Virus Detection
In some cases, the occasion calls to detect whether data has been tampered with, and encrypt some kind of 'checksum' into the data stream itself. This is useful not only for authorization codes but for programs themselves. A virus that infects such a 'protected' program would no doubt neglect the encryption algorithm and authorization/checksum signature. The program could then check itself each time it loads, and thus detect the presence of file corruption. Naturally, such a method would have to be kept highly in secret, as virus programmers represent the worst of the code breakers: those who willfully use information to do damage to others. As such, the use of encryption is mandatory for any decent anti-virus protection scheme.
A cyclic redundancy check is one typically used checksum method. It uses bit rotation and an XOR mask to generate a 16-bit or 32-bit value for a data stream, such that one missing bit or 2 interchanged bits are more or less guaranteed to cause a 'checksum error'. This method has been used for file transfers for a long time, such as with XMODEM-CRC. The method is somewhat well documented, and standard. But, a deviation from the standard CRC method might be useful for the purpose of detecting a problem in an encrypted data stream, or within a program file that checks itself for viruses.
E-Commerce
Digital Copyright Protection
Virus Detection