Abstract: A method of authenticating input data including user data and pin code data for an access control system. The method includes the steps of receiving the user data and the pin code data from a keyboard and/or a card reader, combining an offset data with the user data, making a modulo adjustment of said offset user data, obtaining one or more variables from the offset user data, calculating a matrix index by bringing certain variables into a predetermined equation, determining a constant from a matrix by the matrix index and comparing the constant with a corresponding digit value of the pin code data. If the constant matches the digit value, a validation signal is generated and the next constant is obtained from the matrix by the next matrix index obtained by use of a predetermined second equation so that the next digit value of the pin code data is compared with this constant.