To provide an authentication system and an authentication method for making it difficult to generate the illegal analysis of a cryptographic arithmetic expression.

A plurality of vehicle keys 2 to be authenticated based on an immobilizer system can be registered in one vehicle 1. Then, a table 18 in which a registration key and a cryptographic arithmetic expression F(x) are associated with each other is prepared for the vehicle 1. The table 18 is configured to define which cryptographic arithmetic expression (x) should be used by the registration key registered in the vehicle 1. In the case of performing challenge response authentication, the cryptographic arithmetic expression F(x) to be used for authentication is changed for each registration key registered in the vehicle 1.