S-Tools (Steganography Tools) is a program written by Andy Brown. It is perhaps the most widely recognized steganography tool available today. BMP, GIF, and WAV files can be used as the cover files that conceal the secret messages. It is easy to use, with simple dragging and dropping of the files. S-Tools will hide the secret message within the cover file via random available bits. These available bits are determined through the use of a pseudorandom number generator. This nonlinear insertion makes the presence and extraction of secret messages more difficult.