Abstract: Reverse engineering and piracy of software is prevented by encrypting code blocks of a program. A program is modified to include additional protective code, including a protective code launcher which is launched with the program. Decryption and execution code is also provided for the protective code launcher and one or more code blocks of the program. A given code block is encrypted using a key which is based on a previous code block, and the previous code block is encrypted using a key which is based on a further previous code block, and so forth. If a hacker modifies the program, such as to avoid a message which requires the user to purchase the program, the program will be disabled. The program can also be encrypted based on computer hardware such as a hard disk serial number so that it will only operate on a particular computer.