Network Working Group H. Marques
Internet-Draft pEp Foundation
Intended status: Standards Track November 03, 2020
Expires: May 7, 2021
pretty Easy privacy (pEp): Email Formats and Protocols
draft-pep-email-01
Abstract
The proposed pretty Easy privacy (pEp) protocols for email are based
upon already existing email and encryption formats (as PGP/MIME) and
designed to allow for easily implementable and interoperable
opportunistic encryption. The protocols range from key distribution,
secret key synchronization between own devices, to mechanisms of
metadata and content protection. The metadata and content protection
is achieved by moving the whole message (not only the body part) into
the PGP/MIME encrypted part. The proposed pEp Email Formats not only
achieve simple forms of metadata protection (like subject
encryption), but also allow for sending email messages through a
mixnet. Such enhanced forms of metadata protection are explicitly
discussed within the scope of this document.
The purpose of pEp for email is to simplify and automate operations
in order to make usage of email encryption a viability for a wider
range of Internet users, with the goal of achieving widespread
implementation of data confidentiality and privacy practices in the
real world.
The proposed operations and formats are targeted towards to
Opportunistic Security scenarios and are already implemented in
several applications of pretty Easy privacy (pEp).
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."
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This Internet-Draft will expire on May 7, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Relationship to other pEp documents . . . . . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.3. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Opportunistic Security and Privacy for Email . . . . . . . . 5
2.1. Privacy by Default . . . . . . . . . . . . . . . . . . . 5
2.2. Data Minimization . . . . . . . . . . . . . . . . . . . . 6
2.3. Metadata Protection . . . . . . . . . . . . . . . . . . . 6
2.4. Interoperability . . . . . . . . . . . . . . . . . . . . 7
2.5. End-to-End . . . . . . . . . . . . . . . . . . . . . . . 7
2.6. Peer-to-Peer . . . . . . . . . . . . . . . . . . . . . . 7
2.7. User Experience (UX) . . . . . . . . . . . . . . . . . . 7
2.8. Identity System . . . . . . . . . . . . . . . . . . . . . 7
2.8.1. Address . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.2. Key . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.3. User . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.4. Identity . . . . . . . . . . . . . . . . . . . . . . 8
2.8.5. Alias . . . . . . . . . . . . . . . . . . . . . . . . 9
2.9. pEp Email Formats . . . . . . . . . . . . . . . . . . . . 9
2.9.1. Unencrypted pEp Format . . . . . . . . . . . . . . . 10
2.9.2. pEp Email Format 1.0 . . . . . . . . . . . . . . . . 11
2.9.3. pEp Email Format 2.0 . . . . . . . . . . . . . . . . 15
2.9.4. pEp Email Format 2.1 . . . . . . . . . . . . . . . . 18
2.9.5. Protocol Negotiation for Format Selection . . . . . . 21
2.9.6. Saving Messages . . . . . . . . . . . . . . . . . . . 21
3. Key Management . . . . . . . . . . . . . . . . . . . . . . . 21
3.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 21
3.2. Private Keys . . . . . . . . . . . . . . . . . . . . . . 22
3.2.1. Storage . . . . . . . . . . . . . . . . . . . . . . . 22
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3.2.2. Passphrase . . . . . . . . . . . . . . . . . . . . . 22
3.2.3. Private Key Export / Import . . . . . . . . . . . . . 22
3.3. Public Key Distribution . . . . . . . . . . . . . . . . . 22
3.4. Key Reset . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Trust Management . . . . . . . . . . . . . . . . . . . . . . 22
4.1. Privacy Status . . . . . . . . . . . . . . . . . . . . . 25
4.2. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3. Trust Rating . . . . . . . . . . . . . . . . . . . . . . 25
5. Synchronization . . . . . . . . . . . . . . . . . . . . . . . 25
5.1. Private Key Synchronization . . . . . . . . . . . . . . . 25
5.2. Trust Synchronization . . . . . . . . . . . . . . . . . . 25
6. Interoperability . . . . . . . . . . . . . . . . . . . . . . 25
7. Options in pEp . . . . . . . . . . . . . . . . . . . . . . . 25
7.1. Option "Passive Mode" . . . . . . . . . . . . . . . . . . 25
7.2. Option "Disable Protection" . . . . . . . . . . . . . . . 26
7.3. Option "Extra Keys" . . . . . . . . . . . . . . . . . . . 26
7.4. Option "Blacklist Keys" . . . . . . . . . . . . . . . . . 26
7.5. Option "Trusted Server" . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 27
11.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 27
11.2. Current software implementations of pEp . . . . . . . . 27
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
13.1. Normative References . . . . . . . . . . . . . . . . . . 28
13.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. Document Changelog . . . . . . . . . . . . . . . . . 31
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
This document contains propositions for implementers of Mail User
Agents (MUAs) seeking to support pretty Easy privacy (pEp)
specifically for email [RFC5322]. All the propositions of
[I-D.birk-pep] also apply to pEp for email. In this document,
requirements are outlined for MUAs wanting to establish
interoperability and/or to implement pEp for email.
pEp for email builds upon the cryptographic security services offered
by PGP/MIME [RFC3156]. The primary goals of pEp for email are:
(1) Maximization of email privacy for Internet actors deploying and
using the pretty Easy privacy approach.
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(2) Compatibility with legacy or other automatic email encryption
solutions in order to preserve privacy to the greatest extent
possible.
