Abstract: A full-featured e-mail system is used in both Internet-based and client-side (personal computer) forms. In each form, either basic e-mail service is provided to system subscribers or a secure, premium service with authentication, concealment, integrity, and non-repudiation functions for electronic messaging services is provided. In either form and at either level of service, subscribers can work off-line on their own computers with proprietary software loaded or, alternatively, on-line on any computer with an Internet connection. The system is interoperable, to preserve security, with all S/MIME compliant software applications, even for those users not subscribing to a service implementing the disclosed system. Digital certificates can be provided as a security service of the disclosed system, rather than requiring a second source with separate verification procedures.