pretty Easy privacy (pEp): Header Protection
draft-luck-lamps-pep-header-protection-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
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|
|---|---|---|---|
| Authors | Claudio Luck , Bernie Hoeneisen | ||
| Last updated | 2019-03-08 | ||
| RFC stream | (None) | ||
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| Additional resources | |||
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draft-luck-lamps-pep-header-protection-00
Network Working Group C. Luck
Internet-Draft pEp Foundation
Intended status: Informational B. Hoeneisen
Expires: September 9, 2019 Ucom.ch
March 08, 2019
pretty Easy privacy (pEp): Header Protection
draft-luck-lamps-pep-header-protection-00
Abstract
Issues with email header protection in S/MIME have been recently
raised in the IETF LAMPS Working Group. The need for amendments to
the existing specification regarding header protection was expressed.
The pretty Easy privacy (pEp) implementations currently use a
mechanism quite similar to the currently standardized message
wrapping for S/MIME. The main difference is that pEp is using PGP/
MIME instead. In LAMPS also voices have been expressed, that
whatever mechanism will be choosen, it should not be limited to
S/MIME, but also applied to PGP/MIME.
This document aims to contribute to this discussion and share pEp
implementation experience with email header protection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 9, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The OpenPGP Radix-64 . . . . . . . . . . . . . . . . . . 4
2.1.1. Radix-64 in the Context of MIME Messages . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Interactions . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Protection Levels . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. General Requirements . . . . . . . . . . . . . . . . . . 7
4.1.1. Sending Side . . . . . . . . . . . . . . . . . . . . 7
4.1.2. Receiving Side . . . . . . . . . . . . . . . . . . . 7
4.2. Additional Requirements for Backward-Compatibility With
Legacy Clients Unaware of Header Protection . . . . . . . 7
4.2.1. Sending side . . . . . . . . . . . . . . . . . . . . 8
4.2.2. Receiving side . . . . . . . . . . . . . . . . . . . 8
4.3. Additional Requirements for Backward-Compatibility with
Legacy Header Protection Systems (if supported) . . . . . 8
4.3.1. Sending Side . . . . . . . . . . . . . . . . . . . . 8
4.3.2. Receiving Side . . . . . . . . . . . . . . . . . . . 8
5. Message Format for Header Protection . . . . . . . . . . . . 8
5.1. Preparing a Message for Header Protection . . . . . . . . 11
5.1.1. Requirements for the Original Message . . . . . . . . 12
5.1.2. Building the Inner Message . . . . . . . . . . . . . 12
5.1.3. Building the Outer Message for Signed Inner Messages 14
5.1.4. Building the Outer Message for Signed and Encrypted
Inner Messages . . . . . . . . . . . . . . . . . . . 15
5.1.5. S/MIME Compatibility . . . . . . . . . . . . . . . . 16
6. Candidate Header Fields for Header Protection . . . . . . . . 16
7. Stub Outside Headers . . . . . . . . . . . . . . . . . . . . 16
8. Processing Incoming Email with Protected Headers . . . . . . 17
8.1. Detecting Header Protection in Incoming Email . . . . . . 17
8.2. Resolving Conflicting Protected and Unprotected Header
Fields . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.3. Processing of Signed-only Email . . . . . . . . . . . . . 18
8.4. Incoming Filter Processing . . . . . . . . . . . . . . . 18
8.4.1. Staged Filtering of Inbound Messages . . . . . . . . 19
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8.5. Outgoing Filter Processing . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 20
10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 20
10.2. Current software implementing pEp . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Document Changelog . . . . . . . . . . . . . . . . . 23
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
A range of protocols for the protection of electronic mail (email)
exist, which allow to assess the authenticity and integrity of the
email headers section or selected header fields from the domain-level
perspective, specifically DomainKeys Identified Mail (DKIM) [RFC6376]
and Sender Policy Framework (SPF) [RFC7208] and Domain-based Message
Authentication, Reporting, and Conformance (DMARC) [RFC7489]. These
protocols, while essential to responding to a range of attacks on
email, do not offer end-to-end protection to the headers section and
are not capable of providing privacy for the information contained
therein.