Interoperability with S/MIME [RFC8551] is a also goal, but at this
time there is no specification or running code that achieves this
goal.
Current tools and implementations have failed to provide a sufficient
level of usability to ordinary Internet users, with the result that
end-to-end email encryption is seldom used.
While OpenPGP [RFC4880] using PGP/MIME [RFC3156] offers good
encryption-for message contents at least-more work is needed to
achieve the following three objectives of pretty Easy privacy (pEp):
1. make email encryption as automatic as possible,
2. protect as much metadata as possible, and
3. provide an easy way to authenticate communication partners.
A reference implementation of pEp for email is available for all
major platforms and it has been ported to many programming languages
(cf. Section 11 for an overview).
1.1. Relationship to other pEp documents
This document describes the pEp for email protocols. While it
specifies details particularly related to pEp for email, it basically
inherits the structure of [I-D.birk-pep], which describes the general
concepts of pEp on a higher level.
For protocol details, constituent pEp mechanisms which also apply to
email can be found in documents like [I-D.marques-pep-handshake]),
which shows how trust between any two pEp users can be established,
[I-D.marques-pep-rating], which describes the privacy indications
that can be helpful for regular Internet users or [I-D.pep-keysync],
which outlines pEp's peer-to-peer protocol to synchronize secret key
material which belongs to the same account and user across various
end-devices.
1.2. Requirements Language
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].
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1.3. Terms
The following terms are defined for the scope of this document:
o pEp Handshake: The process of one user contacting another over an
independent channel in order to verify Trustwords (or fingerprints
as a fallback). This can be done in-person or through established
verbal communication channels, like a phone call.
[I-D.marques-pep-handshake]
o Trustwords: A scalar-to-word representation of 16-bit numbers (0
to 65535) to natural language words. When doing a Handshake,
peers are shown combined Trustwords of both public keys involved
to ease the comparison. [I-D.birk-pep-trustwords]
o Trust On First Use (TOFU): cf. [RFC7435], which states: "In a
protocol, TOFU calls for accepting and storing a public key or
credential associated with an asserted identity, without
authenticating that assertion. Subsequent communication that is
authenticated using the cached key or credential is secure against
an MiTM attack, if such an attack did not succeed during the
vulnerable initial communication."
o Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
form of active wiretapping attack in which the attacker intercepts
and selectively modifies communicated data to masquerade as one or
more of the entities involved in a communication association."
Note: Historically, MITM has stood for '_Man_-in-the-middle'.
However, to indicate that the entity in the middle is not always a
human attacker, MITM can also stand for 'Machine-in-the-middle' or
'Meddler-in-the-middle'.
2. Opportunistic Security and Privacy for Email
In addition to the Protocol's Core Design Principles outlined in
[I-D.birk-pep], the following sections on design principles are
applicable to pEp for email applications.
2.1. Privacy by Default
The pEp formats and protocols aim to maximize privacy. Where privacy
goals contradict with security goals, the privacy goals MUST take
precedence.
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Examples:
o pEp implementers MUST NOT make queries to public key servers by
default. The reason for this is to make it more resource-
intensive for centralized network actors to learn a user's social
graph. This is also problematic security-wise, as centralized
cryptographic key subversion at-scale is made easier. Instead,
key distribution MUST be handled in-band while communicating with
other peers.
o Trust information MUST NOT be attached to the communication
partners' public keys. This is metadata which MUST be held
locally and separately from the keys. Trust is established
between the peers directly (peer-to-peer) and no trust information
is held centrally (no support for the Web of Trust): that is,
while pEp MUST be able to work with OpenPGP keys which carry trust
information, this external trust information MUST NOT be used to
signal any trust level to the pEp user.
o pEp-enabled MUAs MUST either engage in a signed-and-encrypted
communication or in unsigned plaintext communication. While the
signatures attached to plaintext messages can be verified, signed-
only messages neither increase security nor privacy, so long as
the corresponding public key is not authenticated.
2.2. Data Minimization
Data Minimization includes data sparsity and hiding of all
technically concealable information whenever possible.
2.3. Metadata Protection
Email metadata (i.e., headers) MUST either be omitted or encrypted
whenever possible.
The PGP/MIME specification as described in [RFC3156] provides few
facilities for metadata protection: while the email body receives
protection, the header section remains unprotected.
However, it is possible to also protect the information contained in
the header field values by encapsulating the whole message into a
MIME entity to be signed and encrypted.
The S/MIME Message Specification [RFC8551] defines a way to also
protect the header section in addition to the content of a message:
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The sending client MAY wrap a full MIME message in a message/
rfc822 wrapper in order to apply S/MIME security services to
header fields.
2.4. Interoperability
Implementers of pEp SHOULD be liberal in accepting non-pEp formats to
encrypt email contents and metadata. pEp implementations MUST use the
strict and interoperable pEp Email Format 1.0 (cf. Section 2.9.2)
for any outgoing communication to non-pEp users. For communication
between pEp users, more privacy-preserving formats (cf. Section 2.9)
MUST be used. pEp Email Formats 2.0 and newer SHOULD NOT be used
between users who are not recognized as pEp users (cf.
Section 2.9.5), because for non-pEp users those formats are likely to
produce unwanted visual artifacts.
2.5. End-to-End
For interpersonal messaging, an email endpoint in pEp is the MUA on a
user's end-device: that is, encryption and decryption of messages
MUST be executed on a user's end-device and MUST NOT depend on any
third-party network infrastructure (i.e., any infrastructure outside
a user's direct control).
