Omega Secret

Omega Secret (オメガシークレット) is a random number encryption program, winner of an absolute encryption contest held on Academy City's internet.

Principles
The program randomly encrypts files in such a manner that it is impossible to decrypt them afterwards, even for Omega Secret itself. It is said that even an Academy City supercomputer would take 200 years to decrypt a file encrypted by Omega Secret. What's more, every encrypted file from smallest to largest would require 200 years to be decrypted, as the random number method means that there is no decrypting pattern which could be used for all the encrypted files.

Background
Omega Secret was created as part of an absolute encryption contest held on Academy City's internet. This contest wasn't something with actual benefit and was famous for having unpleasant effects. Omega Secret won the contest and gained a reputation for being practically unbreakable.

The Third Friday of September
On the 3rd Friday of September, Misaka Mikoto, while using her powers to hack into Academy City's Bank for an unknown reason, quickly found the data inside becoming encrypted at such a speed that even she could not keep up using her powers, forcing her to withdraw from the system. She deduced from the random manner of encryption that it was the work of Omega Secret.