Leak-resistant cryptographic indexed key update

(57) The method and equipment for increasing the disclosure resistance of a ciphering system using the renewal technique of a key with a summary index are indicated. With 1 enforcement form, a code client device holds a secret key value as a part of the state. The client can use the updating process keep the partial information which might be related secretly and may have been beforehand revealed to the aggressor from already describing the newly updated secret value to be useful, and can update the secret value, for example before each transaction at any time. By repeating and applying this updating process, the information revealed during the code operation collected by the aggressor cannot be used any longer immediately. Therefore, such a system is still safe to the attack in connection with analysis of the measure of information revealed during the power consumption of a device, electromagnetic characteristics, or other transactions (some enforcement forms can prove a safe thing). The present invention can be used about the client and server which use such a protocol. In order to carry out a transaction with a client, a server acquires the present transaction counter of a client. Subsequently, a server carries out a series of operations and it asks for the sequence of conversion required to re-derive the right session key from the initial secret value of a client. These conversion is carried out and a result is used as a transaction session key. The present invention includes a series of renewal processes of a client side of expecting a corresponding significant improvement of the performance of server grabbing.