Hide a message inside a picture, random colours or a photo of your own, by nudging each colour by one step in 256, far below what an eye can see. Save it, send it to a friend, and read it straight back out.
Every pixel is three numbers from 0 to 255: how much red, green and blue. The last binary digit of each is worth 1 out of 255, so changing it moves the colour by one step, an amount nobody can see. Write a message in binary and tuck one bit into the last digit of each colour, and the picture carries it. To read it back, collect those last digits in order and cut them into groups of eight. This is least-significant-bit steganography, and real tools use it on photographs, where millions of pixels can hide a whole file.
Ways to hide it. The message can go in as plain text; scrambled, mixed with a stream of bytes made from a password; or encrypted with AES-256, the cipher banks use. Both locks add a random salt, so the same message and password never hide the same bits twice. Then choose where the bits go: the last one or two binary digits, all three colours or blue alone (eyes notice blue least), in reading order or scattered across the picture by the password, and overwriting each digit or nudging the colour up or down, which hides the fingerprint a well-known detection test (the chi-square attack) looks for. The reader tries every combination by itself; a locked message just needs its password. Everything happens in your browser: no picture, message or password leaves your computer.
A challenge: the green picture on this toy's card is 56 × 24 real pixels, and it's smuggling a message. Download it and open it under Read a picture, or write your own program to dig it out: the bits run red, green, blue, pixel by pixel along the rows from the top left, eight to a letter, first bit first, ending with a zero byte.
Steganography means "covered writing". The word comes from Steganographia, written in 1499 by the German abbot Johannes Trithemius: a book that seemed to be about summoning spirits, and turned out to be a hidden lesson in secret writing. Hiding messages is far older: the Greek historian Herodotus tells of a message tattooed on a messenger's shaved head, then hidden by his hair growing back.