Also the need for means of Data Minimization, which includes data
spareness and hiding of all information, which technically can be
hidden, has grown in importance over the past years.
End-to-end protection for the email headers section or header fields
(HF) is currently not widely implemented - neither for messages
protected by means of S/MIME nor for PGP (Pretty Good Privacy) nor
any other form. A standard exists for S/MIME since version 3.1. (cf.
[RFC5751] and [I-D.ietf-lamps-rfc5751-bis]):
In order to protect outer, non-content-related message header
fields (for instance, the "Subject", "To", "From", and "Cc"
fields), the sending client MAY wrap a full MIME message in a
message/rfc822 wrapper in order to apply S/MIME security services
to these header fields.
No mechanism for header protection has been standardized for PGP/MIME
yet.
At least two variants of header protection are known to be
implemented. A recently submitted Internet-Draft
[I-D.melnikov-lamps-header-protection] discusses the two variants and
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the challenges with header protection for S/MIME. The two variants
are referred to as:
o Option 1: Memory Hole
o Option 2: Wrapping with message/rfc822 or message/global
pEp (pretty Easy privacy) [I-D.birk-pep] for email
[I-D.marques-pep-email] already implements an option quite similar to
Option 2, adapting the S/MIME standards to PGP/MIME.
Existing implementations of pEp have also added inbound support for
"Memory Hole" referred to above as Option 1. On par with other
implementations of "Memory Hole" support for it is currently limited
to the "Subject" header field only.
Interoperability and implementation symmetry between PGP/MIME and
S/MIME is planned by pEp, but still in an early stage of development.
This document lists generic use cases and requirements for header
protection and describes the header protection implemented in "pEp
message format version 2", and how non-pEp mail clients may implement
header protection independently from other pEp standards.
2. Terms
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 [RFC2119].
2.1. The OpenPGP Radix-64
In the examples following in this section, it is a common pattern to
have a MIME encoded mail containing ("wrapping") another signed and
eventually encrypted mail. Such enclosed mails are encoded following
the OpenPGP standard, which specifies an encoding called "Radix-64",
which is 7-bit transport-encoding compatible by design.
The Radix-64 consists of a begin and an end Armor Header Line, a
stream of base64-encoded data limited to 78 characters per line plus
<CR><LF>, and an encoded CRC-24 value.
The following is an example, with some similar lines of base64 output
replaced with ellipsis:
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-----BEGIN PGP MESSAGE-----
hQIMAwusnBHN80H+AQ//cJLQLOl+6hOofKEkQJeu0wedmwt+TkzPx/sCUQ80dzLv
...
j/ES8ndDBftM5mZLzFQ2VatqB9G9cqCgiOVFs6jfTI13nPfLit9IPWRavcVIMdwt
Xd9bdvHx/ReenAk/
=7WaL
-----END PGP MESSAGE-----
To make the examples look more compact and relevant, the above will
be replaced symbolically by:
[[----- OpenPGP Radix-64 Block -----]]
2.1.1. Radix-64 in the Context of MIME Messages
Note that OpenPGP and MIME specifications overlap when a Radix-64
immediately precedes a MIME boundary. The <CR><LF> sequence
immediately preceding a MIME boundary delimiter line is considered to
be part of the delimiter in [RFC2046], 5.1. And in OpenPGP, line
endings are considered a part of the Armor Header Line for the
purposes of determining the content they delimit in [RFC4880], 6.2.
Keeping an empty line between the end Armor Header Line and the MIME
boundary line is suggested.
3. Use Cases
In the following, we show the generic use cases that need to be
addressed independently of whether S/MIME, PGP/MIME or any other
technology is used for which Header Protection (HP) is to be applied
to.
