Concept
A product key, also known as a software key, serial key or activation key, is a specific software-based key for a computer program. It certifies that the copy of the program is original. Product keys consist of a series of numbers and/or letters. This sequence is typically entered by the user during the installation of computer software, and is then passed to a verification function in the program. This function manipulates the key sequence according to a mathematical algorithm and attempts to match the results to a set of valid solutions. Standard key generation, where product keys are generated mathematically, is not completely effective in stopping copyright infringement of software, as these keys can be distributed. In addition, with improved communication from the rise of the Internet, more sophisticated attacks on keys such as cracks (removing the need for a key) and product key generators have become common. Because of this, software publishers use additional product activation methods to verify that keys are both valid and uncompromised. One method assigns a product key based on a unique feature of the purchaser's computer hardware, which cannot be as easily duplicated since it depends on the user's hardware. Another method involves requiring one-time or periodical validation of the product key with an internet server (for games with an online component, this is done whenever the user signs in). The server can deactivate unmodified client software presenting invalid or compromised keys. Modified clients may bypass these checks, but the server can still deny those clients information or communication. Windows 95 retail product keys take the form XXX-XXXXXXX. To determine whether the key is valid, Windows 95 performs the following checks: The first 3 characters must not be equal to 333, 444, 555, 666, 777, 888 or 999. The last 7 characters must all be numbers from 0-8. The sum of the last 7 numbers must be divisible by 7 with no remainder. The fourth character is unchecked. If all checks pass, the product key is valid.
