SecurID tokens are used in two-factor authentication systems. Each user account is linked to a token, and each token generates a pseudo-random number that changes periodically, typically every 30 or 60 seconds. To log in, the user enters a username, password, and the number shown on their token. The authentication server knows what number a particular token should be showing, and so uses this number to prove that the user is in possession of their token."
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