3.1. Interactions
The main interaction case for HP is:
1) Both peers (sending and receiving side) fully support HP
For backward compatibility of legacy clients - unaware of any HP -
the following intermediate interactions need to be considered as
well:
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2) The sending side fully supports HP, while the receiving side does
not support any HP
3) The sending side does not support any HP, while the receiving
side fully supports HP (trivial case)
4) Neither the sending side nor the receiving side supports any HP
(trivial case)
The following intermediate use cases may need to be considered as
well for backward compatibility with legacy HP systems, such as
S/MIME since version 3.1 (cf. [RFC5751] and
[I-D.ietf-lamps-rfc5751-bis]), in the following designated as legacy
HP:
5) The sending side fully supports HP, while the receiving side
supports legacy HP only
6) The sending side supports legacy HP only, while the receiving side
fully supports HP
7) Both peers (sending and receiving side) support legacy HP only
8) The sending side supports legacy HP only, while the receiving side
does not support any HP
9) The sending side does not support any HP, while the receiving side
supports legacy HP only (trivial case)
Note: It is to be decided whether to ensure legacy HP systems do not
conflict with any new solution for HP at all or whether (and to which
degree) backward compatibility to legacy HP systems shall be
maintained.
3.2. Protection Levels
The following protection levels need to be considered:
a) signature and encryption
b) signature only
c) encryption only [[ TODO: verify whether relevant ]]
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4. Requirements
In the following a list of requirements that need to be addressed
independently of whether S/MIME, PGP/MIME or any other technology is
used to apply HP to.
4.1. General Requirements
This subsection is listing the requirements to address use case 1)
(cf. Section 3.1).
G1: Define the format for HP for all protection levels. This includes
MIME structure, Content-Type (including charset and name),
Content-Disposition (including filename), and
Content-Transfer-Encoding. Furthermore, it must be defined, how a
public key should be included.
4.1.1. Sending Side
GS1: Define which HF (Header Fields) should or must be protected for
all protection levels.
GS2: Define which HF should or must appear in clear-text of an
encrypted email.
GS3: Define which HF should or must not appear in clear-text of an
encrypted email.
4.1.2. Receiving Side
GR1: Define which HF are displayed to the user in case of conflicting
information between the protected and unprotected headers.
4.2. Additional Requirements for Backward-Compatibility With Legacy
Clients Unaware of Header Protection
This sub-section addresses the use cases 2) - 4) (cf. Section 3.1)
B1: Depending on the solution, define a means to distinguish between
forwarded messages and encapsulated messages using new HP
mechanism.
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4.2.1. Sending side
BS1: Define how full HP support can be indicated to outgoing
messages.
BS2: Define how full HP support of the receiver can be detected or
guessed
4.2.2. Receiving side
BR1: Define how full HP support can be detected in incoming messages.
4.3. Additional Requirements for Backward-Compatibility with Legacy
Header Protection Systems (if supported)
This sub-section addresses the use cases 5) - 9) (cf. Section 3.1).
LS1: Depending on the solution, define a means to distinguish between
forwarded messages, legacy S/MIME encapsulated messages, and
encapsulated messages using new HP mechanism.
4.3.1. Sending Side
LSS1: Define how legacy HP support can be indicated to outgoing
messages.
LSS2: Define how legacy HP support of the receiver can be detected or
guessed.
4.3.2. Receiving Side
LSR1: Define how legacy HP support can be detected in incoming
messages.
5. Message Format for Header Protection
The pEp message format version 2 is designed such that a receiving
Mail User Agent (MUA), which is OpenPGP-compliant but not pEp-
compliant - and which is not implementing header protection either -,
still has built-in capability to properly decode the mail and display
all information to the user.
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No standard is currently available which enables MUAs to reliably
determine whenever a nested message/rfc822 entity is meant to
override the containing message, or if it was effectively forwarded.
pEp currently intends to implement the proposal described by
[I-D.melnikov-lamps-header-protection], 3.2, which defines a new
Content-Type header field parameter with name "forwarded", for the
MUA to distinguish between a forwarded message and a nested message
for the purpose of header protection, i.e., using "forwarded=no".
