08-02-2013, 11:18 AM
Steganography
1Steganography.ppt (Size: 2.35 MB / Downloads: 223)
Introduction
Clandestine Communication
Cryptography
Scrambles the message into cipher
Steganography
Hides the message in unexpected places
Digital Applications of Steganography
Can be hidden in digital data
MS Word (doc)
Web pages (htm)
Executables (exe)
Sound files (mp3, wav, cda)
Video files (mpeg, avi)
Digital images (bmp, gif, jpg)
Background
Compressed Images
Grayscale jpeg images (jpg)
Joint Photographic Experts Group (jpeg)
Converts image to YCbCr colorspace
Divides into 8x8 blocks
Uses Discrete Cosine Transform (DCT)
Obtain frequency coefficients
Scaled by quantization to remove some frequencies
High quality setting will not be noticed
Huffman Coding
Affects the images statistical properties
Finding and Extracting Messages from Bitmaps
Observation
Only considered linear unencrypted messages
Trial performed on 100 grayscale bitmaps
97 clean
3 stego
Took an average of 9 seconds per image to find with 100% accuracy (no training -- cold)
Occasionally some garbage text at head or tail
Took an average of 3 seconds per image to test with 100% accuracy
Clean images had pattern scores of less than 10
Stego images had pattern scores of 31 or more
Detecting Messages in jpegs
Conclusion
Messages of sufficient size can be detected in stego images with great accuracy
Improved accuracy may be due to a large training set
1000 (800/200)
500 (400/100)
Restricted domain
Many similar images