Privacy enhancement for Internet electronic mail: Part I: Message encipherment and authentication procedures
RFC 1040

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Network Working Group                                    J. Linn (BBNCC)
Request for Comments: 1040                        IAB Privacy Task Force
Obsoletes RFCs: 989                                         January 1988

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

STATUS OF THIS MEMO

   This RFC suggests a proposed protocol for the Internet community, and
   requests discussion and suggestions for improvements.  Distribution
   of this memo is unlimited.

ACKNOWLEDGMENT

   This RFC is the outgrowth of a series of IAB Privacy Task Force
   meetings and of internal working papers distributed for those
   meetings.  I would like to thank the following Privacy Task Force
   members and meeting guests for their comments and contributions at
   the meetings which led to the preparation of this RFC:  David
   Balenson, Curt Barker, Matt Bishop, Danny Cohen, Tom Daniel, Charles
   Fox, Morrie Gasser, Steve Kent (chairman), John Laws, Steve Lipner,
   Dan Nessett, Mike Padlipsky, Rob Shirey, Miles Smid, Steve Walker,
   and Steve Wilbur.

1.  Executive Summary

   This RFC defines message encipherment and authentication procedures,
   as the initial phase of an effort to provide privacy enhancement
   services for electronic mail transfer in the Internet.  Detailed key
   management mechanisms to support these procedures will be defined in
   a subsequent RFC.  As a goal of this initial phase, it is intended
   that the procedures defined here be compatible with a wide range of
   key management approaches, including both conventional (symmetric)
   and public-key (asymmetric) approaches for encryption of data
   encrypting keys.  Use of conventional cryptography for message text
   encryption and/or integrity check computation is anticipated.

   Privacy enhancement services (confidentiality, authentication, and
   message integrity assurance) are offered through the use of
   end-to-end cryptography between originator and recipient User Agent
   processes, with no special processing requirements imposed on the
   Message Transfer System at endpoints or at intermediate relay
   sites.  This approach allows privacy enhancement facilities to be
   incorporated on a site-by-site or user-by-user basis without impact
   on other Internet entities.  Interoperability among heterogeneous

Linn                                                            [Page 1]
RFC 1040        Privacy Enhancement for Electronic Mail     January 1988

   components and mail transport facilities is supported.

2.  Terminology

   For descriptive purposes, this RFC uses some terms defined in the OSI
   X.400 Message Handling System Model per the 1984 CCITT
   Recommendations.  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 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
   sender and recipient UAs.  No privacy enhancements are offered for
   message fields which are added or transformed by intermediate relay
   points.

   Authentication and integrity facilities are always applied to the
   entirety of a message's text.  No facility for confidentiality
   service without authentication is provided.  Encryption facilities
   may be applied selectively to portions of a message's contents; this
   allows less sensitive portions of messages (e.g., descriptive fields)
   to be processed by a recipient's delegate in the absence of the