Header protecton provides both integrity and confidentiality.
Confidentiality requires the same effective key distribution
mechanism to be in-place as for integrity, such that when integrity
can be achieved, also can confidentiality. Integrity and
confidentiality SHOULD always be used together.
The pEp message format version 2 (as used by all the various pEp
implementations, cf. Section 10) is similar to what is standardized
for S/MIME in [RFC5751] and its successor
[I-D.ietf-lamps-rfc5751-bis]:
In order to protect outer, non-content-related message header
fields (for instance, the "Subject", "To", "From", and "Cc"
fields), the sending client MAY wrap a full MIME message in a
message/rfc822 wrapper in order to apply S/MIME security services
to these header fields. It is up to the receiving client to
decide how to present this "inner" header along with the
unprotected "outer" header.
When an S/MIME message is received, if the top-level protected
MIME entity has a Content-Type of message/rfc822, it can be
assumed that the intent was to provide header protection. This
entity SHOULD be presented as the top-level message, [...].
With pEp message format version 2, the original full MIME message is
also wrapped in a message/rfc822 wrapper, but this entity is in turn
wrapped in a multipart/mixed entity. The purpose of the additional
nesting is to allow for public keys of the sender to be stored
alongside the original message while being protected by the same
mechanism. Thus, the top-level entity contains
o exactly one entity of type message/rfc822, and
o at most one entity of type application/pgp-keys
The current pEp message format version 2.0 also adds one entity of
type text/plain where the body part reads "pEp-Wrapped-Message-Info:
OUTER<CR><LF>". It is currently being discussed if this information
should be migrated to the headers section of the top-level entity;
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such an upgrade would be part of the the pEp message format version
2.1.
A pEp message MUST have a text/plain element. The original plaintext
message is prepended by the string "pEp-Wrapped-Message-Info:
INNER<CR><LF>". Also this header may be moved into the headers
section of the entity in message format version 2.1.
[[ TODO: The pEp-Wrapped-Message-Info information is probably not
needed for header protection. ]]
This is an example of the top-level MIME entity, before being
encrypted and signed:
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MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="6b8b4567327b23c6643c986966334873"
--6b8b4567327b23c6643c986966334873
Content-Type: text/plain; charset="utf-8"; name="msg.txt"
Content-Disposition: inline; filename="msg.txt"
pEp-Wrapped-Message-Info: OUTER
--6b8b4567327b23c6643c986966334873
Content-Type: message/rfc822; forwarded="no"
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Example
Date: Fri, 30 Jun 2018 09:55:06 +0200
Message-ID: <05d0526e-41c4-11e9-8828@pretty.Easy.privacy>
X-Pep-Version: 2.0
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"; name="msg.txt"
Content-Disposition: inline; filename="msg.txt"
Content-Transfer-Encoding: quoted-printable
pEp-Wrapped-Message-Info: INNER
p=E2=89=A1p for Privacy by Default.
--6b8b4567327b23c6643c986966334873
Content-Type: application/pgp-keys
Content-Disposition: attachment; filename="pEpkey.asc"
-----BEGIN PGP PUBLIC KEY BLOCK-----
...
-----END PGP PUBLIC KEY BLOCK-----
--6b8b4567327b23c6643c986966334873--
5.1. Preparing a Message for Header Protection
Header protection requires an ideal "original message" to be
transformed into an "inner message", which must be signed and
preferably encrypted according to MIME Security with OpenPGP
[RFC3156], resulting in an "outer message" (not to be confused with
the "inner" and "outer" labels in the above mentioned pEp-Wrapped-
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Message-Info header field). The resulting "outer message" requires
some additional adjustments so that the protected message is properly
handled on all Mail User Agents.
