1. A method to secure communication of data via a messaging system across a computer network, comprising: accessing a certificate at a first messaging client, the certificate sent by a second messaging client via a messaging server, wherein the certificate associates a screen name with a public key value, the screen name being associated with a second user of the second messaging client, wherein a first user of the first messaging client is associated to the screen name of the second user by being listed on at least one list of preferred users of the second user as determined by the messaging server; encrypting an electronic message using the public key value associated with the screen name of the second user; and sending the encrypted message to the messaging server to be sent to the second messaging client.