Encryption is a method of obscuring information by making it unreadable in hopes to protect the contents from being viewed or accessed by anyone other then the intended parties.
Several species used encryption to protect their data and the access to it. The way encryption codes were created differ; some codes were easy to break or to circumvent and other codes were difficult or even impossible to break. The Qomar, a Delta Quadrant species, thought at first that Earth music was a form of encryption. (VOY: "Virtuoso")
Encryption was not only used to protect data, such as personal files, but also for communication protocols. The ops protocol on Deep Space 9 was protected by an encryption system. (DS9: "Battle Lines")
As encryption codes differ in complexity – a fractal-based encryption code could lock out the main computer of a Sovereign-class starship in such a way that even the Borg had trouble bypassing it – encrypting or decrypting such codes might take some time. (Star Trek: First Contact) In the 24th century, encryption programs were used to bypass access protocols. (DS9: "The Quickening") Although an encryption sequence could be broken by a computer a person could also connect themselves directly to a terminal by means of a dataport and so try to break the encryption. (DS9: "A Simple Investigation")
Most 24th century Cardassian encryption codes used a recursive encryption algorithm. (DS9: "In Purgatory's Shadow") Although, when created on some computer systems such as the one on Deep Space 9, an encryption could only contain seven sequences. (DS9: "In the Hands of the Prophets")
Romulans used a progressive encryption lock. This lock consisted of a 43-part cypher key and was used in their encryption entry sequence. Data could not break the 29th level of such an encryption code while Spock could. (TNG: "Unification I")
24th century Starfleet codes were made up of encryption algorithms. (TNG: "Preemptive Strike") Every encryption key had an implementation date, the time and date it was to be used. One encryption key on Voyager had an stardate of 48423, one month after they had left Deep Space 9. (VOY: "Maneuvers")
An encryption circuit contained hardwired security codes. The processing of encryptions was done by encryption subprocessors. 24 isolinear chips with the number 58966/D made up the asymmetric encryption circuits of a Template:ShipClass starship. (DS9: "One Little Ship")