Steganography

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Steganography Exam Review10/21/08
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• What is Steganography?
• Information Hiding
• Specifically, hiding a message in some medium
  such that no one (except the sender and receiver)
  is even aware there is a hidden message
• Steganography literally means covered writing
• A stegosaurus has a covered back

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• What to hide messages in?
• Most commonly, messages are hidden in images

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• What to hide messages in?
• Most commonly, messages are hidden in images

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• Steganography is not new
• Examples of pre-computer steganography
• A Greek shaved the head of a slave, wrote a
  message, then waited for the hair to grow back
  before sending the slave to his destination
• Prior to the Civil War, quilts were sewn with
  special patterns to tell escaping slaves which
  direction to go and what to do
• During WWI there was a cable the read, Father is
  dead. Suspecting a hidden meaning, the censor
  changed it to Father is deceased which caused
  the reply, Is Father dead or deceased?
• During WWII chess by mail was banned, crossword
  puzzles examined, stamps were removed and
  replaced by ones of equal value

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• Hiding data in an image - LSB
• We have a picture, and we want to replace some of
  the data in the picture with parts of our hidden
  message
• The most common method is to replace the Least
  Significant Bit (LSB)
• Each pixel in the mandrill image is composed of 8
  bits and so have a range of values from zero
  (black) to 255 (white)
• SEE NOTES ON BOARD FOR DETAILS

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• Distortion of original image (cover file)
• The Sports Guy was hidden by replacing the 4
  least significant bits of the mandrill pic
• You might be tempted to think, Thats half the
  information!
• No, it is 1/16 the information not noticeable
  to us
• The 4 bits are replaced with the 4 Most
  Significant Bits of the Sports Guy pic

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• Distortion of hidden image (target file)
• The Sports Guy was hidden by replacing the 4
  least significant bits of the mandrill pic
• To reconstruct, use these least 4 bits as the
  MOST SIGNIFICANT bits (still some loss of
  information)

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• Choose your cover image wisely
• Other images with more solid backgrounds would
  NOT provide the same level of imperceptibility
• Using solid white or black image as a cover is
  not a good idea (obv)

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• How many bits to use?
• Here is a progression of hiding in the mandrill
  image

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• You can also hide text in the LSB of an image
• ASCII text is converted from a binary
  representation. This binary representation can
  be hidden in the Least Significant Bits

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• Exam on 10/28 NO MAKEUPS
• Review sheet is here http//docs.google.com/Doc?i
  ddftfgv5c_132cpxwwsdw