The invention relates to a method for checking the integrity of a message transmitted between a sender in a transmitting end and a recipient in a receiving end, in which method an authentication value is calculated for the first message and a random string is generated. According to a method in the transmitting end an authentication value is generated from a message to be sent and the random string. A check code is formed from the authentication value and the random sting. The first message is transferred from a sender to a recipient through a first channel, and the check code is transferred through a second secure channel. In the receiving end a message is received through a first channel and the check code is received through a second secure channel. In the receiving end an authentication check formed at least based of the received message. The integrity of the received message is checked by comparing the predetermined check values in the receiving end.