image steganography

1. Image steganography
EE7150 – Digital Image Processing – Fall 2015
Savitha Bhallamudi

2. Information Hiding -Introduction
 Purpose:
 Hiding sensitive information vital for security purposes
 Securing private files and documents
 Hiding information - 3methods - Cryptography, Steganography, Watermarking.
 Cryptography - Data to be hidden is coded
 several techniques ; sophisticated
 visually obvious to be encrypted; appears non-sensical
 Watermarking
 Applies a copyright label ; prevents illegal use of content
 Improper removal causes degradation of original information
 Steganography -hiding information by using another information as a cover.
 Elements:
 cover component, message component, stego component.

3. Steganography -Introduction
 Types:
 Text steganography
 Image steganography
 Video steganography
 Audio steganography
 Image steganography is widely used.
 Images contain redundant information.
 Easy to manipulate few pixels without affecting visual data observably
 Slight changes in RGB values undetected by human eye (HVS) - Psychovisual
redundancy.
 Types: Text in image ; image in image

4. Image steganography
 Initially used in images sent through emails.
 Presence detected by huge size of the stego-images.
 Various techniques available:
 LSB substitution method
 Blocking method
 Palette modification method
 LSB technique can be used for BMP(bitmap) images.
 Blocking method – DCT and DWT – used for JPEG images
 Palette based method – used for GIF images.

5. Methods:
 LSB substitution method:
 Deals with substituting LSB bits of corresponding pixels in image to store the text
message or another image as a message.
 Block method:
 blocks of the image are considered and DCT (discrete cosine transform) is done in
order to break them.
 Each block is then subdivided into 64 parts (DCT coefficients).
 These coefficients are modified i.e. the color gets modified a little by storing some
text or another image in it.
 Palette modification:
 Every image uses a set of color values.
 Palette Modification replaces the unused colors within an image’s color palette
with colors that represent the hidden message.

6. LSB substitution method -Algorithm:
 In the current project, emphasis is given to this
method.
 Simplicity
 Mostly Imperceivable
 One of the channels among RGB can be chosen, and LSB of its pixels altered
or all the channels could be used.
 Example:
(a) binary form of message to be hidden
(b) few pixel values on cover image
(c) LSB altered

7. Algorithm:
1. Input the message to be hidden. (either image
or text)
2. Check the maximum number of bits that can be
accepted by the cover image
3. Replace the LSB bits of the pixels in cover
image with the binary representation of the
message. => we obtain Stego image
 Either color image (RGB) or grayscale image can
be used as a cover image.
 In case of color image, 24bit image i.e. 8bits are
present for each for RGB so more number
information pixels can be stored.
 Modification can be done to either just the last 1
bit or 2,4 or more bits depending on
requirement.

8. In the above example, image1 represents cover image and image2 is to be
hidden. As stated earlier, using too many or too less bits causes the cover image
to be distorted too much, making the steganography obvious.

9.  Generally ideal to replace only about half of the pixels with the
message in order to keep the stego image visually good.
4. To retrieve the original image,
 Number of bits used to store the secret image should be known.
 The host image is scanned, to pick out the least significant bits
according the number used
 These are then used to create a new image such that the LSB
bits extracted now become the most significant bits in the
retrieved image.

10. References:
1. Johnson, Neil F. "Steganography."
Http://www.jjtc.com/pub/tr_95_11_nfj/sec101.html. N.p., Nov. 1995. Web.
2. Shikha, and Vidhu Kiran Dutt. "International Journal of Advanced Research in
Computer Science and Software Engineering." Http://www.ijarcsse.com/.
N.p., Sept. 2014. Web.
3. Niels Provos, and Peter Honeyman. “Hide and Seek: An Introduction to
Steganography.”
4. IEEE Security & Privacy Magazine, May-June 2013. Web.
5. Nick Nabavian. “Image steganography” Nov. 28, 2007.
http://www1.chapman.edu/~nabav100/ImgStegano/download/ImageStegano
graphy.pdf
6. Matlab help.

11. THANK YOU