Note that pEp email clients are REQUIRED to sign and encrypt the
message as per [I-D.marques-pep-email], while non-pEp clients MAY
encrypt messages.
5.1.1. Requirements for the Original Message
The original message MUST be structured as a valid [RFC5322] message
with a header and a body.
Additionally, the body of the original message MUST be structured in
body parts according to the MIME standard [RFC2046].
The primary entity of type text/plain which is implicitly or
explicitly intended for inline display SHOULD be noted (the "message
entity"). The selection MUST adhere to MIME standards regarding
precedence of parts in multipart structures.
[[ TODO: It is currently undefined how to proceed if no such message
entity exists. ]]
5.1.2. Building the Inner Message
The original message entity is to be substituted with a text/plain
part (and the headers and parameters as specified later), which in
turn will contain a valid [RFC5322] message, where:
o the message SHOULD NOT be structured in MIME parts,
o the body replicates the body of the substituee message entity
decoded according to its eventual Content-Transfer-Encoding header
field value,
o the Content-Type header field is set to "text/plain"
* and the "charset" parameter is set to "UTF-8"
* and the "name" parameter is set to "msg.txt"
* and no other parameter is set on the Content-Type header field,
o the Content-Disposition is set to "inline"
* and the "filename" parameter is set to "msg.txt"
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* and no other parameter is set on the Content-Disposition header
field.
The new body of the message-body (which now contains a valid
[RFC5322] message) must be re-applied a Content-Transfer-Encoding
such that:
o if the message is to be signed and encrypted, the substituted
message-body part results in a valid UTF-8 string not containing
UTF-8 null symbols,
o if the message is to be signed but not encrypted, the substituted
message entity is 7-bit transport-safe.
The Content-Transfer-Encoding previously in place on the substitutee
message-body SHOULD be preferred for the substitued message-body
whenever it is not to be excluded by other criterias.
The inner message is then to be produced such that it can be
represented as a string which consists of only valid UTF-8 symbols
and additionally such that it does not eventually contain the UTF-8
null symbol.
No other Content-Transfer-Encoding other than "7bit", "8bit", or
"binary" is permitted for the root part of the inner message. The
headers section, the MIME boundaries and the headers sections of the
body parts MUST be limited to valid UTF-8 symbols and no UTF-8 null
symbol. Body parts and sub-parts which do not represent a valid
UTF-8 string or MAY include a UTF-8 null symbol, MUST be applied an
appropriate Content-Transfer-Encoding to make their encoded
representation valid in UTF-8 (e.g., with "quoted-printable" or
"base64").
The following shows an example original message and the resulting
message/rfc822 entity for inclusion in the outer multipart/mixed.
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Example
Date: Fri, 30 Jun 2018 09:55:06 +0200
Message-ID: <05d0526e-41c4-11e9-8828@pretty.Easy.privacy>
MIME-Version: 1.0
Content-Type: text/plain
Content-Transfer-Encoding: quoted-printable
p=E2=89=A1p for Privacy by Default.
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MIME-Version: 1.0
Content-Type: message/rfc822; forwarded="no"
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Example
Date: Fri, 30 Jun 2018 09:55:06 +0200
Message-ID: <05d0526e-41c4-11e9-8828@pretty.Easy.privacy>
X-pEp-Version: 2.0
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"; name="msg.txt"
Content-Disposition: inline; filename="msg.txt"
Content-Transfer-Encoding: quoted-printable
pEp-Wrapped-Message-Info: INNER
p=E2=89=A1p for Privacy by Default.
The protected message has the following structure.
+ message/rfc822; forwarded="no";
[
{
all protected headers, overridden by:
Message-ID:
X-pEp-Version: 2.0
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"; name="msg.txt"
Content-Disposition: inline; filename="msg.txt"
Content-Transfer-Encoding: ...
}
[
pEp-Wrapped-Message-Info: INNER (content-transfer-encoded)
[ original body ]
]
]
+ application/pgp-keys (optional)
5.1.3. Building the Outer Message for Signed Inner Messages
The outer message is an email with a body part of type multipart/
signed or multipart/encrypted resulting from applying security
services according to [RFC1847].
