MULTI-PARTY SECURE AUTHENTICATION SYSTEM, AUTHENTICATION SERVER, INTERMEDIATE SERVER, MULTI-PARTY SECURE AUTHENTICATION METHOD, AND PROGRAM

摘要

Even when an intermediate server exists, a plurality of servers simultaneously authenticates a user securely. A user apparatus 1 disperses a password w'. The user apparatus 1 obtains a ciphertext Enc US_i ([w'] i ), which is obtained by encrypting a dispersed value [w'] i . The intermediate server 2 transmits the ciphertext Enc US_i ([w'] i ) to an authentication server 3. The authentication server 3 decrypts the ciphertext Enc US_i ([w'] i ) to obtain the dispersed value [w'] i . The authentication server 3 determines a verification value q a_i (W) . The authentication server 3 obtains a ciphertext Enc WS_a_i (q a_i (W) ). The intermediate server 2 decrypts the ciphertext Enc WS_a_i (q a_i (W) ) to obtain the verification value q a_i (W) . The intermediate server 2 verifies whether a sum total of the verification values is equal to 0 or not. The authentication server 3 determines a verification value q a_i (a_j) . The authentication server 3 obtains a ciphertext Enc S_a_iS_a_j (q a_i (a_j) ). The authentication server 3 decrypts the ciphertext Enc S_a_iS_a_j (q a_j (a_i) ) to obtain the verification value q a_i (a-j) . The authentication server 3 verifies whether a sum total of the verification values is equal to 0 or not.