FTL hides files within other files, using some kind of steganography, or perhaps it'd be called a watermarking process. Whatever, it changes some of the file's bytes where such changes might ideally be invisible (unless the picture has very large patterns of bytes all set to the same value, as in a picture with large areas filled with a plain color and no details, noise or dithering). The process gives the best results with images saved in 8-bit grayscale, 256 colors, or 24-bit truecolor. It is also possible to hide stuff in audio records (.wav, .au) recorded in 8 bits per sample. The hidden stuff could be a simple watermark for a copyright notice, or something longer. Yeah, because FTL acts on uncompressed files, and in such files there is often a lot of space to hide binary data. So it could equally hide other small pictures, or even small executable files, within a single "mask file". Or even, hiding a smaller picture in a larger picture, where in the smaller picture there's ANOTHER hidden content, such as a short message. The example picture above, once saved as a bitmap (and NOT an LZ-compressed GIF file), could be examined throught FTL. If you give the most idiot 10-digit access code, 1234567890, it will reveal the hidden message. One final observation about FTL: its encoding mechanism is probably invulnerable, there shouldn't absolutely be a way to extract the hidden file without knowing the access code. Access codes are 10-digit numbers, so it'd take a lot of time to brute-force them. Especially if you consider hiding pictures in pictures, using 2 different 10-digit codes, giving 100 billions of billion possible combinations.