RFC 1421 (rfc1421) - Page 2 of 42
Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures
Alternative Format: Original Text Document
RFC 1421 Privacy Enhancement for Electronic Mail February 1993
endpoints or at intermediate relay sites. This approach allows
privacy enhancement facilities to be incorporated selectively on a
site-by-site or user-by-user basis without impact on other Internet
entities. Interoperability among heterogeneous components and mail
transport facilities is supported.
The current specification's scope is confined to PEM processing
procedures for the RFC-822 textual mail environment, and defines the
Content-Domain indicator value "RFC 822" to signify this usage.
Follow-on work in integration of PEM capabilities with other
messaging environments (e.g., MIME) is anticipated and will be
addressed in separate and/or successor documents, at which point
additional Content-Domain indicator values will be defined.
For descriptive purposes, this RFC uses some terms defined in the OSI
X.400 Message Handling System Model per the CCITT Recommendations.
This section replicates a portion of (1984) X.400's Section 2.2.1,
"Description of the MHS Model: Overview" in order to make the
terminology clear to readers who may not be familiar with the OSI MHS
In the [MHS] model, a user is a person or a computer application. A
user is referred to as either an originator (when sending a message)
or a recipient (when receiving one). MH Service elements define the
set of message types and the capabilities that enable an originator
to transfer messages of those types to one or more recipients.
An originator prepares messages with the assistance of his or her
User Agent (UA). A UA is an application process that interacts with
the Message Transfer System (MTS) to submit messages. The MTS
delivers to one or more recipient UAs the messages submitted to it.
Functions performed solely by the UA and not standardized as part of
the MH Service elements are called local UA functions.
The MTS is composed of a number of Message Transfer Agents (MTAs).
Operating together, the MTAs relay messages and deliver them to the
intended recipient UAs, which then make the messages available to the
The collection of UAs and MTAs is called the Message Handling System
(MHS). The MHS and all of its users are collectively referred to as
the Message Handling Environment.