Fractal encryption code

From Memory Alpha, the free Star Trek reference.

A fractal encryption code is a method by which the main computer of a Federation starship can be isolated, blocking an intruding force from taking control of all ship's functions.

Having traveled to the year 2063 from the year 2373, Lieutenant Commander Data of the USS Enterprise-E, entered a fractal encryption code into the ship's computer to prevent takeover by a contingent of Borg drones. Stored in the Android's neural net, the codes could neither be force-ably removed nor assimilated by the Borg. Nevertheless continuing their attempt to gain control of the Enterprise computer, the Borg Queen used unconventional means of persuasion to extract the codes. (Star Trek: First Contact)

A fractal encryption code may serve a similar function as the invasive program devised by Data and the crew of the USS Enterprise-D in "I, Borg" in defense against the Borg; fractals generally representing recursive geometric shapes that cannot be deciphered through traditional Euclidean geometric language.