Why copyright?
- Incentive to create stuff (US) and to promote cultural progress
- Authors can sell their copyrights
- Benefit to author weighed against other public goods.
Law grants exclusive rights to the author
- Copy and distribute
- Publicly perform
- Make a derivative work (translation, film based on novel)
- Covers expression, but no ideas
- Subject to “fair use” exceptions—parody, brief quotations. Factors: purpose and character of the use, nature of work, amount of material used relative to the whole, effect on market for original work.
First sale doctrine: allows resale, lending, etc.
These rules are for physical copies: Digital copies introduce new problems.
Transition to digital technology threatens balance.
- From buyer: ease of copying/redistribution
- From copyright owner: technologies of control, licensing
“Copy protection/prevention”/DRM.
Some general correspondence between legal rights and what technology enforces.
Common factors
- threat model assumes that file sharing is widely used.
- “Break once, infringe anywhere”—once you can crack something, it can be copied or distributed everywhere. If someone can break your copy protection technology, anybody anywhere can infringe on copyright.
I.e. copy-protected CD’s. Lame approach, because player manufacturers will try to fix their players so that consumers who buy the drives can actually play their CD’s on the drives.
Distribute data in encrypted form.
Allow only authorized devices to learn the key.
- Pro: even if people get data, they can’t decrypt them.
- Device decrypts before playing content.
- Typical: each player device has own private key.
- Work is encrypted for a single player OR encrypted under session key, which is encrypted for a single player.
- Problem: no end-to-end protection. Content is vulnerable after decryption.
- Analog hole
- Problem: if someone can get the key, someone can build their own device to decrypt the data. Need tamper-resistant technology.
- Problem: installed base. People don’t buy new audio/video equipment very often.
Every time you sell, you put a serial number that uniquely identifies the copy.
Track who bought each copy so you know who to blame.
- Problem: Mark must be indelible (see later in lecture)
- Problem: practical details aren’t plausible.
- Hard to identify people
- Rely on store record clerks to record consumer names and ID’s accurately.
- What do you do when there are a million copies?
- Can’t be sure that if a person is accused, they are really guilty—what if stuff is stolen?
Mark attached to work, carries instructions about what to allow
- Player devices must protect the mark and obey it.
- Variants: Simple mark (bits in header) “broadcast flag”
- No attempt to prevent the removal of this flag.
- Watermark
- Subtle mark that’s in the content.
- Requirements: imperceptible, indelible (can’t remove w/out trashing the content), detectable by its plague
- Perceptual issues force per-medium solution
- Watermarking—end to end solution, i.e. if camcorder says “nope, I’m not recording THAT” to movie in the theater
- Theoretically dubious
- Bad track record