[[ *TODO*: Add enterprise settings with Key Escrow / Extra Keys ]]
2.6. Peer-to-Peer
All relevant pEp mechanisms and state information about other peers
MUST be held locally, on a peer's end-device. There MUST NOT be any
reliance on an email server or even a centralized network component
to hold relevant information for peers to be able to communicate or
to authenticate themselves. Email servers (like, SMTP or IMAP) are
only used as transport infrastructure for messages, but MUST not be
relevant to hold actual state between peers.
[[ TODO: Make clear there is a way that synchronizes trust in a peer-
to-peer fashion, by using the Trust Sync mechanism. ]]
2.7. User Experience (UX)
[[ *TODO*: Add here what is specific to email ]]
2.8. Identity System
In pEp for email, a user is a person or group who can have one or
more identities, each represented by email addresses. Every identity
has an own key attached to it. An email address can also be an alias
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for an already existing identity, in which case the same key is
attached to it.
All information about communication partners, like identities, keys
and aliases MUST be held on a user's end-device as state information.
This SHOULD be done using a structured format, to facilitate the
synchronization of state information across various devices, taking
into account multi-device scenarios, which are common today.
In pEp's reference implementation (cf. Section 11), keys are held
using the key store of the cryptographic library, while peer-specific
state information, including trust information, is held in a simple
relational database.
[[ *TODO*: Check optimal order for the following sections. ]]
2.8.1. Address
In pEp for email, the SMTP address (e.g., mailto:alice@example.org)
constitutes the network address.
2.8.2. Key
For now, a key in pEp for email is an OpenPGP key. Each identity has
a default key attached for that identity. This is the public key to
be used to encrypt communications to it.
2.8.3. User
A user in pEp for email is a specific person or group. A user has at
least one identity, but can have more.
2.8.4. Identity
An identity in pEp for email is represented by an email URI, like
mailto:alice@example.org. Identities are represented by email
address URIs because a user may have multiple URIs. For example, if
Alice uses mailto:alice@example.org for private purposes, but also
wishes to have a public address, she may create another email
address, such as anonymous@example.com. Because this is a new URI
(mailto:anonymous@example.com), it is considered a new identity for
Alice.
By default, pEp-enabled MUAs MUST generate a new key pair during new
account configuration, so that a user's respective identities are not
correlated to each other. However, if Alice wants her URIs to be
handled as a single identity with one key, she may configure her
respective identities as aliases.
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For other email URIs pointing to the same identity, see the alias
(cf. Section 2.8.5) concept.
2.8.5. Alias
Aliases share the same key and identity, e.g., the same key might be
used for mailto:alice@example.org as well as for
mailto:alice@example.net. That is, both addresses refer to the same
identity.
2.9. pEp Email Formats
The pEp Email Formats 1.0, 2.0 and 2.1 are restricted MIME-based
email formats, which ensure messages to be signed and encrypted. In
accordance with pEp's privacy (and not security) focus, signed-only
messages MUST NOT be produced (cf. Section 2.1). pEp-enabled clients
MUST be able to render all pEp Email Formats properly: for outgoing
communications, the most privacy-preserving format available is to be
used, taking interoperability (cf. Section 2.4) into account.
Since pEp Email Format 2.0, a compatibility format (i.e., pEp Email
Format 1.0, cf. Section 2.9.2) exists, which SHOULD be applied to
non-pEp users, for which trustworthy public keys are available
according to the local database.
In case no trustworthy encryption key is available, an unencrypted,
unsigned MIME email is sent out. As in all pEp formats, also this
(unprotected) message MUST contain the sender's public key, unless
Passive Mode (cf. Section 7.1) is active.
All pEp Email Formats include a "pEpkey.asc" file attachment holding
the sender's OpenPGP public key in ASCII-armored format, which is
suitable for manual key import by non-pEp users. Thus, a user of any
OpenPGP-enabled MUA is able to manually import the public key and
engage in end-to-end encryption with the pEp sender. MUA
implementers of PGP-capable email clients, even when not fully
supporting pEp's protocols, are encouraged to automatically import
the key such that the user can immediately engage in opportunistic
encryption.
In pEp's reference implementation the subject is set to "pEp" (or
alternatively to its UTF-8 representation as "=?utf-
8?Q?p=E2=89=A1p?="). However, the subject's value of the outer
message MUST be ignored. Therefore, the subject can be set to any
value (e.g., "..." as used in other implementations).
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2.9.1. Unencrypted pEp Format
This is the format to be used when unencrypted messages are sent out.
The unencrypted pEp format is a "multipart/mixed" MIME format, which
by default ensures the delivery of the sender's public key as an
attachment ("Content-Disposition: attachment").
A simple plaintext email looks like the following:
From: Alice <alice@example.org>
To: Bob <bob@example.org>
Date: Tue, 31 Dec 2019 05:05:05 +0200
X-pEp-Version: 2.1
MIME-Version: 1.0
Subject: Saying Hello
Content-Type: multipart/mixed; boundary="boundary"
--boundary
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Hello Bob
If you reply to this email using a pEp-enabled client, I will
be able to send you that sensitive material I talked to you
about.
Have a good day!
Alice
--
Sent with pEp for Android.
--boundary
Content-Type: application/pgp-keys; name="pEpkey.asc"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="pEpkey.asc"; size=2639
-----BEGIN PGP PUBLIC KEY BLOCK-----
[...]
