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 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,
Product keys also present new ways for distribution to go wrong. If a product is shipped with missing or invalid keys, then the product itself is useless. For example, all copies of originally shipped to Australia without product 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 key is used with an expansion pack, the server may ban legitimate 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.