Device and authentication system

An authentication system comprising a first device and a second device,

• the first device including:
• a challenge generating unit that generates a challenge;
• a response receiving unit that receives a response;
• and
• an authentication processing unit that performs authentication by using the challenge and the response,
• one of the first device and the second device including:
• a storage unit that stores generated challenges which are challenges previously generated;
• a determination unit that determines whether a newly generated challenge matches any of the generated challenges or not;
• an output unit that outputs the newly generated challenge as an unused challenge when the determination unit determines that the newly generated challenge does not match any of the generated challenges;
• and
• a registration unit that stores the newly generated challenge as a new generated challenge in the storage unit when the determination unit determines that the newly generated challenge does not match any of the generated challenges, and
• the second device including:
• a response generating unit that generates the response by using the unused challenge;
• and
• a response transmitting unit that transmits the response.