Security 3 Distributed Systems Lecture # 16

Overview Steganography Digital Watermarking

Steganography Taken from An Analysis of Steganographic Techniques by Popa

Steganography First evidence in 440BC Histiaeus persian revolution messages Apparently neutral � s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects for pretext embargo on by-products, ejecting suets and vegetable oils. � � Pershing sails for NY June 1 � can be retrieved

Steganography Integrity of hidden information must be preserved Stego object must not visibly change Changes in the stego object must not affect the watermark Always assume the presence of watermark is well-known

Steganography Private vs public Private: Require secret key Public: Can look at watermark, but cannot tamper it Private Type I: Require original and modified objects, ouput watermark Type II: Require Orginal, Modified and Watermark, answer yes or no

Fingerprinting vs watermarking Fingerprints are user specific Encrypted checksum + user ID Watermarks are vendor specific Independent of users Protect against illegal copies Watermarks more feasible for mass produced CDs/disks

Techniques Audio Frequency masking Temporal masking Source code a = 5 b = 7 c = 9 Move around unordered statements to to encode a detectable pattern Useful for binary code as well

Techniques Text-based techniques Line-shift coding Word-shift coding Feature coding White-space Content-based

Image-based Visible watermarking taken from Jonathan et al univ of birmingjam

Least-significant bit encoding Host Pixel: Secret Pixel: New Image Pixel:

More techniques Image: DCT Round off coefficient Sound MP3: use parity bits Video Lots of room for sneaking in additional information

Example Results of StirMark. Taken from Information Hiding - A Survey by Peticolas et al.

Mosaic Attack

More attacks Intrepretation Attacks Confuse the watermark checker Implementation Attacks Digimarc was compromised by using a debugger to crack the ID checking part