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Method for changing the decryption key disclosure time securely for the time-controlled cryptosystem. Disclosure Number: IPCOM000013236D
Original Publication Date: 1999-Nov-01
Included in the Prior Art Database: 2003-Jun-18

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Yasuhisa Gotoh


Disclosed is a secure method to change the decryption key disclosure time for the time-controlled cryptosystem. The time-controlled cryptosystem is a system that guarantees a digital data being kept encrypted until a certain time will come. The time-controlled cryptosystem realizes its function as follows. 1) The data owner requests an encryption key to the time-controlled cryptosystem by specifying a certain time, called decryption key disclosure time, which is the time until when a digital data should be kept encrypted. 2) The time-controlled cryptosystem receives the request, prepares an asymmetric key (a pair of a public key and a private key), associates them with the specified time, store those data in the system, and publishes only the public key to the data owner. 3) The data owner encrypts a digital data by using the given public key. 4) The time-controlled cryptosystem guarantees that the private key will never be disclosed outside the system before the decryption key disclosure time. Then the encrypted digital data will be safe until the decryption key disclosure time. 5) After the decryption key disclosure time has arrived, the time-controlled cryptosystem allows to disclose the private key outside the system. 6) Then, the encrypted digital data can be decrypted using the private key. In order to change the decryption key disclosure time securely after the public key is published outside the time-controlled cryptosystem for some reasons, the following algorithm should be applied.