-----END PGP PUBLIC KEY BLOCK-----
--boundary--
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2.9.2. pEp Email Format 1.0
pEp Email Format 1.0 (PEF-1.0) is an encrypted and signed MIME
format, which by default ensures:
o a signed and encrypted message, with subject encryption
o delivery of the sender's public key
PEF-1.0 has a "multipart/encrypted" MIME node on-the-wire format with
an OpenPGP encrypted and signed filename "msg.asc", and "Content-
Disposition: inline" attribute."
The subject is the only header field that can be protected with PEF-
1.0. To achieve this protection, the real subject value is added to
the top for the content section for the very first MIME entity with
media type "text/plain", that is encrypted, e.g.:
Subject: Credentials
For example, an email with "Subject: Credentials" becomes "Subject:
pEp" on-the-wire once encryption has taken place, with the true
subject appended to the beginning of the message text once decrypted.
Thus, legacy clients not aware of nor able to parse pEp's subject
encryption, still display the actual subject (in the above example:
"Credentials") to the user. Whenever the first encrypted "text/
plain" MIME entity contains such a subject line, the MUAs
implementing pEp MUST render it to the user. Note that also the
lines starting with "subject:" or "SUBJECT:" are to be rendered (as
with header fields, this is case-insensitive).
A pEp-enabled MUA MUST add the "X-pEp-Version" header field with its
highest value (preferably with value "2.1" as for pEp Email Format
2.1 Section 2.9.4) when producing this format. Here, a pEp-enabled
MUA declares its capability to receive and render more privacy-
preserving formats. Upgrading both sides to the highest version of
the pEp Email Format allows pEp-enabled MUAs for best possible
protection of metadata. For non-pEp MUAs it is OPTIONAL to add the
"X-pEp-Version: 1.0" header field. However, this format is
implicitly assumed even if this header field is not present.
Please note that for messages between pEp- and non-pEp clients the
subject encryption MAY be disabled, sacrificing usability over
privacy by avoiding artefacts for non-pEp recipients.
PEF-1.0 is also considered pEp's compatibility format towards non-pEp
clients.
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A PEF-1.0 example looks as follows:
From: Alice <alice@example.org>
To: Bob <bob@example.org>
Date: Wed, 1 Jan 2020 23:23:23 +0200
X-pEp-Version: 1.0
MIME-Version: 1.0
Subject: pEp
Content-Type: multipart/encrypted; boundary="boundary1";
protocol="application/pgp-encrypted"
--boundary1
Content-Type: application/pgp-encrypted
Version: 1
--boundary1
Content-Type: application/octet-stream
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="msg.asc"
-----BEGIN PGP MESSAGE-----
[...]
-----END PGP MESSAGE-----
--boundary--
Decrypting the enclosed "msg.msc" part yields the following:
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MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary2"
--boundary2
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline; filename="msg.txt"
Subject: Credentials
Dear Bob
Please use "bob" with the following password to access the wiki site:
correcthorsebatterystaple
Please reach out if there are any issues and have a good day!
Alice
--boundary2
Content-Type: application/pgp-keys
Content-Disposition: attachment; filename="pEpkey.asc"
-----BEGIN PGP PUBLIC KEY BLOCK-----
[...]
-----END PGP PUBLIC KEY BLOCK-----
--boundary2--
Note that the user-intended subject value is encrypted in the first
"text/plain" MIME entity under the "multipart/mixed" MIME node.
2.9.2.1. Deprecated variant of PEF-1.0
An earlier variant of PEF-1.0 started with a "multipart/mixed" MIME
node, which in case of a simple text-only email without attachments
and other MIME entities has
(1) a "text/plain" MIME entity with the PGP-encrypted content, and
(2) the sender's transferable public key at the very end.
This variant MUST NOT be produced anymore.
An example of this deprecated variant of PEF-1.0 looks as follows:
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From: Alice <alice@example.org>
To: Bob <bob@example.org>
Date: Wed, 1 Jan 2020 23:23:23 +0200
X-pEp-Version: 1.0
MIME-Version: 1.0
Subject: pEp
Content-Type: multipart/mixed; boundary="boundary"
--boundary
Content-Type: application/pgp-encrypted
Version: 1
--boundary
Content-Type: text/plain; charset="utf8"
Content-Transfer-Encoding: 7bit
-----BEGIN PGP MESSAGE-----
[...]
-----END PGP MESSAGE-----
--boundary
Content-Type: application/pgp-keys; name="pEpkey.asc"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="pEpkey.asc"
-----BEGIN PGP PUBLIC KEY BLOCK-----
[...]
-----END PGP PUBLIC KEY BLOCK-----
--boundary--
There, decrypting the PGP encrypted text/plain element yields a text
like the following; most obviously, the intended subject line is now
visible:
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Subject: Credentials
Dear Bob
Please use "bob" with the following password to access the wiki site:
correcthorsebatterystaple
Please reach out if there are any issues and have a good day!
Alice
2.9.3. pEp Email Format 2.0
pEp Email Format 2.0 (PEF-2.0) is a strict MIME format, which by
default ensures:
o a signed and encrypted message, with full email encapsulation
o delivery of the sender's public key
In PEF-2.0, the actual email (inner message) is encapsulated by a
MIME entity ("Content-Type: message/rfc822"), which is the second
part of a "multipart/mixed" MIME node. The first part of this MIME
node contains a "text/plain" MIME entity, including a marker text
"pEp-Message-Wrapped-Info: OUTER" (in its MIME content). This is
used for proper displaying and mapping of the nested message and its
encrypted header fields. Like with the PEF-1.0 (cf. Section 2.9.2),
the third (and last) part of the "multipart/mixed" MIME node MUST
contain the sender's public key.
