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To encode something or decode it
A key is used to encipher something. The same key can be used to decipher it.
embitter, encipher, emperor, empower, engineer, exporter, extensor
Encipher is a pretty close match. It's a fairly technical word.
Obfuscate is kind of close.
Germany used an enigma machine to encipher and decipher messages before World War II, and with military operations during the War.