INFORMATION PROCESSOR, MOBILE BODY APPARATUS, AND COMMUNICATION SYSTEM

An information processor of an aspect of the disclosure includes a protection section protecting first communication between a first device and a second device and second communication between a third device and the second device. The protection section executes the following (1) to (4):(1) deriving or receiving a first session key;(2) using the first session key for encryption or decryption and message authentication of the first communication;(3) using the first communication protected by the first session key to receive a second session key; and(4) using the second session key for encryption, decryption, or message authentication of the second communication.Here, the total number of times of communication or the total amount of communication data from the start of use to the end of the use of the second session key differs between the first communication and third communication between the first device and the third device.