The "multipart/mixed" MIME node is encrypted inside yet another MIME
node ("Content-Type: multipart/encrypted", cf. [RFC1847] /
[RFC3156]), which is the body part of the outer message.
Thus, the whole header section of the inner message can be fully
preserved, not only encrypted, but also signed. In the outer
message, however, when communicating with pEp users all header fields
that are not needed MUST be omitted to the fullest extent possible.
Once encrypted, only the outer message consisting of the (minimal)
outer header section and the "multipart/encrypted" MIME entity as
body with an application/octet-stream "Content-Type" with name
"msg.asc" is visible on the wire.
If the receiving side is not a known pEp-enabled MUA, but there is a
trustworthy public key available, PEF-1.0 (cf. Section 2.9.2) MUST
be used to send the email.
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In any case, the "X-pEp-Version" header field MUST be set to version
2.0, as the highest version that the sender supports.
The following example shows a PEF-2.0 multipart/encrypted email,
signed and encrypted, as an 7bit octet stream with a filename
"msg.asc", with "Content-Disposition: inline". Within that, the
original email message is fully contained in encrypted form (like
this, also the subject line gets encrypted). The support of version
2.0 is announced in the "X-pEp-Version" header field (in this
example, 2.0 is the newest pEp Email Format the pEp-enabled MUA is
able to produce and render):
From: Alice <alice@example.org>
To: Bob <bob@example.org>
Date: Wed, 1 Jan 2020 23:23:23 +0200
X-pEp-Version: 2.0
MIME-Version: 1.0
Subject: pEp
Content-Type: multipart/encrypted; boundary="boundary1";
protocol="application/pgp-encrypted"
--boundary1
Content-Type: application/pgp-encrypted
Version: 1
--boundary1
Content-Type: application/octet-stream
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="msg.asc"
-----BEGIN PGP MESSAGE-----
[...]
-----END PGP MESSAGE-----
--boundary--
Decrypting "msg.asc" results in a multipart/mixed node, with three
elements:
(1) a text part indicating this is the encapsulated message
(2) the original message encapsulated by a "message/rfc822" MIME
entity, and
(3) the transferable sender's public key in ASCII-armored format.
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An unwrapped example looks like this:
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary2"
--boundary2
Content-Type: text/plain; charset="utf-8"
Content-Disposition: inline; filename="msg.txt"
pEp-Wrapped-Message-Info: OUTER
--boundary2
Content-Type: message/rfc822
Message-ID: <pEp.1234>
Date: Wed, 1 Jan 2020 23:23:23 +0200
Subject: Credentials
X-pEp-Version: 2.0
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary3"
--boundary3
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
pEp-Wrapped-Message-Info: INNER
Dear Bob
Please use "bob" with the following password to access the wiki site:
correcthorsebatterystaple
Please reach out if there are any issues and have a good day!
Alice
--boundary3--
--boundary2
Content-Type: application/pgp-keys
Content-Disposition: attachment; filename="pEpkey.asc"
-----BEGIN PGP PUBLIC KEY BLOCK-----
[...]
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-----END PGP PUBLIC KEY BLOCK-----
--boundary2--
2.9.4. pEp Email Format 2.1
pEp Email Format 2.1 (PEF-2.1) introduces further pEp-specific header
fields to the inner message, which help to determine the behavior
between pEp users.
In normal interpersonal messaging those additional header fields are:
(1) "X-pEp-Wrapped-Message-Info: INNER" header field stating that the
message carrying this is to be considered the most inner message
containing the original email (this is particularly relevant for
mixnet or other scenarios of nested messaging; cf. [pEp.mixnet])
(2) "X-pEp-Sender-FPR" header field with the value set to sender's
full 160-bit public key fingerprint (e.g.,
"1234567890ABCDEF1234567890ABCDEF12345678"), and
(3) the "X-pEp-Version" header field set to version "2.1".
As with PEF-2.0 Section 2.9.3, in PEF-2.1 the actual email (inner
message) is encapsulated by a MIME entity ("Content-Type: message/
rfc822"), which is the second part of a "multipart/mixed" MIME node.
The first part of this MIME node contains a "text/plain" MIME entity,
which SHOULD be used to inform about the nature of this format (in
case a non-pEp client encounters in the mailbox). It MAY be used to
carry the intended subject of the inner message (which is not done in
current reference implementations). Like with the PEF-1.0 (cf.
Section 2.9.2) and PEF 2.0 (cf. Section 2.9.3), the third (and last)
part of this "multipart/mixed" MIME node MUST contain the sender's
public key.
This "multipart/mixed" MIME node is encrypted inside yet another MIME
node ("Content-Type: multipart/encrypted", cf. [RFC1847] /
[RFC3156]), which is the body part of the outer message.
A caveat of PEF-2.1 is that message rendering varies considerably
across different MUAs. This is relevant as it might happen that a
non-pEp MUA encounters a PEF-2.1 message (e.g., if a pEp-enabled
client was used in the past). No standard is currently available
which enables MUAs to reliably determine whenever a nested "message/
rfc822" MIME entity is meant to render the contained email message,
or if it was effectively intended to be forwarded as an attachment,
where a user needs to click on in order to see its content. To help
unaware MUAs, a Content-Type header field parameter with name
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"forwarded" as per [I-D.melnikov-iana-reg-forwarded] is added to the
Content-Type header field. MUAs can use this to distinguish between
a forwarded message and a nested message (i.e., using
"forwarded=no").
