Another Method for Attaining Security

Practical approaches to constructing public key cryptosystems secure against chosen ciphertext attacks were rst initiated by Damgard and further extended by Zheng and Seberry. In this paper we rst point out that in some cryptosystems proposed by Zheng and Seberry the method for adding authentication capability may fail just under known plaintext attacks. Next, we present a new method for immunizing public key cryptosystems against adaptively chosen ciphertext attacks. In the proposed immunization method, the deciphering algorithm rst checks that the ciphertext is legitimate and then outputs the matching plaintext only when the check is successful. This is in contrast with the Zheng and Seberry's methods, where the deciphering algorithm rst recovers the plaintext and then outputs it only when the checking condition on it is satissed. Such a ciphertext-based validity check will be particularly useful for an application to group-oriented cryptosystems, where almost all deciphering operations are performed by third parties, not by the actual receiver.