals with the enemy communication during a (fictitious) operation against Zendia, a totalitarian island state."
(Lambros D. Callimahos, 1959)
Traffic Analysis And The Zendian Problem. By Lambros D. Callimahos
ISBN: 0-89412-161-8
Aegean Park Press, P.O.Box 2837, Laguna Hills, CA 92654

## The Zendian Problem:An Exercise in Communication Intelligence Operations

By the morning of 23 December, radio intelligence units were operative and began an all-out effort to intercept and solve the encrypted Zendian traffic. The traffic intercepted on 23 December totalled 375 messages.

The messages can be found in file
"messages.zip".

The top line of the message heading is the intercept line, consisting of the receiving and transmitting call signs, the frequency, the intercept date/time, and the intercept station designation; also included is a teleprinter serial number (which can serve as a worksheet number in subsequent processing) used in forwarding the traffic by teleprinter.
The second line of the heading is the message preamble which consists of eight 4-digit groups; this is followed by the message text, invariably transmitted in groups of five characters. In all messages, the first two groups are repeated at the end.

• identify sending and receiving stations,
• construct an overview over the net of radio stations,
• identify the cipher methods used,
• decrypt and read the messages.

Results of an intial analysis of the messages:

• The first three digits of the preamble constitute the station serial number, the next three a message center number, the next six the file date and time, followed by two groups whose meaning has not yet been determined;
• apparently the next (the sixth) 4-digit group denotes the originator of the message. The fourth digit of this group is a check digit; the sum of the four digits of this group equals 9 mod 10.
• the next (the seventh) 4-digit group denotes the addressee. Here, too, the fourth digit is a check digit; the sum of the four digits of this group equals 0 mod 10.
• in the last group of the preamble, the first two digits constitute the group count, and the last two digits are still unidentified.
• The first two groups of the cipher text are repeated at the end of the message.
• Apparently the first group, the discriminant, is an indicator of the cipher method used to encrypt the message.

It can easily been seen that there are three different types of discriminants: "ab c ab", ab c ba" and "aa b cc".

 Group Iab c ab Group IIab c ba Group IIIaa b cc ABCAB FBHFB AEFEA CCFII ADEAD FEAFE CFIFC DDHAA AGHAG GBIGB DAEAD FFBGG AIJAI GDAGD DCGCD IIHFF AJAAJ GEBGE DJDJD BCEBC GHEGH ECHCE BEGBE HAIHA EGBGE BFHBF HBJHB GIFIG BGIBG HDBHD HBJBH CBECB JAAJA JAAAJ CEHCE JBBJB 28082 CIBCI JCCJC 50505 CJCCJ JDDJD 68486 DCGDC JEEJE 80808 DFJDF JFFJF 95459 DHBDH JGGJG EDIED JHHJH EGBEG JIIJI EIDEI number ofmessages: 69 149 157

 A diagram shows the radio station net together with call signs and discriminants used on the various radio connections. This net is identical to the ordre-de-bataille of the Zendian army. The system of call signs (except the call signs of battalion level) is given an analysis. Further cryptanalysis is done according to the discriminant, the first cipher text group.

You can read the up to now decrypted messages, sorted according to the originator:

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