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As Stenographia ( "secret writing with light"; also referred to as Cryptologia ) called Athanasius Kircher developed by his system of secret communication . This system used - in contrast to the optical telegraphy of antiquity - a concave mirror , which is labeled with the characters to be transmitted. With this procedure, military commands could be transmitted "bug-proof" over a distance of around three and a half kilometers. Kircher described this process in 1671 in his Ars magna lucis et umbrae .

See also: telegraphy , communications engineering , cryptography