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Signing, but not encrypting, a message with MIME Security with
OpenPGP ([RFC4880] and [RFC3156]), yields a message with the
following basic MIME structure. If any part directly below
multipart/signed is of type message/rfc822, then the property
forwarded="no" SHOULD be set.
= multipart/signed; protocol="application/pgp-signature";
+ multipart/mixed
+ message/rfc822; forwarded="no";
|
[ protected message ]
+ application/pgp-keys
{
Content-Disposition: attachment;
filename="pEpkey.asc"
}
+ application/pgp-signature
No additional requirements exist for a signed but not encrypted
message with header protection.
5.1.4. Building the Outer Message for Signed and Encrypted Inner
Messages
Signing and encrypting a message with MIME Security with OpenPGP
[RFC3156], yields a message with the following basic MIME structure:
= multipart/encrypted; protocol="application/pgp-encrypted";
+ application/pgp-encrypted [ Version: 1 ]
+ application/octet-stream; name="msg.asc"
{
Content-Disposition: inline; filename="msg.asc";
}
|
[ opaque encrypted structure ]
|
+ multipart/mixed
+ text/plain [ pEp-Wrapped-Message-Info: OUTER ]
+ message/rfc822; forwarded="no";
|
[ protected message ]
+ application/pgp-keys
The header fields of the sub-part of type application/octet-stream
must be modified to ensure that:
o the Content-Type header field's
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* "name" parameter is set to the value "msg.asc", and
* parameter "forwarded" is set to "no", and
o the Content-Disposition header field value is set to "inline"
* and the "filename" parameter is set to "msg.asc".
5.1.5. S/MIME Compatibility
Interoperability and implementation symmetry between PGP/MIME and
S/MIME is on the roadmap of pEp.
6. Candidate Header Fields for Header Protection
By default, all headers of the original message SHOULD be protected,
with one exception:
o the header field "Bcc" MUST NOT be added to the protected headers.
7. Stub Outside Headers
The outer message requires a minimal set of headers to be in place
for being eligible for transport. This includes the "From", "To",
"Cc", "Bcc", "Subject" and "Message-ID" header fields. The protocol
hereby defined also depends on the "MIME-Version", "Content-Type",
"Content-Disposition" and eventually the "Content-Transport-Encoding"
header field to be present.
Submission and forwarding based on SMTP carries "from" and
"receivers" information out-of-band, so that the "From" and "To"
header fields are not strictly necessary. Nevertheless, "From",
"Date", and at least one destination header field is mandatory as per
[RFC5322]. They SHOULD be conserved for reliability.
The following header fields should contain a verbatim copy of the
header fields of the original message:
o Date
o From
o To
o Cc (*)
o Bcc (*)
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The entries with an asterisk mark (*) should only be set if also
present in the original message.
If signing, but no encryption is applied to the inner message, all
other headers of the original message SHOULD be copied verbatim to
the outer message as well.
Clients which follow pEp standards MUST set the header field value
for "Subject" to "=?utf-8?Q?p=E2=89=A1p?=" or "pEp". Clients which
do not adhere to all pEp standards should set the header field value
of "Subject" to a descriptive stub value. An example used in
practice is
o Subject: Encrypted message
The following header fields should be initialized with proper values:
o Message-ID
o Content-Type
o Content-Disposition
o Content-Transport-Encoding (if necessary)
8. Processing Incoming Email with Protected Headers
[[ TODO ]]
8.1. Detecting Header Protection in Incoming Email
[[ TODO: Reverse of above. Multiple equivalent specs available. ]]
8.2. Resolving Conflicting Protected and Unprotected Header Fields
For the purpose of selecting messages based on search criteria, or
just for displaying them, pEp clients may have to temporarily rebuild
the unprotected representation of the email (pEp clients may
implement a caching mechanism to avoid rebuilding these messages
repeatedly, provided they can use a trusted storage for the cache).
