File Encryption

Putting Marks vs. Dynamic Binding

The introduction on purpose of unique marks to the storage medium is a simple task in itself, and is widely used. Those whose financial resources are limited simply scratch the disc with a sharp needle. Those with access to more cash can afford to disfigure it with a laser. More sophisticated systems of protection use pits of non-standard shapes , or carry out complicated manipulations of the density and/or pattern of the spiral track. None of these methods , however, are without their own drawbacks. First, they require specialized equipment. Second, unique characteristics of the storage medium are considered as such because they can t just be reproduced on demand. On the contrary, they are formed in process of medium creation. This means that, at present, when the disc is removed from the burner , the protection mechanism doesn t know the medium characteristics, to which it is bound. When the burning process has been completed, it s already too late to report these characteristics to the protection, since the disc is already burnt and CD-ROM drives do not support the writing of additional information. In theory, encoded marks can be placed into the diskette supplied along with the protected CD-ROM. Users, however, are unlikely to be enthusiastic about this approach. Finally, these unique marks that are placed on the master copy of the CD are useless against the CD fabrication plants that prefer to steal part of the printed lot.

Therefore, it is much better not to introduce any marks to the disc. A much better approach is to use those that already exist and try to detect their uniqueness on the fly. Is this possible? Actually, it s easy! The protection mechanism measures the disc characteristic that is most vulnerable to the maximum scatter from one medium instance to another (as a rule, this is the read timing characteristic). Then, according to a specially designed algorithm, this characteristic is converted into a certain code, which is reported to the disc owner. The owner passes this code to the program developer and receives (naturally, at a price) the registration number derived from this code (for the sake of simplicity, let s assume that the registration code is equal to the disc characteristic code multiplied by 0x666 ). After the user enters the registration number, the protection mechanism carries out an inverse operation over this code and compares the result to the code of the disc characteristic (as a variant, the protection can compute the appropriate registration code for the code of the disc characteristic on its own, and then compare it to the registration number entered by the user). If the results match, then everything is OK. If not, the program refuses to operate ^[i] .

The first advantage of this mechanism is that the creation of a protected disc does not require any specialized equipment. Any CD recorder is suitable for this purpose. The copying of a protected disc is accomplished without any problems. All duplicates, however, automatically lose their registration status, because the code of the disc characteristic changes. These duplicate copies can be registered legally, however, if the user contacts the program developer. This is the second advantage!

Naturally, the algorithm used for generating the registration number must be chosen in such a way as to ensure that no dependence between the registration number and the characteristic code can be traced. Furthermore, the procedure itself must complicate its investigation using disassemblers and debuggers as much as possible. Otherwise, the protection mechanism can be cracked easily and efficiently .

^[i] Some programs react rudely to the failed attempts at their cracking. For example, Alcohol 120% displays messages with obscene words;).