When the receiving peer was registered as being only PEF-2.0-capable,
the message must be sent in PEF-2.0 (cf. Section 2.9.3). The reason
for this is that pEp-enabled MUAs which are only PEF-2.0-capable rely
on the plaintext "pEp-Message-Wrapped-Info: OUTER" and "pEp-Message-
Wrapped-Info: INNER" markers to properly display and map the nested
message and its encrypted header fields.
As with PEF-1.0, if the receiving side is not a known pEp-enabled
MUA, but there is a trustworthy public key available, PEF-1.0 (cf.
Section 2.9.2) MUST be used to send the email.
In any case, the "X-pEp-Version" header field MUST be set to version
2.1, as the highest version that the sender supports.
This is an example of what the format looks like between two PEF-
2.1-capable clients:
From: Alice <alice@example.org>
To: Bob <bob@example.org>
Date: Wed, 1 Jan 2020 23:23:23 +0200
X-pEp-Version: 2.1
MIME-Version: 1.0
Subject: pEp
Content-Type: multipart/encrypted; boundary="boundary1";
protocol="application/pgp-encrypted"
--boundary1
Content-Type: application/pgp-encrypted
Version: 1
--boundary1
Content-Type: application/octet-stream
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="msg.asc"
-----BEGIN PGP MESSAGE-----
[...]
-----END PGP MESSAGE-----
--boundary1--
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Unwrapping the "multipart/encrypted" MIME node, yields this:
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary2"
--boundary2
Content-Type: text/plain; charset="utf-8"
Content-Disposition: inline; filename="msg.txt"
This message was encrypted with pEp (https://pep.software). If you
are seeing this message, your client does not support raising message
attachments. Please click on the message attachment to view it,
or better yet, consider using pEp!
--boundary2
Content-Type: message/rfc822; forwarded="no"
Message-ID: <pEp.1234>
Date: Wed, 1 Jan 2020 23:23:23 +0200
Subject: Credentials
X-pEp-Version: 2.1
X-pEp-Wrapped-Message-Info: INNER
X-pEp-Sender-FPR: 1234567890ABCDEF1234567890ABCDEF12345678
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary3"
--boundary3
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Dear Bob
Please use "bob" with the following password to access the wiki site:
correcthorsebatterystaple
Please reach out if there are any issues and have a good day!
Alice
--boundary3--
--boundary2
Content-Type: application/pgp-keys
Content-Disposition: attachment; filename="pEpkey.asc"
-----BEGIN PGP PUBLIC KEY BLOCK-----
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[...]
-----END PGP PUBLIC KEY BLOCK-----
--boundary2--
[[ TODO: Clarify on the "raising message" term in the example. ]]
2.9.5. Protocol Negotiation for Format Selection
To be able to decide which email format to generate, the pEp-enabled
MUA REQUIRES to record state on a per-identity basis. Once a "X-pEp-
Version" header field is discovered, the user MUST be recorded as a
pEp user and the corresponding pEp Version it supports (according to
the highest value of the "X-pEp-Version" header field encountered).
2.9.6. Saving Messages
In accordance with the Privacy by Default principle, messages sent or
received in encrypted form MUST be saved with the identity's
respective public key.
Messages sent or received in unencrypted form, SHOULD NOT be saved in
encrypted form on the email server: this reflects the Privacy Status
the user encountered when sending or receiving the email and thus
meets the user's expectations.
Instead, message drafts MUST always be saved with the identity's
public key.
Other messages sent and received MUST be saved encrypted by default:
for most end-user scenarios, the servers users work with, are
considered untrusted.
For trusted environments (e.g., in organizations) and to conform to
legally binding archiving regulations, pEp implementations MUST
provide a "Trusted Server" option. With the user's explicit consent
(opt-in), unencrypted copies of the Messages MUST be held on the mail
servers controlled by the organization.
3. Key Management
3.1. Key Generation
A pEp-enabled Mail User Agent MUST consider every email account as an
new identity: for each identity, a different key pair MUST be created
automatically if no key material with sufficient length is available.
By default, RSA-4096 key pairs for OpenPGP encryption [RFC4880]
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SHOULD be generated automatically for each email account. However,
the key length MUST be at least 2048 bits. Elliptic curve keys with
at least 256 bits MUST be supported, but SHOULD NOT yet be generated
and announced by default for interoperability reasons.
If for an identity there's an RSA key pair with less than 2048 bits,
new keys MUST be generated.
3.2. Private Keys
[[ TODO: Add here what is specific to email ]]
3.2.1. Storage
[[ TODO: Add here what is specific to email ]]
3.2.2. Passphrase
[[ TODO: Add here what is specific to email ]]
3.2.3. Private Key Export / Import
3.3. Public Key Distribution
By default, public keys MUST always be attached to any outgoing
message as described in Section 2.9. If this is undesired, Passive
Mode (cf. Section 7.1) can be activated.
3.4. Key Reset
[[ TODO: Add here what is specific to email ]]
4. Trust Management
The following example roughly describes a pEp email scenario with a
typical initial message flow to demonstrate key exchange and basic
trust management:
1. Alice - knowing nothing of Bob - sends an email to Bob. As Alice
has no public key from Bob, this email is sent out unencrypted.
