A system for communicating a message securely between a sender and a receiver. The sender provides a key server with a string specifying the receiver. The key server obtains a message key and a particular envelope encryption key corresponding with a particular envelope decryption key, encrypts the message key with the envelope encryption key (creating the envelope), and provides the envelope to the sender-client. The sender-client encrypts the message with the message key and provides it and the envelope to the receiver. The receiver-client receives these and asks an authentication server for the envelope decryption key. The authentication server obtains the envelope decryption key and provides it to the receiver. The receiver then decrypts the envelope with the envelope decryption key, to get the message key, and decrypts the message.