The invention relates to an arrangement and a method for reliably identifying a user in a computer 
system. The method utilizes a 
mobile station for communicating with the 
system. The method comprises generating a first one-time 
password in the 
mobile station by utilizing a known 
algorithm on the basis of the identification number of the user, subscriber-specific identifier, device-specific identifier of the 
mobile station, and time. The 
password obtained and the subscriber-specific identifier of the user are encoded and transmitted to an 
authentication server of the computer 
system, comprising identifying the user on the basis of the subscriber-specific identifier, searching a 
database for the personal identifier number of the user and the device-specific identifier of the mobile 
station associated with the user, generating a second 
password at the 
authentication server by utilizing the same predetermined 
algorithm on the basis of the 
personal identification number of the user, subscriber-specific identifier, device-specific identifier of the mobile 
station and time, comparing the first and the second passwords with each other at the 
authentication server, and if the passwords match, enabling the telecommunication connection between the mobile 
station and the computer system.