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RFC 3156 - MIME Security with OpenPGP


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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   (including the since withdrawn "application/pgp" content type) prior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".

   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
      avoid compatibility problems with PGP implementations which
      predate the OpenPGP specification.

      Note: If any line begins with the string "From ", it is strongly
      suggested that either the Quoted-Printable or Base64 MIME encoding
      be applied.  If Quoted-Printable is used, at least one of the
      characters in the string should be encoded using the hexadecimal
      coding rule.  This is because many mail transfer and delivery
      agents treat "From " (the word "from" followed immediately by a
      space character) as the start of a new message and thus insert a
      right angle-bracket (>) in front of any line beginning with
      "From " to distinguish this case, invalidating the signature.

   Data that is ONLY to be encrypted is allowed to contain 8-bit
   characters and trailing whitespace and therefore need not undergo the
   conversion to a 7bit format, and the stripping of whitespace.

      Implementor's note: It cannot be stressed enough that applications
      using this standard follow MIME's suggestion that you "be
      conservative in what you generate, and liberal in what you
      accept."  In this particular case it means it would be wise for an
      implementation to accept messages with any content-transfer-
      encoding, but restrict generation to the 7-bit format required by
      this memo.  This will allow future compatibility in the event the
      Internet SMTP framework becomes 8-bit friendly.

4.  OpenPGP encrypted data

   Before OpenPGP encryption, the data is written in MIME canonical
   format (body and headers).

   OpenPGP encrypted data is denoted by the "multipart/encrypted"
   content type, described in [2], and MUST have a "protocol" parameter
   value of "application/pgp-encrypted".  Note that the value of the
   parameter MUST be enclosed in quotes.

   The multipart/encrypted MIME body MUST consist of exactly two body
   parts, the first with content type "application/pgp-encrypted".  This
   body contains the control information.  A message complying with this
   standard MUST contain a "Version: 1" field in this body.  Since the
   OpenPGP packet format contains all other information necessary for
   decrypting, no other information is required here.

   The second MIME body part MUST contain the actual encrypted data.  It
   MUST be labeled with a content type of "application/octet-stream".

   Example message:

      From: Michael Elkins <elkins@aero.org>
      To: Michael Elkins <elkins@aero.org>
      Mime-Version: 1.0

      Content-Type: multipart/encrypted; boundary=foo;
         protocol="application/pgp-encrypted"

      --foo
      Content-Type: application/pgp-encrypted

      Version: 1

      --foo
      Content-Type: application/octet-stream

      -----BEGIN PGP MESSAGE-----
      Version: 2.6.2

      hIwDY32hYGCE8MkBA/wOu7d45aUxF4Q0RKJprD3v5Z9K1YcRJ2fve87lMlDlx4Oj
      eW4GDdBfLbJE7VUpp13N19GL8e/AqbyyjHH4aS0YoTk10QQ9nnRvjY8nZL3MPXSZ
      g9VGQxFeGqzykzmykU6A26MSMexR4ApeeON6xzZWfo+0yOqAq6lb46wsvldZ96YA
      AABH78hyX7YX4uT1tNCWEIIBoqqvCeIMpp7UQ2IzBrXg6GtukS8NxbukLeamqVW3
      1yt21DYOjuLzcMNe/JNsD9vDVCvOOG3OCi8=
      =zzaA
      -----END PGP MESSAGE-----

      --foo--

5.  OpenPGP signed data

   OpenPGP signed messages are denoted by the "multipart/signed" content
   type, described in [2], with a "protocol" parameter which MUST have a
   value of "application/pgp-signature" (MUST be quoted).

   The "micalg" parameter for the "application/pgp-signature" protocol
   MUST contain exactly one hash-symbol of the format "pgp-<hash-
   identifier>", where <hash-identifier> identifies the Message
   Integrity Check (MIC) algorithm used to generate the signature.
   Hash-symbols are constructed from the text names registered in [1] or
   according to the mechanism defined in that document by converting the
   text name to lower case and prefixing it with the four characters
   "pgp-".

   Currently defined values are "pgp-md5", "pgp-sha1", "pgp-ripemd160",
   "pgp-md2", "pgp-tiger192", and "pgp-haval-5-160".

