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Reference.org
Zendian problem
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<p>The Zendian problem was an exercise in communication intelligence operations (mainly <a href="/facts/Traffic_analysis/ReJ8of8l">traffic analysis</a> and <a href="/facts/Cryptanalysis/7CPEQlJI">cryptanalysis</a>) devised by <a href="/facts/Lambros_D._Callimahos/QqlVqAsg">Lambros D. Callimahos</a> as part of an advanced course, CA-400, that Callimahos taught to <a href="/facts/National_Security_Agency/Sg6MGV1o">National Security Agency</a> <a href="/facts/Cryptanalysts/7CPEQlJI">cryptanalysts</a> starting in the 1950s. </p>