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Steganography. Leo Lee CS 265, Section 2 Dr. Stamp April 5, 2004. Outline. What is Steganography? Historical Examples LSB Embedding Concept Implementation Analysis. What is Steganography?. Literally means “covered writing” Goals: Hide a secret message within some other object

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steganography

Steganography

Leo Lee

CS 265, Section 2

Dr. Stamp

April 5, 2004

outline
Outline
  • What is Steganography?
  • Historical Examples
  • LSB Embedding
    • Concept
    • Implementation
    • Analysis
what is steganography
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
historical examples
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)
shaved head steg
Shaved head Steg

mm…

Beer…

null cipher
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
lsb embedding terminology
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
lsb concept
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!
lsb 24 bit bitmaps
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
lsb 24 bit bitmaps example
LSB 24-bit Bitmaps Example
  • Message = ‘f’ = 0110 01102
  • Cover Image:

FF FF FF 00 00 00 FF FF …

  • Stego-image:

FE FF FF 00 00 01 FF FE …

my own implementation
My Own Implementation

Which is the stego-image and which the cover?

Cover Image

Stego-Image

8 bit
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
approach 1 ezstego
Approach 1 - EzStego
  • Use cover image with similar colors
  • Experts recommend gray-scale images
  • Arrange palette so adjacent colors are similar
approach 2 s tools
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
lsb analysis the good
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
lsb analysis the bad
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
lsb analysis the ugly
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
lsb analysis conclusion
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.
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