   The multipart/signed body MUST consist of exactly two parts.  The
   first part contains the signed data in MIME canonical format,
   including a set of appropriate content headers describing the data.

   The second body MUST contain the OpenPGP digital signature.  It MUST
   be labeled with a content type of "application/pgp-signature".

      Note: Implementations can either generate "signatures of a
      canonical text document" or "signatures of a binary document", as
      defined in [1].  The restrictions on the signed material put forth
      in section 3 and in this section will make sure that the various
      MIC algorithm variants specified in [1] and [5] will all produce
      the same result.

   When the OpenPGP digital signature is generated:

   (1)   The data to be signed MUST first be converted to its content-
         type specific canonical form.  For text/plain, this means
         conversion to an appropriate character set and conversion of
         line endings to the canonical <CR><LF> sequence.

   (2)   An appropriate Content-Transfer-Encoding is then applied; see
         section 3.  In particular, line endings in the encoded data
         MUST use the canonical <CR><LF> sequence where appropriate
         (note that the canonical line ending may or may not be present
         on the last line of encoded data and MUST NOT be included in
         the signature if absent).

   (3)   MIME content headers are then added to the body, each ending
         with the canonical <CR><LF> sequence.

   (4)   As described in section 3 of this document, any trailing
         whitespace MUST then be removed from the signed material.

   (5)   As described in [2], the digital signature MUST be calculated
         over both the data to be signed and its set of content headers.

   (6)   The signature MUST be generated detached from the signed data
         so that the process does not alter the signed data in any way.

      Note: The accepted OpenPGP convention is for signed data to end
      with a <CR><LF> sequence.  Note that the <CR><LF> sequence
      immediately preceding a MIME boundary delimiter line is considered
      to be part of the delimiter in [3], 5.1.  Thus, it is not part of
      the signed data preceding the delimiter line.  An implementation
      which elects to adhere to the OpenPGP convention has to make sure
      it inserts a <CR><LF> pair on the last line of the data to be
      signed and transmitted (signed message and transmitted message
      MUST be identical).

   Example message:

         From: Michael Elkins <elkins@aero.org>
         To: Michael Elkins <elkins@aero.org>
         Mime-Version: 1.0

         Content-Type: multipart/signed; boundary=bar; micalg=pgp-md5;
           protocol="application/pgp-signature"

         --bar
      & Content-Type: text/plain; charset=iso-8859-1
      & Content-Transfer-Encoding: quoted-printable
      &
      & =A1Hola!
      &
      & Did you know that talking to yourself is a sign of senility?
      &
      & It's generally a good idea to encode lines that begin with
      & From=20because some mail transport agents will insert a greater-
      & than (>) sign, thus invalidating the signature.
      &
      & Also, in some cases it might be desirable to encode any   =20
      & trailing whitespace that occurs on lines in order to ensure  =20
      & that the message signature is not invalidated when passing =20
      & a gateway that modifies such whitespace (like BITNET). =20
      &
      & me

      --bar

      Content-Type: application/pgp-signature

      -----BEGIN PGP MESSAGE-----
      Version: 2.6.2

      iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
      jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
      uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
      HOxEa44b+EI=
      =ndaj
      -----END PGP MESSAGE-----

      --bar--

   The "&"s in the previous example indicate the portion of the data
   over which the signature was calculated.

   Upon receipt of a signed message, an application MUST:

   (1)   Convert line endings to the canonical <CR><LF> sequence before
         the signature can be verified.  This is necessary since the
         local MTA may have converted to a local end of line convention.

   (2)   Pass both the signed data and its associated content headers
         along with the OpenPGP signature to the signature verification
         service.

6.  Encrypted and Signed Data

   Sometimes it is desirable to both digitally sign and then encrypt a
   message to be sent.  This protocol allows for two methods of
   accomplishing this task.

6.1.  RFC 1847 Encapsulation

   In [2], it is stated that the data is first signed as a
   multipart/signature body, and then encrypted to form the final
   multipart/encrypted body.  This is most useful for standard MIME-
   compliant message forwarding.

