MIME Security with OpenPGP
RFC 3156

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Network Working Group                                          M. Elkins
Request for Comments: 3156                      Network Associates, Inc.
Updates: 2015                                               D. Del Torto
Category: Standards Track                        CryptoRights Foundation
                                                               R. Levien
                                    University of California at Berkeley
                                                             T. Roessler
                                                             August 2001

                       MIME Security with OpenPGP

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document describes how the OpenPGP Message Format can be used to
   provide privacy and authentication using the Multipurpose Internet
   Mail Extensions (MIME) security content types described in RFC 1847.

1.  Introduction

   Work on integrating PGP (Pretty Good Privacy) with MIME [3]
   (including the since withdrawn "application/pgp" content type) prior
   to RFC 2015 suffered from a number of problems, the most significant
   of which is the inability to recover signed message bodies without
   parsing data structures specific to PGP.  RFC 2015 makes use of the
   elegant solution proposed in RFC 1847, which defines security
   multipart formats for MIME.  The security multiparts clearly separate
   the signed message body from the signature, and have a number of
   other desirable properties.  This document revises RFC 2015 to adopt
   the integration of PGP and MIME to the needs which emerged during the
   work on the OpenPGP specification.

   This document defines three content types for implementing security
   and privacy with OpenPGP: "application/pgp-encrypted",
   "application/pgp-signature" and "application/pgp-keys".

Elkins, et al.              Standards Track                     [Page 1]
RFC 3156               MIME Security with OpenPGP            August 2001

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

2.  OpenPGP data formats

   OpenPGP implementations can generate either ASCII armor (described in
   [1]) or 8-bit binary output when encrypting data, generating a
   digital signature, or extracting public key data.  The ASCII armor
   output is the REQUIRED method for data transfer.  This allows those
   users who do not have the means to interpret the formats described in
   this document to be able to extract and use the OpenPGP information
   in the message.

   When the amount of data to be transmitted requires that it be sent in
   many parts, the MIME message/partial mechanism SHOULD be used rather
   than the multi-part ASCII armor OpenPGP format.

3.  Content-Transfer-Encoding restrictions

   Multipart/signed and multipart/encrypted are to be treated by agents
   as opaque, meaning that the data is not to be altered in any way [2],
   [7].  However, many existing mail gateways will detect if the next
   hop does not support MIME or 8-bit data and perform conversion to
   either Quoted-Printable or Base64.  This presents serious problems
   for multipart/signed, in particular, where the signature is
   invalidated when such an operation occurs.  For this reason all data
   signed according to this protocol MUST be constrained to 7 bits (8-
   bit data MUST be encoded using either Quoted-Printable or Base64).
   Note that this also includes the case where a signed object is also
   encrypted (see section 6).  This restriction will increase the
   likelihood that the signature will be valid upon receipt.

   Additionally, implementations MUST make sure that no trailing
   whitespace is present after the MIME encoding has been applied.

      Note: In most cases, trailing whitespace can either be removed, or
      protected by applying an appropriate content-transfer-encoding.
      However, special care must be taken when any header lines - either
      in MIME entity headers, or in embedded RFC 822 headers - are
      present which only consist of whitespace: Such lines must be
      removed entirely, since replacing them by empty lines would turn
      them into header delimiters, and change the semantics of the
      message.  The restrictions on whitespace are necessary in order to
      make the hash calculated invariant under the text and binary mode
      signature mechanisms provided by OpenPGP [1].  Also, they help to