How this really works

The encoding algorithm creates a new image that changes the bit with the lowest exponent (2^0) according to the bit value of the text you are trying to encode. For example, if you want to encode the text "Hello, World" into an image using this algorithm, then the program will follow these steps:

• Try to open the image
• Convert your text to a string of "ones" and "zeroes" and add a delimiter of 64 "ones" to it.
  The delimiter is used so the decoding algorithm will know when to stop reading characters from the encoded image.
• Change the bit value of each pixel to the bit of the text. To encode "Hello, World" first you need to convert it to bits. The bit value with the delimiter is: "0100100001100101011011000110110001101111001011000010000001010111011011110111001001101100011001001111111111111111111111111111111111111111111111111111111111111111"
• The algorithm reads each letter of the message and then appends it's bit value to a string.
• Then it changes the bit value of the color of the pixels needed. For example if the first pixel of the first row is [235, 245, 100] then it converts each of the colors to bits.
  • 235 is "11101011". The last bit (the bit with the lowest power) is "1", but the first bit of our message is "0". We change it to a 0. So 235 ("11101011") becomes 234 ("11101010").
  • 245 is "11110101". The last bit is a "1" and the bit we need to write is a "1", so we don't need to change it.
  • 100 is "01100100". The last bit is a "0" and the bit we need to write is also a "0", so we don't need to change it.

This happens to all the bits of our message until the message is over. After the message is done, the algorithm just writes the normal values to the image, so nothing is changed.

The bits needed to write can't be bigger than the bytes of the image. Because we can only change 1 bit of every byte, we can only write as many bits as the image has bytes.

An image with 200 bytes can hold 200 bits of text but the algorithm appends a delimiter of 64 bits, leaving you with 136 bits to write.

This is a PHP implementation of the Python Steganography script at https://github.com/atbuy/steganography.