Steganography

Leo LeeCS 265, Section 2

Dr. StampApril 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

Do so in such a way that the presence of the message is not discernable

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

mm…Beer…

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

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 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 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

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

Which is the stego-image and which the cover?

Cover Image Stego-Image

The Message

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

Use cover image with similar colors

Experts recommend gray-scale images

Arrange palette so adjacent colors are similar

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 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 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 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 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.