A method performed by a server computing system , the method comprising: associating an indication of a first authentication process with a client computing system , wherein the first authentication process takes as input a timestamp and generates as output a number that is based on the timestamp; receiving a first number from the client computing system , wherein the first number is generated by the client computing system by applying the first authentication process to a first reduced precision timestamp based on a clock of the client computer system; in response to receiving the first number, determining a second reduced precision timestamp based on a clock of the server computing system; applying a second authentication process, wherein the second authentication process is the same as the first authentication process, to the second reduced precision timestamp to generate a second number based on the second reduced precision timestamp; determining a third reduced precision timestamp that is prior to the second reduced precision timestamp; applying the second authentication process to the third reduced precision timestamp to generate a third number based on the third reduced precision timestamp; and authenticating the client computing system when the first number matches the second number or when the first number matches the third number. associating an indication of a first authentication process with a client computing system , wherein the first authentication process takes as input a timestamp and generates as output a number that is based on the timestamp; receiving a first number from the client computing system , wherein the first number is generated by the client computing system by applying the first authentication process to a first reduced precision timestamp based on a clock of the client computer system; in response to receiving the first number, determining a second reduced precision timestamp based on a clock of the server computing system; applying a second authentication process, wherein the second authentication process is the same as the first authentication process, to the second reduced precision timestamp to generate a second number based on the second reduced precision timestamp; determining a third reduced precision timestamp that is prior to the second reduced precision timestamp; applying the second authentication process to the third reduced precision timestamp to generate a third number based on the third reduced precision timestamp; and authenticating the client computing system when the first number matches the second number or when the first number matches the third number. determining a second reduced precision timestamp based on a clock of the server computing system; applying a second authentication process, wherein the second authentication process is the same as the first authentication process, to the second reduced precision timestamp to generate a second number based on the second reduced precision timestamp; determining a third reduced precision timestamp that is prior to the second reduced precision timestamp; applying the second authentication process to the third reduced precision timestamp to generate a third number based on the third reduced precision timestamp; and authenticating the client computing system when the first number matches the second number or when the first number matches the third number. <EOS>