Patent ID: 7272230
Filing Date: 2007-09-18
Classification: H04L

Abstract:
1. A cryptographic system in which a key management server comprising a management database for storing, with respect to each of users, an inherent ID, an inherent key, and an inherent public key which are inherent in the user, and a decryption object ID which is an inherent ID of a creator of an enciphered file decipherable by the user, and a client comprising inherent data storing means for storing said inherent ID and an inherent secret key paired with said inherent public key, and session key generating means are connected to each other through a network, wherein said client comprises plaintext data entering means for accepting the entry of plaintext data, first transmitting means for transmitting to the key management server the inherent ID stored in the inherent data storing means when the plaintext data is accepted, enciphered data creating means for generating a session key by said session key generating means, and enciphering the accepted plaintext data with the generated session key, to create enciphered data, enciphered file creating means for deciphering an enciphered inherent key transmitted from the key management server with the inherent secret key stored in the inherent data storing means, to obtain an inherent key, enciphering said session key with the obtained inherent key, to generate a first enciphered session key, and using the inherent ID stored in the inherent data storing means as a creator ID, to add the creator ID and the generated first enciphered session key to the enciphered data, to create an enciphered file, enciphered file entering means for accepting the entry of the enciphered file, second transmitting means for using, when the enciphered file is accepted, the inherent ID stored in the inherent data storing means as a decryptor ID, to transmit to the key management server the decryptor ID, and the creator ID and the first enciphered session key in the accepted enciphered file, and deciphering means for deciphering a second enciphered session key transmitted from said key management server with the inherent secret key stored in the inherent data storing means, to obtain a session key, and deciphering the enciphered data in the accepted enciphered file with the obtained session key, to obtain plaintext data, and said key management server comprises first transmitting means for enciphering the inherent key stored in the management database in correspondence with said inherent ID transmitted from the client with the inherent public key stored in the management database in correspondence with said inherent ID, to generate an enciphered inherent key, and transmitting the generated enciphered inherent key to the client, judging means for judging whether or not the creator ID, together with the decryptor ID, transmitted from the client is stored as a decryption object ID in said management database in correspondence with the decryptor ID transmitted from said client, and second transmitting means for deciphering, when said judging means judges that said creator ID is stored as the decryption object ID in the management database, the first enciphered session key, together with said decryptor ID and said creator ID, transmitted from the client with the inherent key stored in the management database in correspondence with said creator ID, to obtain a session key, enciphering the obtained session key with the inherent public key stored in the management database in correspondence with said decryptor ID, to generate a second enciphered session key, and transmitting the generated second enciphered session key to the client.