Key recovery method and system

Abstract

A key recovery method and system capable of key recovery without informing a third party of one's own secret key are disclosed. For realization of the method, a transmitting information processor generates a data value satisfying a relational expression by which if one of data obtained by converting a first public key and used as a cipher text generating parameter and data obtained by converting at least one second public key is decided, the other can be determined. The transmitting processor transmits a cipher text applied with the generated data value to a first receiving information processor which has a secret key paired with the first public key and at least one second receiving information processor which has a secret key paired with the second public key. The first receiving processor, even in the case of loss of the secret key paired with the first public key, can determine the data obtained by converting the first public key in such a manner that data obtained from the second receiving processor by converting the second public key and determined from the secret key paired with the second public key and the data value applied to the cipher text sent from the transmitting processor are introduced into the above relational relationship. Thereby, the cipher text can be deciphered into the original message.