Symmetric Public-Key Encryption

Public-key encryption would seem to be inherently assymmetric, in that only messages sent to a user can be encrypted using his public key. We demonstrate that the use of interactive protocols for sending encrypted messages enables a symmetric use of public keys; we give cryptographic protocols for the following tasks: 1.Probabilistic encryption, using the same public key, both of messages that are sent to a particular user as well as of messages that the user sends to others, without compromising the key. We propose a public-key cryptosystem based on these protocols which has only one key, owned by a cryptographic server.2.Authentication both of the sender and of the receiver of a probabilistically encrypted message.3.Probabilistic encryption which is provably secure against both chosen-message and chosen-ciphertext attack.