The invention relates to a method (50) for authenticating a user to a computer system (70), the method comprising the following steps executed in a token (10): generating (52) a counter value (20, 22) by utilizing a counting unit (12) implemented in the token (10), wherein at least a portion of generated counter values (20, 22) forms a strictly monotonous sequence, generating (54) a message (30) depending on the generated counter value (20, 22), signing (56) the generated message (30) by utilizing a private key (24) of the user, wherein the private key (24) is stored in the token (10), and wherein the private key (24) or a copy thereof is not provided to the computer system (70), and transmitting (58) the signed message to the computer system (70). The invention further relates to a token (10) for authenticating a user to a computer system (70) and to a method (60) executed on a computer system (70) for authenticating a user. The token (10A, 10B) according to the invention may be implemented in hardware or software in a mobile device (80A, 80B). The invention further relates to a computer program product comprising instructions which, when executed on a processor (72, 82), implement the steps of the inventive methods.