Every pEp wrapper email MUST contain exactly one multipart/encrypted
MIME part, which contains the protected signed-and-encrypted email as
an application/octet-stream encoded as a OpenPGP Radix-64. Such a
protected email MAY be in turn a pEp wrapper email and contain
another protected email which the client MUST try to decrypt
recursively. Through recursion, intermediate protected emails will
be encountered and header fields encountered therein, protected or
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not, MUST be ignored for the purpose of rebuilding the unprotected
representation of the email.
[[ TODO: Describe what happens when the messages do not validate;
difference between have-no-key-for-it, and broken-according-to-key-
we-have. ]]
Values of protected header fields always override the header fields
defined in the outer context. A single protected header field
requires to discard ALL header fields from the outer context with the
same header name.
A header field defined in the wrapper message and not in the
protected header section, or alternatively present in the protected
header section and not in the wrapper message, MUST be present in the
unprotected representation of the email.
For the purpose of rebuilding the unprotected email, the selection of
headers in DKIM is not relevant. The unprotected representation of
the email MAY NOT validate to DKIM or SPF rules anymore.
8.3. Processing of Signed-only Email
pEp either engages in a signed-and-encrypted communication or in an
unsigned plaintext communication. Inbound signatures attached to
plaintext messages are duly verified but cannot enhance the perceived
quality of the message in the user interface (an invalid signature
degrades the perception) ([I-D.birk-pep]).
8.4. Incoming Filter Processing
The Mail User Agent may implement outgoing filtering of mails, which
may veto, alter, redirect or replicate the messages. The
functionality may be implemented on the mailbox server and be
configurable through a well-known protocol, e.g., by means of The
Sieve Mail-Filtering Language [RFC5490], or be implemented client-
side, or in a combination of both.
A mailbox server, which is required to process the full range of
possible filters, is requiring plaintext access to the header fields.
In an end-to-end-encryption context, which pEp enforces by default,
upon first reception of the message the mailbox server is limited to
see the transport- relevant headers of the outer wrapper message. A
pEp client configued to trust the server ("trusted server" setting
[I-D.marques-pep-email]) will later download the encrypted message,
decrypt it and replace the copy on the server by the decrypted copy.
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8.4.1. Staged Filtering of Inbound Messages
Inbound messages are expected to be delivered to the inbox while
still being encrypted. At this point in time, server-side filtering
can only evaluate the unprotected header fields in the wrapper
message.
In an end-to-end-encryption context, which pEp enforces by default,
the mailbox server is limited to see the transport-relevant headers
of the outer wrapper message only upon first delivery. A pEp client
configued to trust the server ("trusted server" setting
[I-D.marques-pep-email]) will eventually download the encrypted
message, decrypt it locally and replace the copy on the server by the
decrypted copy. Server-side message filters SHOULD be able to detect
such post-processed messages, and apply the pending filters. The
client SHOULD easily reflect the post-filtered messages in the user
interface.
8.5. Outgoing Filter Processing
The Mail User Agent may implement outgoing filtering of emails, which
may veto, alter or replicate the email. They may also hint the MUA
to store a copy in an alternate "Sent" folder.
Filters which veto the sending or do alter the mail or replicate it
(e.g., mass-mail generators) SHOULD be completed priorly to applying
protection, and thus also priorly to applying header protection.
Redirection to alternate "Sent" folders MUST NOT be decided priorly
to applying protection, but MUAs MAY abide from this restriction if
they implement the "trusted server" option and the option is selected
for the specific mailbox server; in this case, MUAs MUST NOT allow to
redirect a message to an untrusted server by these rules, to prevent
information leakage to the untrusted server.
