Practical Approaches to Attaining Security against Adaptively Chosen Ciphertext

This paper presents three methods for strengthening pub lic key cryptosystems in such a way that they become secure against adaptively chosen ciphertext attacks In an adaptively chosen ciphertext attack an attacker can query the deciphering algorithm with any cipher texts except for the exact object ciphertext to be cryptanalyzed The rst strengthening method is based on the use of one way hash functions the second on the use of universal hash functions and the third on the use of digital signature schemes Each method is illustrated by an example of a public key cryptosystem based on the intractability of computing discrete logarithms in nite elds Two other issues namely applications of the methods to public key cryptosystems based on other intractable problems and enhancement of information authentication capability to the cryptosystems are also discussed