Patent ID: 7620806

Claim:
A decrypting method in a decrypting device for decrypting an encrypted message, wherein: public keys and secret keys are set in advance corresponding to each individual decrypting device, for decrypting the encrypted message, wherein said decrypting device: generates node associating information for associating each of a plurality of first terminal nodes in a first tree structure connecting a plurality of nodes with each of a plurality of decrypting devices in relation to a group of said decrypting devices enabled to decrypt the encrypted message; generates node information for associating a plurality of second terminal nodes in a second tree structure connecting the plurality of second terminal nodes with each of a plurality of decrypting devices; creates the structure of the first tree structure and the second tree structure based on characteristics of users of the decrypting devices; extracts a first decryption enabled node from the first tree structure and a second decryption enabled node from the second tree structure to remove the nodes, in which said decryption devices are enabled to decrypt the encrypted message; and decrypts said encrypted message by use of a first node decryption key based on the product of secret keys associated with each decrypting device that are not associated with each first terminal node of descendents of the first decryption enabled node; wherein said decrypting method includes: storing a device decryption key determined based on the product of secret keys of the decrypting devices other than a specified decrypting device among the plurality of decrypting devices; acquiring node associating information generated in relation to a group of decrypting devices enabled to decrypt the encrypted message; specifying a first terminal node, which is associated with the specified decrypting device, based on the node associating information; generating a first node decryption key based on a product of the secret keys corresponding to the decrypting device not belonging to the group of said decrypting devices enabled to decrypt the encrypted message, based on the public keys corresponding to the respective decrypting devices other than the specified decrypting device corresponding to respective descendent first terminal nodes; and decrypting the encrypted message using the first node decryption key, on condition that any of nodes ranging from a first terminal node associated with the specified decrypting device to a root node of the first tree structure is the first decryption enabled node.