   Example:

         Content-Type: multipart/encrypted;
            protocol="application/pgp-encrypted"; boundary=foo

         --foo
         Content-Type: application/pgp-encrypted

         Version: 1

         --foo
         Content-Type: application/octet-stream

         -----BEGIN PGP MESSAGE-----
      & Content-Type: multipart/signed; micalg=pgp-md5
      &     protocol="application/pgp-signature"; boundary=bar
      &
      & --bar
      & Content-Type: text/plain; charset=us-ascii
      &
      & This message was first signed, and then encrypted.
      &
      & --bar
      & Content-Type: application/pgp-signature
      &
      & -----BEGIN PGP MESSAGE-----
      & Version: 2.6.2
      &
      & iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
      & jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
      & uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn

      & HOxEa44b+EI=
      & =ndaj
      & -----END PGP MESSAGE-----
      &
      & --bar--
        -----END PGP MESSAGE-----

        --foo--

   (The text preceded by '&' indicates that it is really encrypted, but
   presented as text for clarity.)

6.2.  Combined method

   The OpenPGP packet format [1] describes a method for signing and
   encrypting data in a single OpenPGP message.  This method is allowed
   in order to reduce processing overhead and increase compatibility
   with non-MIME implementations of OpenPGP.  The resulting data is
   formatted as a "multipart/encrypted" object as described in Section
   4.

   Messages which are encrypted and signed in this combined fashion are
   REQUIRED to follow the same canonicalization rules as
   multipart/signed objects.

   It is explicitly allowed for an agent to decrypt a combined message
   and rewrite it as a multipart/signed object using the signature data
   embedded in the encrypted version.

7.  Distribution of OpenPGP public keys

   Content-Type: application/pgp-keys
   Required parameters: none
   Optional parameters: none

   A MIME body part of the content type "application/pgp-keys" contains
   ASCII-armored transferable Public Key Packets as defined in [1],
   section 10.1.

8.  Security Considerations

   Signatures of a canonical text document as defined in [1] ignore
   trailing white space in signed material.  Implementations which
   choose to use signatures of canonical text documents will not be able
   to detect the addition of whitespace in transit.

   See [3], [4] for more information on the security considerations
   concerning the underlying protocols.

9.  IANA Considerations

   This document defines three media types: "application/pgp-encrypted",
   "application/pgp-signature" and "application/pgp-keys".  The
   following sections specify the IANA registrations for these types.

9.1.  Registration of the application/pgp-encrypted media type

   MIME media type name: application
   MIME subtype name: pgp-encrypted
   Required parameters: none
   Optional parameters: none

   Encoding considerations:

      Currently this media type always consists of a single 7bit text
      string.

   Security considerations:

      See Section 8 and RFC 2440 Section 13.

   Interoperability considerations: none

   Published specification:

      This document.

   Additional information:

      Magic number(s): none
      File extension(s): none
      Macintosh File Type Code(s): none

   Person & email address to contact for further information:

      Michael Elkins
      Email: me@cs.hmc.edu

   Intended usage: common

   Author/Change controller:

      Michael Elkins
      Email: me@cs.hmc.edu

9.2.  Registration of the application/pgp-signature media type

   MIME media type name: application
   MIME subtype name: pgp-signature
   Required parameters: none
   Optional parameters: none

   Encoding considerations:

      The content of this media type always consists of 7bit text.

   Security considerations:

      See Section 8 and RFC 2440 Section 13.

   Interoperability considerations: none

   Published specification:

      RFC 2440 and this document.

   Additional information:

      Magic number(s): none
      File extension(s): asc, sig
      Macintosh File Type Code(s): pgDS

   Person & email address to contact for further information:

      Michael Elkins
      Email: me@cs.hmc.edu

   Intended usage: common

   Author/Change controller:

      Michael Elkins
      Email: me@cs.hmc.edu

9.3.  Registration of the application/pgp-keys media type

   MIME media type name: application
   MIME subtype name: pgp-keys
   Required parameters: none
   Optional parameters: none

   Encoding considerations:

      The content of this media type always consists of 7bit text.

   Security considerations:

      See Section 8 and RFC 2440 Section 13.

   Interoperability considerations: none

   Published specification:

      RFC 2440 and this document.

