A secure instant messaging (IM) system integrates secure instant messaging into existing instant messaging systems. A certificate authority (CA) issues security certificates to users binding the user's IM screen name to a public key, used by sending users to encrypt messages and files for the user. The CA uses a subscriber database to keep track of valid users and associated information, e.g. user screen names, user subscription expiration dates, and enrollment agent information. A user sends his certificate to an instant messaging server which publishes the user's certificate to other users. Users encrypt instant messages and files using an encryption algorithm and the recipient's certificate. A sending user can sign instant messages using his private signing key. The security status of received messages is displayed to recipients.