RFC 1421 (rfc1421) - Page 3 of 42

Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures

RFC 1421        Privacy Enhancement for Electronic Mail    February 1993


3.  Services, Constraints, and Implications

   This RFC defines mechanisms to enhance privacy for electronic mail
   transferred in the Internet. The facilities discussed in this RFC
   provide privacy enhancement services on an end-to-end basis between
   originator and recipient processes residing at the UA level or above.
   No privacy enhancements are offered for message fields which are
   added or transformed by intermediate relay points between PEM
   processing components.

   If an originator elects to perform PEM processing on an outbound
   message, all PEM-provided security services are applied to the PEM
   message's body in its entirety; selective application to portions of
   a PEM message is not supported. Authentication, integrity, and (when
   asymmetric key management is employed) non-repudiation of origin
   services are applied to all PEM messages; confidentiality services
   are optionally selectable.

   In keeping with the Internet's heterogeneous constituencies and usage
   modes, the measures defined here are applicable to a broad range of
   Internet hosts and usage paradigms.  In particular, it is worth
   noting the following attributes:

        1.  The mechanisms defined in this RFC are not restricted to a
            particular host or operating system, but rather allow
            interoperability among a broad range of systems.  All
            privacy enhancements are implemented at the application
            layer, and are not dependent on any privacy features at
            lower protocol layers.

        2.  The defined mechanisms are compatible with non-enhanced
            Internet components.  Privacy enhancements are implemented
            in an end-to-end fashion which does not impact mail
            processing by intermediate relay hosts which do not
            incorporate privacy enhancement facilities.  It is
            necessary, however, for a message's originator to be
            cognizant of whether a message's intended recipient
            implements privacy enhancements, in order that encoding and
            possible encryption will not be performed on a message whose
            destination is not equipped to perform corresponding inverse
            transformations.  (Section 4.6.1.1.3 of this RFC describes a
            PEM message type ("MIC-CLEAR") which represents a signed,
            unencrypted PEM message in a form readable without PEM
            processing capabilities yet validatable by PEM-equipped
            recipients.)

        3.  The defined mechanisms are compatible with a range of mail
            transport facilities (MTAs).  Within the Internet,



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