pretty Easy privacy (pEp): Privacy by Default
draft-birk-pep-03
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 "Replaced".
|
|
|---|---|---|---|
| Authors | Hernâni Marques , Bernie Hoeneisen | ||
| Last updated | 2019-03-07 | ||
| Replaced by | draft-pep-general | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-birk-pep-03
Network Working Group H. Marques
Internet-Draft pEp Foundation
Intended status: Standards Track B. Hoeneisen
Expires: September 8, 2019 Ucom.ch
March 07, 2019
pretty Easy privacy (pEp): Privacy by Default
draft-birk-pep-03
Abstract
The pretty Easy privacy (pEp) protocols describe a set of conventions
for the automation of operations traditionally seen as barriers to
the use and deployment of secure end-to-end interpersonal messaging.
These include, but are not limited to, key management, key discovery,
and private key handling (including peer-to-peer synchronization of
private keys and other user data across devices). pEp also introduces
means to verify communication peers and proposes a trust-rating
system to denote secure types of communications and signal the
privacy level available on a per-user and per-message level.
Significantly, the pEp protocols build on already available security
formats and message transports (e.g., PGP/MIME), and are written with
the intent to be interoperable with already widely-deployed systems
in order to facilitate and ease adoption and implementation. This
document outlines the general design choices and principles of 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
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Internet-Drafts are draft documents valid for a maximum of six months
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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 8, 2019.
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Copyright Notice
Copyright (c) 2019 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
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
1.1. Relationship to other pEp documents . . . . . . . . . . . 5
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol's Core Design Principles . . . . . . . . . . . . . . 6
3.1. Privacy by Default . . . . . . . . . . . . . . . . . . . 6
3.2. Data Minimization . . . . . . . . . . . . . . . . . . . . 6
3.3. Interoperability . . . . . . . . . . . . . . . . . . . . 6
3.4. Peer-to-Peer (P2P) . . . . . . . . . . . . . . . . . . . 7
3.5. User Experience (UX) . . . . . . . . . . . . . . . . . . 7
4. Specific Elements in pEp . . . . . . . . . . . . . . . . . . 8
4.1. pEp identity system . . . . . . . . . . . . . . . . . . . 8
4.2. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Key . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.2. User . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.3. Identity . . . . . . . . . . . . . . . . . . . . . . 9
4.2.4. Address . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Example: Difference between pEp and OpenPGP . . . . . . . 9
5. Key Management . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 10
5.2. Private Keys . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Storage . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.2. Passphrase . . . . . . . . . . . . . . . . . . . . . 11
5.2.3. Private Key Export / Import . . . . . . . . . . . . . 11
5.3. Public Key Distribution . . . . . . . . . . . . . . . . . 11
5.4. Key Reset . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Trust Management . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Privacy Status . . . . . . . . . . . . . . . . . . . . . 15
6.2. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. Trust Rating . . . . . . . . . . . . . . . . . . . . . . 15
6.4. Trust Revoke . . . . . . . . . . . . . . . . . . . . . . 16
7. Synchronization . . . . . . . . . . . . . . . . . . . . . . . 16
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7.1. Private Key Synchronization . . . . . . . . . . . . . . . 16
7.2. Trust Synchronization . . . . . . . . . . . . . . . . . . 16
8. Interoperability . . . . . . . . . . . . . . . . . . . . . . 16
9. Options in pEp . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Option "Passive Mode" . . . . . . . . . . . . . . . . . . 16
9.2. Option "Disable Protection" . . . . . . . . . . . . . . . 17
9.3. Option "Extra Keys" . . . . . . . . . . . . . . . . . . . 17
9.4. Option "Blacklist Keys" . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
12. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
12.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 18
12.2. Current software implementing pEp . . . . . . . . . . . 18
12.3. Reference implementation of pEp's core . . . . . . . . . 19
12.4. Abstract Crypto API examples . . . . . . . . . . . . . . 20
13. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Code Excerpts . . . . . . . . . . . . . . . . . . . 23
A.1. pEp Identity . . . . . . . . . . . . . . . . . . . . . . 23
A.1.1. Corresponding SQL . . . . . . . . . . . . . . . . . . 24
A.2. pEp Communication Type . . . . . . . . . . . . . . . . . 25
A.3. Abstract Crypto API examples . . . . . . . . . . . . . . 26
A.3.1. Encrypting a Message . . . . . . . . . . . . . . . . 26
A.3.2. Decrypting a Message . . . . . . . . . . . . . . . . 27
A.3.3. Obtain Common Trustwords . . . . . . . . . . . . . . 29
Appendix B. Document Changelog . . . . . . . . . . . . . . . . . 29
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
Secure and private communications are vital for many different
reasons, and there are particular properties that privacy-preserving
protocols need to fulfill in order to best serve users. In
particular, [RFC8280] has identified and documented important
principles such as data minimization, the end-to-end principle, and
interoperability as integral properties for access to the Internet
for human rights purposes. While (partial) implementations of these
concepts are already available, today's applications widely lack
privacy support that ordinary users can easily handle. The pretty
Easy privacy (pEp) protocols generally conform with the principles
outlined in [RFC8280] as a matter of course, and as such can be used
to facilitate the adoption and correct usage of secure and private
communications technology.
