ISAAC is a cryptographically secure pseudo-random number generator (CSPRNG) and stream cipher. It was developed by Bob Jenkins from 1993-1996 and placed in the Public Domain. ISAAC is fast - especially when optimised - and portable to most architectures in nearly all programming and scripting languages.
After more than 20 years of existence ISAAC has not been broken, and the little program you have here allows you to use it to encrypt and decrypt text of arbitrary length, files of any type...