Nonmalleable Cryptography

The notion of non-malleable cryptography, an extension of semantically secure cryptography , is deened. Informally, the additional requirement is that given the ciphertext it is impossible to generate a diierent ciphertext so that the respective plaintexts are related. The same concept makes sense in the contexts of string commitment and zero-knowledge proofs of possession of knowledge. Non-malleable schemes for each of these three problems are presented. The schemes do not assume a trusted center; a user need not know anything about the number or identity of other system users.