A method and system for authentication of sites in a land mobile radio (LMR) system and encryption of messages exchanged by the sites. In some embodiments, the method includes transmitting a certificate created by a trusted authority by applying a function to a first site public key using the trusted authority's private key to generate a reduced representation, which is encrypted with the trusted authority's private key. Other sites may receive the certificate, decrypt it using the trusted authority's public key, and authenticate the first site. The method may further include generating a session key, encrypting it with the public key of the first site, and transmitting the encrypted session key to the first site. The first site decrypts the encrypted session key with the first site's private key, and transmits a message encrypted with the shared session key to other sites for decryption using the session key.