This paper introduces a non-interactive zero-knowledge proof of knowledge (NIZKPoK) and demonstrates how it can be used to construct a public-key cryptosystem secure against chosen ciphertext attacks. The authors show that a non-interactive zero-knowledge proof system for NP, as introduced by Blum, Feldman, and Micali, can be used to build a secure cryptosystem. They define a stronger model of chosen ciphertext attack than the "lunchtime" attack considered by Naor and Yung, and prove that their proposed cryptosystem is secure against this attack. The paper discusses the challenges of constructing non-interactive proofs of knowledge and the difficulties of defending against chosen ciphertext attacks. It introduces a model where each sender has a secret associated with a public key, and a message's "putative" sender is determined by the receiver based on information in the message. In this model, a chosen ciphertext attack allows an attacker to send chosen ciphertext messages to the receiver and receive their correct decryptions. The paper proposes a solution that involves encrypting a message using both the sender's and receiver's public keys, and providing a non-interactive zero-knowledge proof that the two encryptions encrypt the same plaintext. The authors also define three types of chosen ciphertext attacks: general, attacker-specific, and message-restricted. They show that their proposed cryptosystem is secure against all three types of attacks. The paper concludes that the choice of an appropriate definition and solution to the problem of chosen ciphertext attacks is a first step toward formalizing and solving the broader problem of securing public-key cryptographic communication in a multiparty setting.This paper introduces a non-interactive zero-knowledge proof of knowledge (NIZKPoK) and demonstrates how it can be used to construct a public-key cryptosystem secure against chosen ciphertext attacks. The authors show that a non-interactive zero-knowledge proof system for NP, as introduced by Blum, Feldman, and Micali, can be used to build a secure cryptosystem. They define a stronger model of chosen ciphertext attack than the "lunchtime" attack considered by Naor and Yung, and prove that their proposed cryptosystem is secure against this attack. The paper discusses the challenges of constructing non-interactive proofs of knowledge and the difficulties of defending against chosen ciphertext attacks. It introduces a model where each sender has a secret associated with a public key, and a message's "putative" sender is determined by the receiver based on information in the message. In this model, a chosen ciphertext attack allows an attacker to send chosen ciphertext messages to the receiver and receive their correct decryptions. The paper proposes a solution that involves encrypting a message using both the sender's and receiver's public keys, and providing a non-interactive zero-knowledge proof that the two encryptions encrypt the same plaintext. The authors also define three types of chosen ciphertext attacks: general, attacker-specific, and message-restricted. They show that their proposed cryptosystem is secure against all three types of attacks. The paper concludes that the choice of an appropriate definition and solution to the problem of chosen ciphertext attacks is a first step toward formalizing and solving the broader problem of securing public-key cryptographic communication in a multiparty setting.