Steganography Leo Lee CS 265, Section 2 Dr. Stamp April 5, 2004.
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Transcript of Steganography Leo Lee CS 265, Section 2 Dr. Stamp April 5, 2004.
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Steganography
Leo LeeCS 265, Section 2
Dr. StampApril 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 person’s 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 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 1Pershing 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
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
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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, there’re 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 can’t tell it’s a stego-image… right?
Human vision can’t tell but vulnerable to statistical analysis
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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
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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.