University of Virginia, Department of Computer Science
CS588: Cryptology - Principles and Applications, Fall 2001

Manifest: Wednesday 7 November 2001

Assignments Due


Next Wednesday's lecture (14 November) will be a guest lecture by Professor Daniel Ortiz from the UVA Law School. The attached reading is the decision from the Napster copyright case. Read the first 9 pages closely, and skim the rest and use it for reference.

While you read it, consider the following scenario:

After the decision, Napster decides to disband and form two independent companies, Retspan and Padster. Each vigilantly enforces copyright laws and refuses to allow any pirated recordings on its network. Instead of distributing normal MP3 files, they both distribute a recording in two pieces such that each piece contains (apparently) random bits and the XOR of the pieces produces the original MP3 file.

Alice's band has just finished their latest recording, "Alice and the Cryptones" and wants to distribute it using Retspan. She selects a random L and calculates M such that the original MP3 file, R = L XOR M.

Bob has finished his latest solo album, "Unencrypt My Heart" and wants to distribute it using Padster. Since he had to give his computer to Cathy to cover a poker debt, he sends her the MP3 file, Q, and asks her to calculate the shares for him.

Cathy downloads Alice's L and M from Retspan. She has a pirated MP3 file, P for The Slim Shady LP. Cathy selects NM such that P = M XOR NM. Then, she calculates T such that Q = NM XOR T and sends NM and T to Bob.

Bob checks that NM XOR T is his original recording, and distributes NM and T on Padster.

Cathy sends email to her friends telling them to download Alice and Bob's latest recordings from Retspan and Padster respectively. Cathy's friends download Alice's recording from Retspan (L and M) and Bob's from Padster (NM and T. Knowing that Alice and Bob are better cryptographers than musicians, they XOR pieces from each recording, M and NM to get the pirated recording.

Are Retspan and Padster breaking the law? Who's the copyright infringer?



CS 655 University of Virginia
Department of Computer Science
CS 588: Cryptology - Principles and Applications
David Evans