An in-vehicle computer system provided in a vehicle, the in-vehicle computer system comprising: a first in-vehicle computer including a first arithmetic processing device, a secure element (13, 14), and a first storage unit; and a second in-vehicle computer including a second arithmetic processing device, a secure hardware extension, SHE,, and a second storage unit, wherein the first in-vehicle computer and the second in-vehicle computer are connected to a communication network provided in the vehicle, wherein an initial key is preset in the SHE, the initial key being usable in a generation process and a verification process for a message authentication code, wherein a master key is preset in the secure element (13, 14), the master key being used together with an identifier of the second in-vehicle computer for generating the message authentication code to be used as the initial key, wherein: the first arithmetic processing device is configured to cause the message authentication code for the identifier of the second in-vehicle computer received from the second in-vehicle computer to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to execute a process of authenticating validity of the second in-vehicle computer by using the generated message authentication code, the first arithmetic processing device is further configured to cause a MASTER_ECU_KEY, MEK, key of the SHE to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to cause a key registration message for registering a key exchange key in a KEY_2 key usable in an encryption process and a decryption process of the SHE or a KEY_3 key usable in the generation process and the verification process for the message authentication code of the SHE to be generated using the MEK key through the secure element (13, 14) and transmit the key registration message to the second in-vehicle computer, and the second arithmetic processing device is configured to register the key exchange key in the SHE according to the key registration message. a first in-vehicle computer including a first arithmetic processing device, a secure element (13, 14), and a first storage unit; and a second in-vehicle computer including a second arithmetic processing device, a secure hardware extension, SHE,, and a second storage unit, wherein the first in-vehicle computer and the second in-vehicle computer are connected to a communication network provided in the vehicle, wherein an initial key is preset in the SHE, the initial key being usable in a generation process and a verification process for a message authentication code, wherein a master key is preset in the secure element (13, 14), the master key being used together with an identifier of the second in-vehicle computer for generating the message authentication code to be used as the initial key, wherein: the first arithmetic processing device is configured to cause the message authentication code for the identifier of the second in-vehicle computer received from the second in-vehicle computer to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to execute a process of authenticating validity of the second in-vehicle computer by using the generated message authentication code, the first arithmetic processing device is further configured to cause a MASTER_ECU_KEY, MEK, key of the SHE to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to cause a key registration message for registering a key exchange key in a KEY_2 key usable in an encryption process and a decryption process of the SHE or a KEY_3 key usable in the generation process and the verification process for the message authentication code of the SHE to be generated using the MEK key through the secure element (13, 14) and transmit the key registration message to the second in-vehicle computer, and the second arithmetic processing device is configured to register the key exchange key in the SHE according to the key registration message. the first arithmetic processing device is configured to cause the message authentication code for the identifier of the second in-vehicle computer received from the second in-vehicle computer to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to execute a process of authenticating validity of the second in-vehicle computer by using the generated message authentication code, the first arithmetic processing device is further configured to cause a MASTER_ECU_KEY, MEK, key of the SHE to be generated using the master key through the secure element (13, 14), the first arithmetic processing device is further configured to cause a key registration message for registering a key exchange key in a KEY_2 key usable in an encryption process and a decryption process of the SHE or a KEY_3 key usable in the generation process and the verification process for the message authentication code of the SHE to be generated using the MEK key through the secure element (13, 14) and transmit the key registration message to the second in-vehicle computer, and the second arithmetic processing device is configured to register the key exchange key in the SHE according to the key registration message.