Some text with length from 16 to 30 symbols (depending on complexity level) is written into the BMP file with 24-bit color scheme.
Archive contains image and program, which injected some text in this image. Reveal this text and send it as solution file.
At each byte of image, which encodes pixel color, some changes in low bits are almost not influence on human's reception and it may be used for text information storage (steganography).
The choice of address of the next byte for recording information is performed by a pseudo-random number generator with pre-selected coefficients to avoid overlaps.
Example of such generator:
X = (A * X + B) mod L where X - address of the next byte to write A,B - integers L - size of image in bytes
All low bits of all bytes, which contains pixel colors, are filled randomly each time, which does not allow to use a white image to detect places of storing useful information.