First vehicle-based terminal (NN1) of a group of terminals of a radio communication network, the group of terminals having a second vehicle-based terminal (NN2), wherein the first vehicle-based terminal (NN1) comprises at least one processor, at least one memory containing computer program code, at least one communication module and at least one antenna, the computer program code being configured such that it uses the at least one processor, the at least one communication module and the at least one antenna to cause the first vehicle-based terminal (NN1) to determine (202; 302; 402; 502) at least one symmetrical group key that is associated with the group of terminals, to encrypt (204; 304; 404; 504) the at least one symmetrical group key using a symmetrical pair key that is associated with only the first and the second vehicle-based terminal (NN1, NN2), to send (206; 306; 406; 506) the encrypted symmetrical group key towards the second vehicle-based terminal (NN2), to receive (212; 312; 412; 512) an encrypted message from the second vehicle-based terminal (NN2), to decrypt (214; 314; 414; 514) the encrypted message on the basis of the determined symmetrical group key, to determine a new symmetrical group key, to encrypt the new symmetrical group key on the basis of the symmetrical pair key that is associated with the second vehicle-based terminal (NN2), and to send the encrypted new symmetrical group key towards the second vehicle-based terminal (NN2), to receive a positive acknowledgement of receipt that acknowledges receipt of the new symmetrical group key by the second vehicle-based terminal (NN2), to determine a new symmetrical pair key, which is associated with only the first and the second vehicle-based terminal (NN1, NN2), on the basis of receipt of the positive acknowledgement of receipt, and to select the new symmetrical pair key as the valid symmetrical pair key. to determine (202; 302; 402; 502) at least one symmetrical group key that is associated with the group of terminals, to encrypt (204; 304; 404; 504) the at least one symmetrical group key using a symmetrical pair key that is associated with only the first and the second vehicle-based terminal (NN1, NN2), to send (206; 306; 406; 506) the encrypted symmetrical group key towards the second vehicle-based terminal (NN2), to receive (212; 312; 412; 512) an encrypted message from the second vehicle-based terminal (NN2), to decrypt (214; 314; 414; 514) the encrypted message on the basis of the determined symmetrical group key, to determine a new symmetrical group key, to encrypt the new symmetrical group key on the basis of the symmetrical pair key that is associated with the second vehicle-based terminal (NN2), and to send the encrypted new symmetrical group key towards the second vehicle-based terminal (NN2), to receive a positive acknowledgement of receipt that acknowledges receipt of the new symmetrical group key by the second vehicle-based terminal (NN2), to determine a new symmetrical pair key, which is associated with only the first and the second vehicle-based terminal (NN1, NN2), on the basis of receipt of the positive acknowledgement of receipt, and to select the new symmetrical pair key as the valid symmetrical pair key. <EOS>