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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 illegally copied. 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. ==Effectiveness== 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. The overall effectiveness of product keys in enforcing software copyrights requires further study. 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 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「product key」の詳細全文を読む スポンサード リンク
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