   Additional information:

      Magic number(s): none
      File extension(s): asc
      Macintosh File Type Code(s): none

   Person & email address to contact for further information:

      Michael Elkins
      Email: me@cs.hmc.edu

   Intended usage: common

   Author/Change controller:

      Michael Elkins
      Email: me@cs.hmc.edu

10.  Notes

   "PGP" and "Pretty Good Privacy" are registered trademarks of Network
   Associates, Inc.

11.  Acknowledgements

   This document relies on the work of the IETF's OpenPGP Working
   Group's definitions of the OpenPGP Message Format.  The OpenPGP
   message format is currently described in RFC 2440 [1].

   Special thanks are due: to Philip Zimmermann for his original and
   ongoing work on PGP; to Charles Breed, Jon Callas and Dave Del Torto
   for originally proposing the formation of the OpenPGP Working Group;
   and to Steve Schoenfeld for helpful feedback during the draft
   process.  The authors would also like to thank the engineers at
   Pretty Good Privacy, Inc (now Network Associates, Inc), including
   Colin Plumb, Hal Finney, Jon Callas, Mark Elrod, Mark Weaver and
   Lloyd Chambers, for their technical commentary.

   Additional thanks are due to Jeff Schiller and Derek Atkins for their
   continuing support of strong cryptography and PGP freeware at MIT; to
   Rodney Thayer of Sable Technology; to John Noerenberg, Steve Dorner
   and Laurence Lundblade of the Eudora team at QUALCOMM, Inc; to Bodo
   Moeller for proposing the approach followed with respect to trailing
   whitespace; to John Gilmore, Hugh Daniel and Fred Ringel (at
   Rivertown) and Ian Bell (at Turnpike) for their timely critical
   commentary; and to the international members of the IETF's OpenPGP
   mailing list, including William Geiger, Lutz Donnerhacke and Kazu
   Yamamoto.  The idea to use multipart/mixed with multipart/signed has
   been attributed to James Galvin.  Finally, our gratitude is due to
   the many members of the "Cypherpunks," "Coderpunks" and "pgp-users"
   <http://cryptorights.org/pgp-users> mailing lists and the many users
   of PGP worldwide for helping keep the path to privacy open.

12.  Addresses of the Authors and OpenPGP Working Group Chair

   The OpenPGP working group can be contacted via the current chair:

   John W. Noerenberg II
   Qualcomm, Inc.
   5775 Morehouse Dr.
   San Diego, CA 92121 USA

   Phone: +1 619 658 3510
   EMail: jwn2@qualcomm.com

   The principal authors of this document are:

   Dave Del Torto
   CryptoRights Foundation
   80 Alviso Street, Mailstop: CRF
   San Francisco, CA 94127 USA

   Phone: +1.415.334.5533, vm: #2
   EMail: ddt@cryptorights.org, ddt@openpgp.net

   Michael Elkins
   Network Associates, Inc.
   3415 S. Sepulveda Blvd Suite 700
   Los Angeles, CA 90034 USA

   Phone: +1.310.737.1663
   Fax:   +1.310.737.1755
   Email: me@cs.hmc.edu, Michael_Elkins@NAI.com

   Raph Levien
   University of California at Berkeley
   579 Soda Hall
   Berkeley, CA 94720 USA

   Phone: +1.510.642.6509
   EMail: raph@acm.org

   Thomas Roessler
   Nordstrasse 99
   D-53111 Bonn, Germany

   Phone: +49-228-638007
   EMail: roessler@does-not-exist.org

References

   [1]   Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
         Message Format", RFC 2440, November 1998.

   [2]   Galvin, J., Murphy, G., Crocker, S. and N. Freed, "Security
         Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
         RFC 1847, October 1995.

   [3]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types", RFC 2046, November
         1996.

   [4]   Galvin, J., Murphy, G., Crocker, S. and N. Freed, "MIME Object
         Security Services", RFC 1848, October 1995.

   [5]   Atkins, D., Stallings, W. and P. Zimmermann, "PGP Message
         Exchange Formats", RFC 1991, August 1996.

   [6]   Elkins, M., "MIME Security with Pretty Good Privacy (PGP)", RFC
         2015, October 1996.

   [7]   Freed, N., "Gateways and MIME Security Multiparts", RFC 2480,
         January 1999.

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