Abstract: A method and apparatus for verifying a personal identification number and checking a number series put onto an identification medium, containing information readable by machine in the form of a card and account number and permanent information in the form of a permanently inscribed number, include a first unit (2,4,22,24,6,8,10,12,14,16,18,20) for encryption of the card or account number (PAN) with an algorithm defined by a first key. The unit includes elements (6,8,10,12,14,16,18) for forming the first key of the personal identification number (PIN) in combination with a first key number (K1). The apparatus further contains a second unit (30,32,42,34,36,12,14,40,38) for generating a personal check number (PCN) by decryption of coded information obtained from the first unit with the aid of an algorithm defined by a second key.