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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.