[[ TODO: Mention implicit filter for minimal color-rating for message
replication. ]]
[[ TODO: How to produce key-export-mails manually this way? That is,
what about non-pEp-mode? ]]
9. Security Considerations
[[ TODO ]]
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10. Implementation Status
10.1. Introduction
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "[...] this will allow reviewers and working
groups to assign due consideration to documents that have the benefit
of running code, which may serve as evidence of valuable
experimentation and feedback that have made the implemented protocols
more mature. It is up to the individual working groups to use this
information as they see fit."
10.2. Current software implementing pEp
The following software implementing the pEp protocols (to varying
degrees) already exists:
o pEp for Outlook as add-on for Microsoft Outlook, release
[SRC.pepforoutlook]
o pEp for Android (based on a fork of the K9 MUA), release
[SRC.pepforandroid]
o Enigmail/pEp as add-on for Mozilla Thunderbird, release
[SRC.enigmailpep]
o pEp for iOS (implemented in a new MUA), beta [SRC.pepforios]
pEp for Android, iOS and Outlook are provided by pEp Security, a
commercial entity specializing in end-user software implementing pEp
while Enigmail/pEp is pursued as community project, supported by the
pEp Foundation.
All software is available as Free Software and published also in
source form.
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11. Acknowledgements
Special thanks go to Krista Bennett for valuable input to this draft
and Hernani Marques for reviewing.
12. References
12.1. Normative References
[I-D.birk-pep]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Privacy by Default", draft-birk-pep-03 (work in progress),
March 2019.
[I-D.ietf-lamps-rfc5751-bis]
Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", draft-ietf-lamps-rfc5751-bis-12
(work in progress), September 2018.
[I-D.marques-pep-email]
Marques, H., "pretty Easy privacy (pEp): Email Formats and
Protocols", draft-marques-pep-email-02 (work in progress),
October 2018.
[I-D.melnikov-lamps-header-protection]
Melnikov, A., "Considerations for protecting Email header
with S/MIME", draft-melnikov-lamps-header-protection-00
(work in progress), October 2018.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847,
October 1995, <https://www.rfc-editor.org/info/rfc1847>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156,
DOI 10.17487/RFC3156, August 2001,
<https://www.rfc-editor.org/info/rfc3156>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5490] Melnikov, A., "The Sieve Mail-Filtering Language --
Extensions for Checking Mailbox Status and Accessing
Mailbox Metadata", RFC 5490, DOI 10.17487/RFC5490, March
2009, <https://www.rfc-editor.org/info/rfc5490>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
12.2. Informative References
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
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[SRC.enigmailpep]
"Source code for Enigmail/pEp", March 2019,
<https://enigmail.net/index.php/en/download/source-code>.
[SRC.pepforandroid]
"Source code for pEp for Android", March 2019,
<https://pep-security.lu/gitlab/android/pep>.
[SRC.pepforios]
"Source code for pEp for iOS", March 2019,
<https://pep-security.ch/dev/repos/pEp_for_iOS/>.
[SRC.pepforoutlook]
"Source code for pEp for Outlook", March 2019,
<https://pep-security.lu/dev/repos/pEp_for_Outlook/>.
Appendix A. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
o draft-luck-lamps-pep-header-protection
* Initial version
Appendix B. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication. ]]
o Align with specification for MIME Content-Type message/partial
* We probably have issues and overlapping specifications about
encoding for nested message/rfc822 entities, specified in
[RFC2046]. Further study is needed to find and understand the
issues.
o Signed-only protection needs further study
* pEp only does header protection by applying both signing and
encryption. Technically it is also possible to sign, but not
encrypt the protected messages. This needs futher study.
Authors' Addresses
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Claudio Luck
pEp Foundation
Oberer Graben 4
CH-8400 Winterthur
Switzerland
Email: claudio.luck@pep.foundation
URI: https://pep.foundation/
Bernie Hoeneisen
Ucom Standards Track Solutions GmbH
CH-8046 Zuerich
Switzerland
Phone: +41 44 500 52 44
Email: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen AT ucom.ch)
URI: https://ucom.ch/
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