Towards Practical Public Key Systems Secure Against Chosen Ciphertext Attacks

We present two efficient constructions aimed at making public key systems secure against chosen ciphertext attacks. The first one applies to any deterministic public key system and modifies it into a system that is provably as hard to break under a passive attack as the original one, but has the potential of making a chosen ciphertext attack useless to an enemy. The second construction applies to the El Gamal/Diffie-Hellman public key system. Again, the modified system is provably as hard to break under a passive attack as the original one, and under an additional cryptographic assumption, a chosen ciphertext attack is provably useless to an enemy. We also point out a connection between such public-key systems and efficient identification schemes.