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Digital Watermarking

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Blandon Helgason

on 7 July 2010

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Transcript of Digital Watermarking

Digital Watermarking Blandon Helgason
CSCI 6621
Spring 2010 What is it? Anti-Counterfeit technique? Anti-Piracy? Content Authentication Copyright Protection Covert Communications Source Tracking Broadcast Monitoring Applications Proof of Ownership Digital Watermarking the process of embedding information into
a digital signal in a way that is difficult to remove.
The signal may be audio, pictures or video.
Digital watermarking... (′dij·d·l ′wöd·r′märk) Visible Invisible Logos in images or video.
i.e. TV broadcasting. Hidden in images, audio, or video. Idea: Alice wants to protect her photographs. Problem: Can Bob undermine her? Idea: Tamper with the work, tamper with the watermark too. Solution: Asymmetric Crypto with digital signatures. Who touched it last? Hmm..something fishy here... Digital Rights Management (DRM) Secure Digital Music Intiative (SDMI) Steganography Information Hiding Pirate stations Passive versus Active Computer system monitors content, then compares it to database of known works. Passive Active Benefits: Direct, non-intrusive, requires no cooperation from advertisers or broadcasters. Problems: non-trivial, only 95% accurate. Computer-recognizable content transmitted with content. Send through separate signal in VBI/file headers. Problems: VBI/file header transmission is unreliable. Benefits: Simple implementation, no database required. as a Security Mechanism What makes it unique? It sounds like stegonagraphy... ..or just data hiding. Characteristics Watermarks are... Should not be detract from aesthetic of the image ...Imperceptible ...Inseparable Cannot be removed from the work. Should survive conversion. ... and undergo the same transformation as the works. 1. 2. 3. Properties Embedding Effectiveness Fidelity Data Payload Security Cryptography Embedding Effectiveness Probability that the output will be watermarked. Goal: 100% At what cost? Fidelity Similarity between original and watermarked work. Does it effect the quality? Data Payload Number of bits encoded in the work. Audio: Bits per second Video: Bits per frame/second Security Resistance to "hostile" attacks Unauthorized Embedding Unauthorized Removal Unauthorized Detection Cryptography Active Passive Active Attacks that actually modify the cover work Passive Do not modify the work Unauthorized Removal A closer look at attacks... Unauthorized
Goal: Prevent works from being detected Types of attacks: Elimination Attack Watermark is impossible to detect, even with a sophisticated detector Masking Attack Watermark still exists, although it cannot be detected Collusion Attack Obtain several copies of works with different watermarks Combine them to produce a copy with a watermark Embedding Removal Unauthorized Goal: Forgery Embedding illegitimate watermarks into works that should not contain them Unauthorized Usually only a concern with stegonagraphy Detectection Three severity levels HIGH: MED: LOW: ...detect watermarks, distinguish one from another, but cannot decipher. The attacker can... ...determine that a watermark is present, but cannot distinguish or decipher. ...detect and decipher the embedded message. Robustness Watermarking and cryptography. Cryptography revisited.... Knowledge of encryption/decryption algorithm should NOT compromise the system Security is based on keys Keys should be chosen wisely, from a large keyspace ...more on watermarking cryptography Consider it to be an added layer of security. Beneficial to passive attacks only. Notation: C w = Emb (C , a , m , k ) o E The underlying idea... Without knowledge of the key, watermarks should be: undetectable difficult/impossible to remove without degrading the work does it contribute? Robustness How fragile is it? Does it withstand conversions? Robust watermarking should ensure that attackers cannot emit or encode hidden bits. Thank you for your time.

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