A product key, also known as a software key, is a specific software-based key for a computer program. It certifies that the copy of the program is original. Activation is sometimes done offline by entering the key, or with software like Windows 8.1, online activation is required to prevent multiple people using the same key. Not all software has a product key, as some publishers may choose to use a different method to protect their copyright, or in some cases, such as free or open source software, copyright protection is not used.
Computer games use product keys to verify that the game has not been copied without authorization. Likewise, one is not allowed to play online with two identical product keys at the same time.
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.
Because of this, software publishers are increasingly turning to alternative methods of verifying that keys are both valid and uncompromised. One method, product validation, assigns a product key based on a unique feature of the purchaser's computer hardware (such as its MAC address), which cannot be as easily duplicated since it depends on the user's hardware. Other newer methods may involve requiring periodical validation of the CD key with an internet server (for games with an online component, this is done whenever the user signs in). The validation can then be performed on the server side, preventing cracks tampering with it (as they do on the client side). The server can also keep a blacklist of all CD keys known to be invalid or which have explicitly been banned, and deny them access.
Product keys also present new ways for distribution to go wrong. If a product is shipped with missing or invalid keys, then the CD itself is useless. For example, all copies of originally shipped to Australia without CD keys. Australian Pandora Tomorrow CD-Key Problems Shack News
Particularly controversial is the situation which arises when multiple products' keys are bound together. If products have dependencies on other products (as is the case with ), it is common for companies to ban all bound products. For example, if a fake CD key is used with an expansion pack, the server may ban legitimate CD keys from the original game. Similarly, with Valve's Steam service, all products the user has purchased are bound into the one account. If this account is banned, the user will lose access to every product associated with the same account
This "multi-ban" is highly controversial, since it bans users from products which they have legitimately purchased and used.
A common cause of false positives (as with the World of Warcraft case above) is users of unsupported platforms. For example, users of Linux can run Windows applications through compatibility layers such as Wine and Cedega. This software combination sometimes triggers the game's server anti-cheating software, resulting in a ban due to Wine or Cedega being a Windows API compatibility layer for Linux, so it is considered third-party (cheating) software by the game's server.