Immunizing Public Key Cryptosystems Against Chosen Ciphertext Attacks

This paper presents three methods for strengthening public key cryptosys tems 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 ciphertexts except for the exact object cipher text 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 illus trated by an example of a public key cryptosystem based on the intractability of computing discrete logarithms in nite elds Security of the three example cryptosystems is formally proved 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