RFC 989 (rfc989) - Page 2 of 23
Privacy enhancement for Internet electronic mail: Part I: Message encipherment and authentication procedures
Alternative Format: Original Text Document
RFC 989 February 1987
2 Terminology
For descriptive purposes, this RFC uses some terms defined in the OSI
X.400 Message Handling System Model. This section replicates a
portion of 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 Model.
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 User
Agent. A User Agent (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
intended recipients.
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.
3 Services, Constraints, and Implications
This RFC's goal is to define 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 sender and recipient UAs. No privacy
enhancements are offered for message fields which are added or
transformed by intermediate relay points. Two distinct privacy
enhancement service options are supported:
1. an option providing sender authentication and integrity
verification
2. an option providing sender authentication and integrity
verification in addition to confidentiality service through
encryption
No facility for confidentiality service in the absence of
authentication is provided. Encryption and authentication facilities
may be applied selectively to portions of a message's contents; this
allows less sensitive portions of messages (e.g., descriptive fields)
Linn, Privacy Task Force