1. Fun Stuff: Digital Media
Cryptography Lecture 3: Chantilly Academy
Poorvi Vora
Department of Computer Science
George Washington University

2. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

3. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

4. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

5. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

6. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

7. Application: Robust, imperceptible watermarking Prevention of digital media piracy
Physical media piracy
Quality deterioration with each copy
Distribution visible and expensive, legal enforcement easier
Digital media piracy
Perfect copies
Distribution unsupervised; no clear laws; no means of enforcing them, most important no easy way of locating distribution centers
Current OS and digital copies: Any object that needs a PC for viewing can be copied perfectly through file copies, and imperfectly through screen captures
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

8. Copyright protection with a robust, invisible watermark
Owner looks for the watermark - either all over the Internet, or on an individual picture that she suspects was stolen from her.
Original picture to be sold on the Internet contains an invisible mark:
If she finds it with/being sold by an unauthorized person, she could seek legal recourse.
`watermark’ - something that identifies owner or copyright owner
All existing methods can be busted by free software available on the Internet; it is not a preventive measure, it merely aids in finding the culprit after the fact;
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

9. Watermark should survive accidental attacks
Original, legal, buyer performs operations on image - sharpening, brightness/colour adjustment, filtering, cropping, resizing, rotation - while retaining perceptual quality of image.
Watermark should survive such operations - called accidental attacks - and should remain invisible; ie.e watermark should be robust
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

10. Image quality degrades if watermark removed (intentional attack)
Illegal reseller trys to destroy watermark before reselling. Watermark should damage perceptual quality of image when being removed (detection procedure does not detect the existence of a watermark) or sufficiently altered (the information contained in it is altered sufficiently - identity of owner is changed). Intentional attack.
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

11. aCd9Tof3trefgu Evariste Galois aCd9Tof3trefgu Evariste Galois
Encapsulated Content: Better Security, More Expensive – Prevents Unauthorized Use
Content encrypted during transmission and in storage on buyer’s local disk
User without decryption sees garbage
aCd9Tof3trefgu
Evariste Galois
aCd9Tof3trefgu
Content decrypted in hardware or software to allow viewing and manipulation. Pay per view possible
Evariste Galois
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

12. Crypto Lecture 3: Spring 07
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

13. Crypto Lecture 3: Spring 07
Another problem: Authentication of digital media given cheap access to capable processing systems
Was this image changed in any way after it was captured?
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07

14. Authentication with a fragile, invisible watermark
In a court of law, a judge checks the watermark hidden in the presented image to see if it is associated in the same way with each pixel of the image presented as evidence
Original picture contains an invisible mark inserted at `source’:
If she finds that the hidden watermark is not what it should be for the presented image, the presented image is not authentic and has been changed since it was obtained.
`watermark’ - something that is associated with the value of each pixel of the image
5/9/2019
Chantilly Academy
Crypto Lecture 3: Spring 07