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Network Working Group                                          M. ElkinsRequest for Comments: 2015                     The Aerospace CorporationCategory: Standards Track                                   October 1996MIME Security with Pretty Good Privacy (PGP)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.Abstract   This document describes how Pretty Good Privacy (PGP) can be used to   provide privacy and authentication using the Multipurpose Internet   Mail Extensions (MIME) security content types described inRFC1847.1.  Introduction   Previous work on integrating PGP with MIME (including the since   withdrawn application/pgp content type) has suffered from a number of   problems, the most significant of which is the inability to recover   signed message bodies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.  This work makes use of the elegant solution proposed inRFC1847, 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 is styled afterRFC 1848, which defines MIME Object Security   Services (MOSS) for providing security and authentication.   This document defines three new content types for implementing   security and privacy with PGP: application/pgp-encrypted,   application/pgp-signature and application/pgp-keys.1.1  Compliance   In order for an implementation to be compliant with this   specification, is it absolutely necessary for it to obey all items   labeled as MUST or REQUIRED.Elkins                      Standards Track                     [Page 1]

RFC 2015                 MIME Security with PGP             October 19962.  PGP data formats   PGP can generate either ASCII armor (described in [3]) 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 extract and use the PGP 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 multipart ASCII armor PGP 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 [1].   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   should 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.   Data that is ONLY to be encrypted is allowed to contain 8-bit   characters and therefore need not be converted to a 7-bit format.     Implementor's note: It cannot be stressed enough that applications     using this standard should 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.  PGP encrypted data   Before encryption with PGP, the data should be written in MIME   canonical format (body and headers).   PGP encrypted data is denoted by the "multipart/encrypted" content   type, described in [1], and MUST have a "protocol" parameter value ofElkins                      Standards Track                     [Page 2]

RFC 2015                 MIME Security with PGP             October 1996   "application/pgp-encrypted".  Note that the value of the parameter   MUST be enclosed in quotes.   The multipart/encrypted MUST consist of exactly two parts.  The first   MIME body part must have a content type of "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 PGP 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.  PGP signed data   PGP signed messages are denoted by the "multipart/signed" content   type, described in [1], with a "protocol" parameter which MUST have a   value of "application/pgp-signature" (MUST be quoted).  The "micalg"Elkins                      Standards Track                     [Page 3]

RFC 2015                 MIME Security with PGP             October 1996   parameter MUST have a value of "pgp-<hash-symbol>", where <hash-   symbol> identifies the message integrity check (MIC) used to generate   the signature.  The currently defined values for <hash-symbol> are   "md5" for the MD5 checksum, and "sha1" for the SHA.1 algorithm.   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 PGP digital signature.  It MUST be   labeled with a content type of "application/pgp-signature".   When the PGP digital signature is generated:   (1)  The data to be signed must first be converted to its        type/subtype 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. Each        line of the encoded data MUST end with the canonical <CR><LF>        sequence.   (3)  MIME content headers are then added to the body, each ending        with the canonical <CR><LF> sequence.   (4)  As described in [1], the digital signature MUST be calculated        over both the data to be signed and its set of content headers.   (5)  The signature MUST be generated detached from the signed data        so that the process does not alter the signed data in any way.   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?     &Elkins                      Standards Track                     [Page 4]

RFC 2015                 MIME Security with PGP             October 1996     & 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     &railing 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.   Though not required, it is generally a good idea to use Quoted-   Printable encoding in the first step (writing out the data to be   signed in MIME canonical format) if any of the lines in the data   begin with "From ", and encode the "F".  This will avoid an MTA   inserting a ">" in front of the line, thus invalidating the   signature!   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 PGP signature to the signature verification        service.Elkins                      Standards Track                     [Page 5]

RFC 2015                 MIME Security with PGP             October 19966.  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.6.1RFC1847 Encapsulation   [1], it is stated that the data should first be signed as a   multipart/signature body, and then encrypted to form the final   multipart/encrypted body, i.e.,    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-----Elkins                      Standards Track                     [Page 6]

RFC 2015                 MIME Security with PGP             October 1996    --foo--    (The text preceded by '&' indicates that it is really    encrypted, but presented as text for clarity.)6.2  Combined method   Versions 2.x of PGP also allow data to be signed and encrypted in one   operation.  This method is an acceptable shortcut, and has the   benefit of less overhead.  The resulting data should be formed as a   "multipart/encrypted" object as described above.   Messages which are encrypted and signed in this combined fashion are   REQUIRED to follow the same canonicalization rules as for   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 PGP public keys   Content-Type: application/pgp-keys   Required parameters: none   Optional parameters: none   This is the content type which should be used for relaying public key   blocks.8.  Notes   PGP and Pretty Good Privacy are trademarks of Philip Zimmermann.9.  Security Considerations   Use of this protocol has the same security considerations as PGP, and   is not known to either increase or decrease the security of messages   using it; see [3] for more information.10.  Author's Address        Michael Elkins        P.O. Box 92957 - M1/102        Los Angeles, CA 90009-2957        Phone: +1 310 336 8040        Fax: +1 310 336 4402Elkins                      Standards Track                     [Page 7]

RFC 2015                 MIME Security with PGP             October 1996References   [1]  Galvin, J., Murphy, G., Crocker, S., and N. Freed, "Security        Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",RFC 1847, October 1995.   [2]  Galvin, J., Murphy, G., Crocker, S., and N. Freed, "MIME Object        Security Services",RFC 1848, October 1995.   [3]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message        Exchange Formats",RFC 1991, August 1996.Elkins                      Standards Track                     [Page 8]