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Digital optical disks are interesting as a storage medium because they combine large inexpensive storage capacity with relatively fast access. They also have a property which appears to make them attractive to the security minded, namely they do not permit bits to be rewritten. In other words, a 0 can be changed to a 1, but a 1 cannot be changed back to a 0. In a recent paper, however, Rivest and Shamir show that this does not prevent the changing of information; in fact, at relatively small additional cost (slightly longer codes), information can be changed as often as is desired. In the present paper we are concerned with issues in the integrity of data stored on digital optical disks or any other write-once storage medium, for that matter. A number of practically useful schemes are presented. In particular, immutable codes are discussed, both fixed length and variable length. These are codes which guarantee that information may not be changed. Also introduced are alternative schemes, such as control key schemes, which guarantee either unconditionally or with a certain probability that information cannot be changed if it is stored on digital optical disks. The main distinguishing feature of these schemes is the price to be paid for implementing them, i. e., the number of additional bits required for obtaining the desired protection against subversion.