Zendian problem
The Zendian problem was an exercise in communication intelligence operations ( mainly traffic analysis and cryptanalysis ) devised by Lambros D . Callimahos as part of an advanced course , CA-400 , that Callimahos taught to National Security Agency cryptanalysts starting in the 1950s . 
<triplet> Zendian problem <subj> Lambros D. Callimahos <obj> author