steganography. means “covered or secret writing” a brief history 440 bc: tattoos on slaves heads...
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Steganography Means “covered or secret writing” A brief history
440 BC: Tattoos on slaves heads Wax covered tablets
WWII Image Substitution Null Ciphers Microdots
Today Technology taking advantage of these old techniques
Null ciphers Coding in images, audio, video and microdot
Possible Uses of Steganography Drawbacks
Combine explanatory information with an image
Could degrade or render an image useless
Embed corrective audio or image data in case of corrosion
Could counteract and be counterproductive with original image
Private Communications Doesn’t hide the fact an e-mail was sent
Posting secret communications on Web to avoid transmission
Third parties with steganography detection and cracking tools can view message
Copywrite protection Steganographic software can’t protect the watermark
Maintaining anonymity Easier to open free Web-based e-mail
What It Does
Hides information needing to be kept secret in unused, redundant parts of data
Data most often hidden in: Text/Word documents Audio files Digital images
Text
Very challenging – small amounts of repetitive data Line-shift encoding: actually shifting the line of text up
or down Value depends on whether the line was up or down
from a stationary line Word-shift encoding: uses horizontal space between
words to obtain a value for the hidden message Feature specific encoding: encoding a secret
message by changing certain attributes such as the length of letters
Audio
The MOST challenging Sample Quantization: 16-bit linear sampling
architecture used for WAV and AIFF formats Temporal Sampling Rate: uses selectable
frequencies; the higher the sample rate the more usable data will become
Perceptual Sampling: usually an MP3 file, changes statistics of audio by only encoding parts listener can perceive Perceived sound maintained but actual signal is
changed
Digital Images
Anything that can be encoded in a bit-stream can be hidden in digital image
Cover Images can be: 8-bit:
256 color choices Small file size
24-bit: 16 million color choices Large file size
Gray-scale: Shades very only slightly from byte to byte
Methods of Image Encoding Least Significant Bit Encoding
Three bits of data encoded into each pixel of 24-bit image; one bit encoded into each pixel of 8-bit image
Ex: Binary A can be hidden into 3 pixels(00100111 11101001 11001000)(00100111 11001000 11101001)(11001000 00100111 11101001)
Inserting Binary value of A (10000011) into three pixels, beginning from top left byte:
(00100111 11101000 11001000)(00100110 11001000 11101000)(11001000 00100111 11101001)
Susceptible to slight changes to image or format 8-bit images less forgiving because of color restrictions
Masking and Filtering Extends image data by putting secret data over the original Immune to manipulation **Important: Watermarking is not Steganography
Watermarks become part of image and are noticeable
Algorithms & Transformations Discrete Cosine Transformation
Used to compress JPEG images Variances will depend on the values and methods used to
calculate DCT Luminance
Uses spread spectrum techniques or redundant pattern matching
Help protect against image processing techniques Fourier Transformations Wavelet Transformations
Hide information more thoroughly than masking Support image manipulation more readily than LSB
Security
Cryptography vs. Steganography Cryptography: scrambles messages to obscure
the messages meaning Steganography: hides the message entirely
Stronger encryption methods result when Steganography and Cryptography are combined
Three Steganography protocols that utilizes concepts similar to Cryptography
Pure Steganography
Least Secure Does not require the exchange of a cipher
key Sender and receiver have to assume no third
parties know of the message
Secret Key Steganography
Secret key must be exchanged prior to communication
Only parties who know the secret key can read the secret message
Exchanging of key makes it more vulnerable
Public Key Steganography
Use of both private and public key to ensure security
Sender uses public key during encoding process
Recipient uses private key during decoding process
Offers multiple levels of security
Steganalysis
Detecting hidden messages using Steganography Majority of stego-images do not reveal visual
clues in the cover-image Possible to detect hidden image with electronic
signatures & Stego software
Signatures
Simplest signature to recognize is increase in file size
Other signatures may become apparent in some form of manipulation to the color palette Large increase or decrease in number of unique colors Palette colors increasing incrementally rather than
randomly Exception: Gray-scale images do increase incrementally Look for disproportionate number of shades of black
Statistical Analysis
Average bytes Variation of bytes Skew Kurtosis Average deviation Differential values
Destroying or Disabling
Convert the image to different format Any formatting:
Cropping Blurring Sharpening, etc.
References
Cole, Eric. Hiding in Plain Sight. Steganography and the Art of Covert Communication. Indianapolis, Indiana: Wiley Publishing, Inc. 2003.
Jajodia, Sushil & Johnson, Neil. Exploring Steganography: Seeing
the Unseen. April 12, 2005. URL: http://www.jjtc.com/pub/r2026.pdf
Radcliff, Deborah. Steganography: Hidden Data. June 10, 2002. Computer World. April 10, 2005. URL: http://www.computerworld.com/printthis/2002/0,4814,71726,00.html