Patent ID: 8270613

Claim:
A system to disable an encryption/decryption device operated by an unauthorized person, comprising: an encryption device comprising: a storage device which stores distribution member set data, a random number, and a public key, the distribution member set data being information on members who are registered members and are permitted to decrypt a ciphertext, the public key including [θ k α i ]G obtained by multiplying G which is an element of an arbitrary additive cyclic group whose order is a composite number and is a generator of the cyclic group, by θ k α i which is a product of θ k and α i , where θ k is an integer randomly generated for every k, k is any integer from 1 to L, L is an arbitrary integer, α i is the i-th power of a constant integer α (i is any integer from 1 to 2L), and each of combinations (k, i) corresponds to a different receiver; and a processing device which, when at least one of a shared key and a message is input, generates an error-containing ciphertext which is a ciphertext containing the shared key or the message, the distribution member set data, and error information which is information different for each of the members, and an error-correcting ciphertext which is a ciphertext of the error information, obtains, when the error-containing ciphertext is being generated, a sum of data for a number of members who do not belong to the distribution member set, and distributes the error-containing ciphertext and the error-correcting ciphertext; and a decryption device connected to the encryption device, the decryption device comprising: a storage device which stores the public key and a secret key corresponding to the decryption device; and a processing device which, when the error-containing ciphertext and the error-correcting ciphertext are received from the encryption device, determines whether the decryption device is included in the distribution member set data, and, when the decryption device is included in the distribution member set data, decrypts the error-containing ciphertext and the error-correcting ciphertext with the public key and the secret key, obtains, when the error-containing ciphertext is decrypted, the sum of the data for the number of members who do not belong to the distribution member set, and removes the error information from the decrypted error-containing ciphertext to output the shared key or the message.