However, Alice's public key is automatically attached.
2. Bob can just reply to Alice and - as he received her public key -
his MUA is now able to encrypt the message. At this point, the
rating for Alice changes to "encrypted" in Bob's MUA, which (UX-
wise) can be displayed using yellow color (cf. Section 4.3).
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3. Alice receives Bob's key. As of now Alice is also able to send
secure emails to Bob. The rating for Bob changes to "encrypted"
(with yellow color) in Alice's MUA (cf. Section 4.3).
4. If Alice and Bob want to prevent man-in-the-middle (MITM)
attacks, they can engage in a pEp Handshake comparing their so-
called Trustwords (cf. Section 4.2) and confirm this process if
those match. After doing so, their identity rating changes to
"encrypted and authenticated" (cf. Section 4.3), which (UX-wise)
can be displayed using a green color.
As color code changes for an identity, this is also reflected to
future messages to/from this identity. Past messages, however, MUST
NOT be altered.
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----- -----
| A | | B |
----- -----
| |
+------------------------+ +------------------------+
| auto-generate key pair | | auto-generate key pair |
| (if no key yet) | | (if no key yet) |
+------------------------+ +------------------------+
| |
+-----------------------+ +-----------------------+
| Privacy Status for B: | | Privacy Status for A: |
| *Unencrypted* | | *Unencrypted* |
+-----------------------+ +-----------------------+
| |
| A sends message to B (Public Key |
| attached) / optionally signed, but |
| NOT ENCRYPTED |
+------------------------------------------>|
| |
| +-----------------------+
| | Privacy Status for A: |
| | *Encrypted* |
| +-----------------------+
| |
| B sends message to A (Public Key |
| attached) / signed and ENCRYPTED |
|<------------------------------------------+
| |
+-----------------------+ |
| Privacy Status for B: | |
| *Encrypted* | |
+-----------------------+ |
| |
| A and B successfully compare their |
| Trustwords over an alternative channel |
| (e.g., phone line) |
|<-- -- -- -- -- -- -- -- -- -- -- -- -- -->|
| |
+-----------------------+ +-----------------------+
| Privacy Status for B: | | Privacy Status for A: |
| *Trusted* | | *Trusted* |
+-----------------------+ +-----------------------+
| |
[[ TODO: Add more of what is specific to email ]]
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4.1. Privacy Status
[[ TODO: Add here what is specific to email ]]
4.2. Handshake
[[ TODO: Add here what is specific to email ]]
4.3. Trust Rating
[[ TODO: Add here what is specific to email ]]
5. Synchronization
As per [I-D.pep-keysync]:
[[ TODO: Add here what is specific to email ]]
5.1. Private Key Synchronization
[[ TODO: Add here what is specific to email ]]
5.2. Trust Synchronization
[[ TODO: Add here what is specific to email ]]
6. Interoperability
[[ TODO: Add here what is specific to email ]]
7. Options in pEp
7.1. Option "Passive Mode"
In email, Passive Mode primarily exists as an option to avoid
potential usability issues in certain environments where Internet
users might get confused by the exposure of public keys in email
attachments. In principle, however, this "problem" can be mitigated
either by training or by MUA implementers displaying public key
material in a more symbolic way or even importing it automatically
and then hiding this attachment altogether (as pEp implementers are
supposed to do, such that regular Internet users do not have to
bother about keys).
Passive Mode has a negative impact on privacy: additional unencrypted
message exchanges are needed until pEp's by-default encryption can
take place.
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Passive Mode MUST only affect unencrypted communications and MUST be
inactive by default. By opting in to Passive Mode, the sender's
public key MUST NOT be attached when sending out unsecure emails. On
the other hand, Passive Mode is without any effect when pEp is able
to send out an encrypted message, because the necessary encryption
key(s) are available.
In this situation, opportunistic by-default encryption MUST take
place: there, the sender's public key is attached in encrypted form
as constituent part of one of pEp's PGP/MIME-based message format
described in Section 2.9.
Additionally, Passive Mode MUST be without effect, if a receiver
learns that an MUA is actually pEp-capable, even if the sender
involved is in Passive Mode, too: this MUST be recognized by the "X-
pEp-Version" header field, as the only clear indicator to detect pEp
users. That means that a pEp-enabled MUA is REQUIRED to attach its
corresponding public key to another pEp user in any case, such that
they can engage in opportunistic encryption.
[[ TODO: Add message examples and a flow chart, if needed ]]
7.2. Option "Disable Protection"
This is an opt-in mechanism to enforce that messages go out
unprotected. Even if encryption keys for recipient(s) are available,
this option MUST enforce that messages are sent in the Section 2.9.1
format.
7.3. Option "Extra Keys"
[[ TODO: Add here what is specific to email ]]
7.4. Option "Blacklist Keys"
[[ TODO: Add here what is specific to email ]]
7.5. Option "Trusted Server"
[[ TODO: Add here what is specific to email ]]
8. Security Considerations
[[ TODO ]]
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9. Privacy Considerations
[[ TODO ]]
10. IANA Considerations
This document has no actions for IANA.
11. Implementation Status
11.1. 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."
