Decryption was the process of making encrypted data legible again.
Hoshi Sato, the communications officer stationed aboard Enterprise, was skilled at decryption. In 2151, an encrypted message was sent to T'Pol, which which Ensign Sato decrypted. (ENT: "Breaking the Ice")
In 2374, USS Voyager received a badly damaged message from Starfleet Command. Captain Janeway tried over fifty decryption algorithms on the message without luck, but managed to read the message after designing a new decryption algorithm in the hopes of dealing with a corrupted message from Starfleet. (VOY: "Hope and Fear")