Practical Approaches to Attaining Security Against Adaptively Chosen Ciphertext Attacks (Extended Abstract)

This paper presents three methods for strengthening public 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.