11.2. Current software implementations of pEp
The following software implementations of the pEp protocols (to
varying degrees) already exists:
o pEp for Outlook as add-on for Microsoft Outlook, release
[SRC.pepforoutlook]
o pEp for iOS (implemented in a new MUA), release [SRC.pepforios]
o pEp for Android (based on a fork of the K9 MUA), release
[SRC.pepforandroid]
o pEp for Thunderbird as a new add-on for Thunderbird, beta
[SRC.pepforthunderbird]
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o Enigmail/pEp as add-on for Mozilla Thunderbird, release (will be
discontinued late 2020/early 2021) [SRC.enigmailpep]
pEp for Android, iOS, Outlook and Thunderbird are provided by pEp
Security, a commercial entity specializing in end-user pEp
implementations while Enigmail/pEp is pursued as community project,
supported by the pEp Foundation.
All software is available as Free and Open Source Software and
published also in source form.
12. Acknowledgements
Special thanks go to Krista Bennett and Volker Birk for the reference
implementation on pEp and the ideas leading to this draft.
The author would like to thank the following people who provided
substantial contributions, helpful comments or suggestions for this
document: Berna Alp, Kelly Bristol, Bernie Hoeneisen, Claudio Luck
and Lars Rohwedder.
This work was initially created by the pEp Foundation, and was
initially reviewed and extended with funding by the Internet
Society's Beyond the Net Programme on standardizing pEp. [ISOC.bnet]
13. References
13.1. Normative References
[I-D.birk-pep]
Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy
privacy (pEp): Privacy by Default", draft-birk-pep-05
(work in progress), November 2019.
[I-D.marques-pep-handshake]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Contact and Channel Authentication through Handshake",
draft-marques-pep-handshake-05 (work in progress), July
2020.
[I-D.marques-pep-rating]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Mapping of Privacy Rating", draft-marques-pep-rating-03
(work in progress), January 2020.
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[I-D.melnikov-iana-reg-forwarded]
Melnikov, A. and B. Hoeneisen, "IANA Registration of
Content-Type Header Field Parameter 'forwarded'", draft-
melnikov-iana-reg-forwarded-00 (work in progress),
November 2019.
[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>.
[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>.
[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>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
13.2. Informative References
[I-D.birk-pep-trustwords]
Hoeneisen, B. and H. Marques, "IANA Registration of
Trustword Lists: Guide, Template and IANA Considerations",
draft-birk-pep-trustwords-05 (work in progress), January
2020.
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[I-D.pep-keysync]
Birk, V., Hoeneisen, B., and K. Bristol, "pretty Easy
privacy (pEp): Key Synchronization Protocol (KeySync)",
draft-pep-keysync-02 (work in progress), July 2020.
[ISOC.bnet]
Simao, I., "Beyond the Net. 12 Innovative Projects
Selected for Beyond the Net Funding. Implementing Privacy
via Mass Encryption: Standardizing pretty Easy privacy's
protocols", June 2017, <https://www.internetsociety.org/
blog/2017/06/12-innovative-projects-selected-for-beyond-
the-net-funding/>.
[pEp.mixnet]
pEp Foundation, "Mixnet", June 2020,
<https://dev.pep.foundation/Mixnet>.
[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>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[SRC.enigmailpep]
"Source code for Enigmail/pEp", November 2020,
<https://enigmail.net/index.php/en/download/source-code>.
[SRC.pepforandroid]
"Source code for pEp for Android", November 2020,
<https://pep-security.lu/gitlab/android/pep>.
[SRC.pepforios]
"Source code for pEp for iOS", November 2020,
<https://pep-security.ch/dev/repos/pEp_for_iOS/>.
[SRC.pepforoutlook]
"Source code for pEp for Outlook", November 2020,
<https://pep-security.lu/dev/repos/pEp_for_Outlook/>.
[SRC.pepforthunderbird]
"Source code for pEp for Thunderbird", November 2020,
<https://pep-security.lu/dev/repos/pEp_for_Thunderbird/>.
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Appendix A. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
o draft-pep-email-01:
* Minor language improvements
o draft-pep-email-00:
* Major restructure of the document
* Major fixes in the description of the various message formats
* Add many open questions and comments inline (TODO)
* Add IANA Considerations section
* Change authors and acknowledgment section
* Add internal references
* Describe Passive Mode
* Better explanation on how this document relates to other pEp
documents
o draft-marques-pep-email-02:
* Add illustrations
* Minor fixes
* Add longer list of Open Issues (mainly by Bernie Hoeneisen)
o draft-marques-pep-email-01:
* Remove an artefact, fix typos and minor editorial changes; no
changes in content
o draft-marques-pep-email-00:
* Initial version
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Appendix B. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication ]]
o Eventually move the many TODOs into this Open Issues section.
o Describe KeyImport to induce the import from secret keys from
other devices
o Describe / Reference KeySync (and other sync, through IMAP)
o Add key pair revocation strategy
o Create clearer relations to the pEp rating draft (draft-marques-
pep-rating), as this plays an important role in how Messages are
rendered and how they need to be presented (after rating) for a
user to have awareness about his privacy status in any given
situation.
o Make document more coherent: check with pEp's general draft pieces
to fill on both sides and how to reference them vice-versa (this
is now pending on the reworked general draft to be published).
o Elaborate more on the X-EncStatus header field and for Trusted
Server situations / mirrors and describe operations.
o Explain Key Mapping (between OpenPGP and S/MIME)
Author's Address
Hernani Marques
pEp Foundation
Oberer Graben 4
CH-8400 Winterthur
Switzerland
Email: hernani.marques@pep.foundation
URI: https://pep.foundation/
Marques Expires May 7, 2021 [Page 32]