Techniques for computer security, and more specifically timestamp-abased authentication, are described. Some implementations provide an authentication method that utilizes an authentication process that is shared as a secret between a first and second computing system. The process provides as output a number that is based on a timestamp. The first computing system executes the authentication process using a timestamp obtained from its clock. The resulting number is transmitted to the second computing system, possibly along with other authentication data, such as a username and/or password. In response, the second computing system executes the authentication process using a timestamp obtained from its clock. If the numbers generated by the first and second computing systems match, the first computing system is authenticated.