Method and control device for securely checking an electronic ticket

A method for securely checking an electronic ticket, in which the electronic ticket is stored as data on a first mobile terminal device allocated to an end consumer, and the electronic ticket is checked using a second terminal device allocated to a ticket inspector or to a control system, wherein the method comprises the following steps:

• obtaining the electronic ticket by the first terminal device, wherein the electronic ticket comprises a first identifier providing an unambiguous identification associated with the first terminal device, which couples the electronic ticket to the first terminal device, and wherein the electronic ticket comprises a digital signature of the service provider to detect manipulation of the electronic ticket as issued;
• sending a request message from the second terminal device to the first terminal device via a first communication channel, the request message comprising a random number of length L;
• sending a response message from the first terminal device to the second terminal device via a second communication channel, the response message being signed by the first terminal device, and the second communication channel either being identical with the first communication channel or different from the first communication channel, the response message comprising the electronic ticket, the random number, and a second digital identifier generated by the first terminal device to match the first digital identifier, wherein the random number prevents the second terminal device from verifying a copy of a response message previously generated;
• verifying, in response to determining the response message includes a representation of the random number generated by the second terminal device, the authenticity of the signed response message by the second terminal device only when the first identifier obtained from the electronic ticket and the second identifier generated by the first terminal device are identical;
• and
• confirming the authenticity of the response message by the second terminal device, if the second terminal device was previously able to verify the authenticity of the signature.