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. A good example is the product keys for Microsoft products; there is no information available anywhere which gives any insights into how these keys actually work. They use alphabetical and numerical characters, with the absence of AEIOULNSZ150 which leaves 24 characters. Those left out appear to be mainly ones likely to be confused with other characters. Having no vowels makes it impossible for a product key to inadvertently spell out words, which could have unintended consequences. So, what we appear to have is a 25-digit base-24 number, which is a very large number indeed; on the order of 4.15 ^180. That is probably larger than the number of atoms in the universe. It is remarkable that Microsoft has managed to never leak any information about how this number is used in this context, since these product keys were introduced with Windows XP.