Network Working Group M. Elkins
Request for Comments: 2015 The Aerospace Corporation
Category: Standards Track October 1996

MIME 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 in RFC1847.

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withdrawn application/pgp content type) has suffered from a number of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 in
RFC1847, 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 after RFC1848, 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.

2. 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 (see section 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 of

"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"

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?
&

& 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.

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 RFC1847 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-----

--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 4402

References

[1] Galvin, J., Murphy, G., Crocker, S., and N. Freed, "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC1847, October 1995.

[2] Galvin, J., Murphy, G., Crocker, S., and N. Freed, "MIME Object
Security Services", RFC1848, October 1995.

[3] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC1991, August 1996.