According to one embodiment, an authenticator which authenticates an authenticatee, which stores first key information (NKey) that is hidden, includes a memory configured to store second key information (HKey) which is hidden, a random number generation module configured to generate random number information, and a data generation module configured to generate a session key (SKey) by using the second key information (HKey) and the random number information. The authenticator is configured such that the second key information (HKey) is generated from the first key information (NKey) but the first key information (NKey) is not generated from the second key information (HKey).