Patent ID: 7724906

Claim:
A decryption apparatus which decrypts a ciphertext, comprising: a decryption apparatus ID storing unit to store an identifier of the decryption apparatus corresponding to a leaf in a tree structure, the identifier indicating a path from a root of the tree structure to the leaf; a secret key storing unit to store a plurality of secret keys, each of which is specified by two nodes in the tree structure, one of the two nodes being an ancestor node of the leaf corresponding to the identifier and the other of the two nodes being a node which is not an ancestor node of the leaf; a first acquiring unit configured to acquire a plurality of ciphertexts, each ciphertext including a ciphertext index information item corresponding to a decryption key for decrypting the respective ciphertext, the ciphertext index information item including an u code and a v code corresponding to a u node and a v node in the tree structure, the u code and the v code indicating paths from the root to the u node and the v node; a second acquiring unit configured to acquire a decryptable ciphertext from the ciphertexts by using the u code and the v code included in each ciphertext index information item and the identifier, the u node of the decryptable ciphertext being an ancestor node of the leaf corresponding to the identifier and the v node of the decryptable ciphertext being a node which is not an ancestor node of the leaf; a selecting unit configured to select, from the secret keys stored in the secret key storing unit, a secret key from which the decryption key is derived by using the u code and the v code included in the ciphertext index information item of the decryptable ciphertext and the identifier; a deriving unit configured to derive the decryption key from the secret key selected, based on the v code included in the ciphertext index information item of the decryptable ciphertext and the identifier; and a decryption unit configured to decrypt the decryptable ciphertext by using the decryption key derived.