To prevent, even if information exchanged between devices that implement authentication is intercepted, illegal authentication using the information in a communication system where a common encryption key is shared between the devices and a result encrypting random number information using the encryption key is exchanged as authentication information to perform authentication.

A verification control device 31 generates a vehicle-side rolling code Cc1 corresponding to the number of authentication times of an electronic key 12. At the same time, a control circuit 21 of the electronic key 12 generates a key-side rolling code Ck1 corresponding to the number of reception times of a challenge code Cch. The verification control device 31 authenticates the electronic key 12 on the basis of an encryption result obtained by encrypting a result obtained by adding the rolling code Ck1, Cc1, that varies each time the authentication is implemented between the verification control device 31 and the electronic key 12, to the challenge code Cch exchanged between them, while using a common encryption key.

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