The Romulan Central Information Net was the primary database of the Romulan Empire. Access to the database was highly restricted and protected with a progressive encryption lock featuring a forty-three part cypher key.
The Klingons spent several years attempting to penetrate the net unsuccessfully prior to 2368, when Data, with help from Ambassador Spock, was able to break the encryption and gain access to the central database, and the proconsul's personal files.
The information retrieved helped foil a planned Romulan invasion of Vulcan and allowed Data and Spock, along with Captain Jean-Luc Picard, to escape from the custody of Romulan Intelligence. (TNG: "Unification II")