Zendian Problem

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The Zendian Problem was a series of problems in traffic analysis and cryptanalysis devised by Lambros D. Callimahos as part of a course taught to National Security Agency cryptanalysts, whose graduates became members of the Dundee Society.

The scenario involves 375 radio messages intercepted on December 23 by the US Army contingent of a United Nations force landed on the fictional island of Zendia in the Pacific Ocean. A typical intercept looks like this:

XYR DE OWN 4235KCS 230620T USM-99/00091

9516 8123 0605 7932 8423 5095 8444 6831

JAAAJ EUEBD OETDN GXAWR SUTEU   EIWEN YUENN ODEUH RROMM EELGE
AEGID TESRR RASEB ENORS RNOMM   EAYTU NEONT ESFRS NTCRO QCEET
OCORE IITLP OHSRG SSELY TCCSV   SOTIU GNTIV EVOMN TMPAA CIRCS
ENREN OTSOI ENREI EKEIO PFRNT   CDOGE NYFPE TESNI EACEA ISTEM
SOFEA TROSE EQOAO OSCER HTTAA   LUOUY LSAIE TSERR ESEPA PHVDN
HNNTI IARTX LASLD URATT OPPLO   AITMW OTIAS TNHIR DCOUT NMFCA
SREEE USSDS DHOAH REEXI PROUT   NTTHD JAAAJ EUEBD

where the first line is provided by the intercept operator, giving callsigns, frequency, time, and reference number. The rest of the message is a transcript of the Morse code transmission. The numeric "header" originally has unknown meaning; the meanings of various components of this header (such as a serial number assigned by the transmitting organization's message center) can largely be puzzled out through traffic analysis. The rest of the message is ciphertext; the first group indicates the cryptosystem used, and often the second group contains a message-specific keying element such as initial rotor settings.

Cryptosystems employed include one-part code and transposition, dinome, and rotor-based machine ciphers; these can be solved without use of a computer. However, solution is not easy, and the practical exercise reinforces many basic principles.

Headers and initial ciphertext groups are also given for intercepts from the next three days; these may be helpful in traffic analysis and in determining certain daily operating procedures.

The problem has been declassified and is available either as part of Military Cryptanalytics or as a book in itself. Both are published by Aegean Park Press.

There is also a Web site which posts partial solutions for the Zendian Problem. It is highly recommended that anyone intending to learn from working on this exercise not look at these solutions.