US Patent 8639628 Token authentication system and method

A method for calculating a One Time Password. A secret is concatenated with a count, where the secret is uniquely assigned to a token. The secret can be a private key or a shared secret symmetric key. The count is a number that increases monotonically at the token with the number of one-time Passwords generated at the token. The count is also tracked at an authentication server, where it increases monotonically with each calculation of a one-time Password at the authentication server. An OTP can be calculated by hashing a concatenated secret and count. The result can be truncated.