Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures
RFC 1421

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Network Working Group                                            J. Linn
Request for Comments: 1421                    IAB IRTF PSRG, IETF PEM WG
Obsoletes: 1113                                            February 1993

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

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Acknowledgements

   This document is the outgrowth of a series of meetings of the Privacy
   and Security Research Group (PSRG) of the IRTF and the PEM Working
   Group of the IETF.  I would like to thank the members of the PSRG and
   the IETF PEM WG, as well as all participants in discussions on the
   "pem-dev@tis.com" mailing list, for their contributions to this
   document.

1.  Executive Summary

   This document defines message encryption and authentication
   procedures, in order to provide privacy-enhanced mail (PEM) services
   for electronic mail transfer in the Internet.  It is intended to
   become one member of a related set of four RFCs.  The procedures
   defined in the current document are intended to be compatible with a
   wide range of key management approaches, including both symmetric
   (secret-key) and asymmetric (public-key) approaches for encryption of
   data encrypting keys.  Use of symmetric cryptography for message text
   encryption and/or integrity check computation is anticipated. RFC
   1422 specifies supporting key management mechanisms based on the use
   of public-key certificates.  RFC 1423 specifies algorithms, modes,
   and associated identifiers relevant to the current RFC and to RFC
   1422.  RFC 1424 provides details of paper and electronic formats and
   procedures for the key management infrastructure being established in
   support of these services.

   Privacy enhancement services (confidentiality, authentication,
   message integrity assurance, and non-repudiation of origin) are
   offered through the use of end-to-end cryptography between originator
   and recipient processes at or above the User Agent level.  No special
   processing requirements are imposed on the Message Transfer System at

Linn                                                            [Page 1]
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 "RFC822" 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.

2.  Terminology

   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
   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 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
   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.

Linn                                                            [Page 2]