1. Steganography
Leo Lee
CS 265, Section 2
Dr. Stamp
April 5, 2004

2. Outline What is Steganography? Historical Examples LSB Embedding
Concept
Implementation
Analysis

3. What is Steganography? Literally means “covered writing” Goals:
Hide a secret message within some other object
Do so in such a way that the presence of the message is not discernable

4. Historical Examples Invisible Ink used in WWII
Microdot: A page of information, the size of a printed period.
Waxed Tablets
A person’s head!
Null-ciphers (unencrypted messages)

5. Shaved head Steg
mm…
Beer…

6. Null cipher Message sent by German spy in WWII
Apparently neutral’s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by-products, ejecting suets and vegetable oils.
Apparently neutral’s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by-products, ejecting suets and vegetable oils.
Pershing sails from NY June 1
Pershing was an American general

7. LSB Embedding - Terminology
Message = the secret information we want to hide
Cover image = image used to hide the message in
Stego-image = the cover image with the message embedded

8. LSB - Concept Which color is different?
In (R,G,B) left and right are (0,255,0)
Center one is (0,254,0)
We can use the LSB to hold info, since it looks the same either way!

9. LSB 24-bit Bitmaps
In 24-bit bmps, each pixel represented by 3 bytes (RGB)
Use lsb of each byte to hold a bit of message

10. LSB 24-bit Bitmaps Example
Message = ‘f’ =
Cover Image:
FF FF FF FF FF …
Stego-image:
FE FF FF FF FE …

11. My Own Implementation Which is the stego-image and which the cover?
Cover Image
Stego-Image

12. The Message

13. 8-bit Don’t hold direct color values Do hold offsets into a palette
Can’t just change lsb, because adjacent colors in palette may not be similar

14. Approach 1 - EzStego Use cover image with similar colors
Experts recommend gray-scale images
Arrange palette so adjacent colors are similar

15. Approach 2 – S-Tools Use only x bits for unique color information.
8 – x bits are for secret message
Example (x = 7):
Can only have 128 unique colors
For each unique color, there’re two similar colors xxxx xxx0 & xxxx xxx1

16. LSB – Analysis – The Good
Simple to implement
Allows for large payload
Max. payload = b * p where;
b = number of bytes per pixel
p = number of pixels of cover image

17. LSB – Analysis – The Bad
Easy for attacker to figure out message if he knows the message is there
But the images look the same, so can’t tell it’s a stego-image… right?
Human vision can’t tell but vulnerable to statistical analysis

18. LSB – Analysis – The Ugly
It’s even easier if the attacker just wants to corrupt the message.
Just randomize the lsbs himself
Even vulnerable to unintentional corruption: image cropping, conversion to jpeg and back, etc.
Integrity is extremely frail

19. LSB – Analysis - Conclusion
Good for cases where only low security is desired, but not necessary.
Added security when coupled with cryptography
Foundation for many variations, which are more secure e.g. not vulnerable to statistical analysis attacks.