Performance evaluation of immutable codes

Commercially available only recently, the optical disk drive uses a sharply focused laser beam to burn impressions onto a disk. It yields storage densities up to 10 times those of magnetic disks. The low cost and high storage capacity of the laser disk make it an attractive storage medium. Laser disks do not permit rewriting of bits, namely a 0 can be changed to a 1, but a 1 cannot be changed to a 0. However, recently Rivest and Shamir have shown that this does not inhibit rewriting of information; in fact at a relatively small additional cost, information can be updated as often as desired. For the security minded then, it becomes imperative to ascertain that information of permanent nature, stored on laser disks, cannot be updated illegally. Information stored on laser disks using conventional codes is not immune to illegal updates. A class of codes known as Immutable Codes, offers one means of guaranteeing the integrity of information stored on laser disks. Fixed-length immutable codes and several kinds of variable-length immutable codes are discussed here, as well as their relative performance for a simulated pseudorandom discrete source.