A method of determining the integrity of a message exchanged between a pair of correspondents. The message is secured by embodying the message in a function of a public key derived from a private key selected by one of the correspondents. The method comprises first obtaining the public key. The public key is then subjected to at least one mathematical test to determine whether the public key satisfies predefined mathematical characteristics. Messages utilizing the public key are accepted if the public key satisfies the predefined mathematical characteristics.