Steganography

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

• What is Steganography?
• Historical Examples
• LSB Embedding
• Concept
• Implementation
• Analysis

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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

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Historical Examples

• Invisible Ink used in WWII
• Microdot A page of information, the size of a
  printed period.
• Waxed Tablets
• A persons head!
• Null-ciphers (unencrypted messages)

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Shaved head Steg
mm Beer
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Null cipher

• Message sent by German spy in WWII
• Apparently neutrals protest is thoroughly
  discounted and ignored. Isman hard hit.
  Blockade issue affects pretext for embargo on
  by-products, ejecting suets and vegetable oils.

• Apparently neutrals 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

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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

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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!

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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

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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

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My Own Implementation
Which is the stego-image and which the cover?
Cover Image
Stego-Image
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The Message
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8-bit

• Dont hold direct color values
• Do hold offsets into a palette
• Cant just change lsb, because adjacent colors in
  palette may not be similar

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Approach 1 - EzStego

• Use cover image with similar colors
• Experts recommend gray-scale images
• Arrange palette so adjacent colors are similar

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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, therere two similar
  colors xxxx xxx0 xxxx xxx1

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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

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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 cant tell its
  a stego-image right?
• Human vision cant tell but vulnerable to
  statistical analysis

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LSB Analysis The Ugly

• Its 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

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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.