Start-up adds locks to media files

But the protection method, based on a decades-old encryption algorithm known as public-key cryptography, could also raise privacy concerns. For it to work, media companies would essentially need to know the identity of users who purchased digital works to create a lock designed for a digital key specific to that user. SigningStation's plan also assumes that digital-media files would incorporate user-specific information, allowing copyright holders to track down anyone who attempted to tamper with the protection scheme.

Unlike traditional coding systems, in which a single key suffices to decode encrypted material, public-key cryptography systems require the use of a pair of keys -- typically known as the public key and the private key. Information encoded with one key can only be decoded with the other. If a user registers a public key in an open database, anyone can use it to encrypt a file that can only be unlocked with that user's key.

In SigningStation's copy-protection method, which it calls Cryptocast, a music company would encode each digital-music file for a given user with his or her public key. The user's private key would be permanently stored in a "dongle," a small piece of hardware that can attach to a personal computer's USB port. Without it, the Cryptocast software won't decode and play the music file. Closely held SigningStation, based in Burbank, Calif., will announce the system at the National Association of Broadcasters meeting Monday in Las Vegas.

Because each digital file would also include additional encoded information particular to a given download session, SigningStation says that anyone wanting to break the copy protection would have to start from scratch on each file, making widespread piracy a much more expensive proposition. Traditional encryption systems using single keys, such as the system used to encode DVD movies, are vulnerable to any attacker who can break the key once.

SigningStation says it is in discussions with Hollywood studios, but has reached no agreements yet. Even if the technology proves feasible, however, it may not benefit SigningStation itself, as the market for such copy-protection schemes has so far failed to generate much money for the companies that devise them.