A multimedia messaging system for receiving/sending multimedia messages, includes: a wireless LAN; and a MMS gateway. The MMS gateway performs: receiving/sending the multimedia message to/from a MMS user device via the wireless LAN; and encrypting the multimedia message. The encryption is performed by: issuing a certificate to the MMS user device; sending a session ID and a master key encrypted by the MMS gateway's private key to the MMS user device in response to a request of the MMS user device having the certificate; generated a shared secret key using an algorithm combining the master key with the MMS user device's phone number and the session ID; and encrypting the multimedia message using the shared secret key.