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Steganography. Ben Lee III Long Truong ECE 478 - Spring 2002. Definition. Steganography: (from Greek steganos, or "covered," and graphie, or "writing") is the hiding of a secret message within an ordinary message and the extraction of it at its destination

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Ben Lee III

Long Truong

ECE 478 - Spring 2002

  • Steganography: (from Greek steganos, or "covered," and graphie, or "writing") is the hiding of a secret message within an ordinary message and the extraction of it at its destination
  • Takes cryptography a step farther by hiding an encrypted message so that no one suspects it exists
  • Ideally, anyone scanning your data will fail to know it contains encrypted data
  • Computer Steganography: two principles
    • Digitized images or sound can be altered without losing their functionality
    • Human inability to distinguish minor changes in image color or sound quality
  • In modern digital steganography, data is first encrypted by the usual means and then inserted, using a special algorithm, into redundant data that is part of a particular file format
  • Changing the value of the least significant bit of the pixel color won’t result in any perceivable change of that color
  • Ancient Greece, text was written on wax covered tablets
  • During WWII, the Germans developed the microdot for the letter i
  • Invisible inks: some messages had to be "developed“ by using special chemicals
  • Unencrypted messages: the real message is "camouflaged" in an innocent sounding message
algorithm pure
Algorithm - Pure
  • Pure steganography
    • No key is necessary
    • E: Ca x M -> Ca
    • D: Ca -> M
    • Relies on the secrecy of the method
  • Example
    • Using the first character in a letter to hide the secret message
example of pure steganography
Example of Pure Steganography
  • Null cipher (unencrypted message)
    • News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The highways are knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday.
  • First letter of each word:
    • Newt is upset because he thinks he is President.
algorithm secret key
Algorithm – Secret Key
  • Secret key steganography
    • Uses a secret stego-key: sender and receiver share stego-key
    • E: Ca x (K x M) -> Ca
    • D: Ca x K -> M
  • Example
    • Encrypt data using shared secret key before hiding within the carrier
example secret key steganography
Example Secret Key Steganography
  • The left image (8.9K) contains no hidden data
  • The right one (11.2K) contains about 5K of password-protected text
algorithm public key
Algorithm – Public Key
  • Public key steganography
    • Uses two keys: public/private key combo
      • Public key encryption
        • E: Ca x (U x M) -> Ca
        • D: Ca x R -> M
      • Private key encryption
        • E: Ca x (R x M) -> Ca
        • D: Ca x U -> M
example programs
Example Programs
  • Various programs offered today
    • Freeware/Shareware include
      • Hide and Seek: hides data in LSB of GIF & BMP
      • Camouflage: hides data (ex: MP3’s) by encrypting and then appending the data to the end of a file
      • S-Tools: LSB color reduction, then inserts additional data to hide color modification
    • Commercial
      • PixelTag: uses pattern block encoding
      • Steganos: multiple products for data hiding
flaws with steganography
Flaws with Steganography
  • Pictures
    • Color degradation
    • Significant file size change
  • Audio
    • Distorted sounds
    • Significant file size change
  • Manipulation of carrier may result in degraded or inability to extract the message
  • Difficulty in hiding large message inside small carriers
  • Visual Attacks
    • LSB
    • Filtering image
  • Digital Manipulation
    • Color palette
    • Modification of carrier
  • Statistical Analysis
    • Mathematical manipulation of possible carrier to determine if a hidden message exists
visual attack lsb
Visual Attack - LSB

Original Image

LSB of original image

visual attack filter
Visual Attack - Filter

Applying a filter to the original image’s LSB shows that there is a hidden message

countermeasure message spreading
Countermeasure – Message Spreading
  • Spreading the message out evenly throughout the carrier conceals the hidden message
  • Filtering of image does not reveal hidden message
countermeasure large carrier
Countermeasure – Large Carrier
  • The use of larger carriers helps to hide the message better

8-bit palette 8-bit steganographic image

countermeasure patterns
Countermeasure - Patterns
  • The use of unrecognizable patterns for the carrier prevents modification from being noticed
  • Extraction of LSB does not reveal existence of a hidden message (no hidden message in this image)
application of steganography
Application of Steganography
  • Enables secret communication
  • Complements regular encryption
    • How will someone decrypt your message if they cannot find it?
    • Harder to break: need to first find the cipher text, then it needs to be decrypted