Introduction to Steganography

1. Introduction to Steganography “Writing between the lines”

2. Steganography • Literally “hidden writing” • Useful for • Secret messages • Watermarking

3. Requirements • Message • Wrapper • Image file • Sound file • Other files with unused bits • Encoding/Decoding scheme

4. Message • A bit string • E.g., 0001 1010 1101 0111

5. Encoding in an Image • Assume a large number of color combinations • Agree upon a repeatable random sequence of pixel locations • While (more bits to encode) • Select the next pixel location • Get the next bit from the message • Set the low-order bit of a color component of the sample at the location to the current bit value of the message

6. Example • Message = 0001 1010 1101 0111 • Image bit stream = …. 01101110 …. • New bit stream = …. 01101111 …. • Color intensity change = 1/255 of its range! Bit affected Bit to be inserted

7. Decoding from an Image • While (there are more bits in the message) • Generate the next pixel location to examine • Extract the bit from the low-order position of the color component • Append the bit to the message bit stream, subdividing when appropriate boundaries occur • Boundaries may not be byte-sized • Messages can be images, sounds, text, programs, or data!

8. Example • Message = 0001 1010 1101 0111 • New image bit stream = …. 01101111 …. To be inserted here Bit to use in message found in image by repeating the random sequence

9. Notes • The wrapper must be larger than the message • There are various encoding schemes to abbreviate the message • The message can (and probably will) be destroyed if a “lossy” compression technique (such as JPEG) is used • To implement in Java the basic bit operations (e.g., &, |, ^,>>,<<) may be useful as well as the Random class (for producing repeatable sequences)