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Network Working Group                                          M. ElkinsRequest for Comments: 3156                      Network Associates, Inc.Updates:2015                                               D. Del TortoCategory: Standards Track                        CryptoRights Foundation                                                               R. Levien                                    University of California at Berkeley                                                             T. Roessler                                                             August 2001MIME Security with OpenPGPStatus 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 inRFC 1847.1.  Introduction   Work on integrating PGP (Pretty Good Privacy) with MIME [3]   (including the since withdrawn "application/pgp" content type) prior   toRFC 2015 suffered from a number of problems, the most significant   of which is the inability to recover signed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 inRFC 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 revisesRFC 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 inRFC 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 (seesection 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 embeddedRFC 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.Elkins, et al.              Standards Track                     [Page 2]

RFC 3156               MIME Security with OpenPGP            August 2001      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: 1.0Elkins, et al.              Standards Track                     [Page 3]

RFC 3156               MIME Security with OpenPGP            August 2001      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".Elkins, et al.              Standards Track                     [Page 4]

RFC 3156               MIME Security with OpenPGP            August 2001      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      insection 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; seesection 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 insection 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: 1.0Elkins, et al.              Standards Track                     [Page 5]

RFC 3156               MIME Security with OpenPGP            August 2001         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.Elkins, et al.              Standards Track                     [Page 6]

RFC 3156               MIME Security with OpenPGP            August 2001   (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/zhdfolT9BrnElkins, et al.              Standards Track                     [Page 7]

RFC 3156               MIME Security with OpenPGP            August 2001      & 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 inSection4.   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.Elkins, et al.              Standards Track                     [Page 8]

RFC 3156               MIME Security with OpenPGP            August 20019.  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:      SeeSection 8 andRFC 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.eduElkins, et al.              Standards Track                     [Page 9]

RFC 3156               MIME Security with OpenPGP            August 20019.2.  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:      SeeSection 8 andRFC 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.edu9.3.  Registration of the application/pgp-keys media type   MIME media type name: application   MIME subtype name: pgp-keys   Required parameters: none   Optional parameters: noneElkins, et al.              Standards Track                    [Page 10]

RFC 3156               MIME Security with OpenPGP            August 2001   Encoding considerations:      The content of this media type always consists of 7bit text.   Security considerations:      SeeSection 8 andRFC 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.eduElkins, et al.              Standards Track                    [Page 11]

RFC 3156               MIME Security with OpenPGP            August 200110.  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 inRFC 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.Elkins, et al.              Standards Track                    [Page 12]

RFC 3156               MIME Security with OpenPGP            August 200112.  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.orgElkins, et al.              Standards Track                    [Page 13]

RFC 3156               MIME Security with OpenPGP            August 2001References   [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)",RFC2015, October 1996.   [7]   Freed, N., "Gateways and MIME Security Multiparts",RFC 2480,         January 1999.Elkins, et al.              Standards Track                    [Page 14]

RFC 3156               MIME Security with OpenPGP            August 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Elkins, et al.              Standards Track                    [Page 15]