> Steganography

Cryptography hides what a message says. Steganography hides that a message exists at all - an encrypted file announces "there's a secret here," a steganographic one doesn't. The word is Greek for "covered writing," and the technique predates computers by millennia: Herodotus records a Greek noble tattooing a message onto a slave's shaved scalp, then waiting for the hair to grow back over it before sending him through enemy territory. The digital version this site's Steganography tool uses - LSB (least-significant-bit) encoding - swaps the lowest, least visually significant bit of each color channel in an image for a bit of the hidden message, nudging every affected pixel's color by an amount too small to see. It's also fragile on purpose: anything that recompresses the image (a JPEG re-save, an upload to a site that reprocesses photos) scrambles those low bits and destroys the message, which is why the tool only ever outputs PNG.