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The pretty Easy privacy (pEp) protocols are propositions to the
Internet community to create software for peers to automatically
encrypt, anonymize (where possible), and verify their daily written
digital communications. This is achieved by building upon already
existing standards and tools and automating each step a user needs to
carry out in order to engage in secure end-to-end encrypted
communications. Significantly, the pEp protocols describe how do to
this without dependence on centralized infrastructures.
In particular, pEp proposes automatation of key management, key
discovery and synchronization of secret key material by an in-band
peer-to-peer approach.
To mitigate man-in-the-middle attacks (MITM) by an active adversary,
and as the only manual step users carry out in the course of the
protocols, the proposed Trustwords [I-D.birk-pep-trustwords]
mechanism uses natural language representations of two peers'
fingerprints for users to verify their trust in a paired
communication channel.
[[ Note: The pEp initiators learned from the CryptoParty movement,
from which the project emerged, that while step-by-step guides can be
helpful for some users to engage in secure end-to-end communications,
for the vast majority of users, it is both more effective and more
convenient to have these step-by-step procedures put into actual code
(as such, following a protocol) and thus automating the initial
configuration and whole usage of cryptographic tools.]]
The privacy-by-default principles that pEp introduces are in
accordance with the perspective outlined in [RFC7435], aiming to
provide opportunistic security in the sense of "some protection most
of the time". This is done, however, with the subtle but important
difference that when privacy is weighed against security, the choice
defaults to privacy. Therefore, data minimization is a primary goal
in pEp (e.g., hiding subject lines and headers unnecessary for email
transport inside the encrypted payload of a message).
The pEp propositions are focused on (but not limited to) written
digital communications and cover asynchronous (offline) types of
communications like email as well as synchronous (online) types such
as chat.
pEp's goal is to bridge different standardized and widely-used
communications channels such that users can reach communications
partners in the most privacy-enhancing way possible.
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1.1. Relationship to other pEp documents
While this document describes the general concept of pEp, other
documents build on top of this. These documents define other parts
of the pEp environment as follows:
1. pEp enhanced applications (e.g., pEp Email
[I-D.marques-pep-email]).
2. Helper functions for interaction with peers (e.g., pEp Handshake
[I-D.marques-pep-handshake]) that assist the user with handling
and understanding cryptographic parts that he/she needs to be
aware of.
3. Helper functions for interactions between a user's own devices
(e.g., pEp Key Sync [E-D.birk-pep-keysync]) that assist the user
to run pEp applications on different devices (such as computer,
mobile phone or tables) at the same time.
In addition, there are documents that do not directly depend on this
one, but provide generic functions needed in pEp, e.g., IANA
Registration of Trustword Lists [I-D.birk-pep-trustwords].
[[At this stage it is not yet clear to us how many of our
implementation details should be part of new RFCs and at which places
we can safely refer to already existing RFCs to make clear on which
RFCs we already rely.]]
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].
o pEp Handshake: The process when Alice - e.g., in-person or via
phone - contacts Bob to verify Trustwords (or by fallback:
fingerprints) is called pEp Handshake.
[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]
o Man-in-the-middle attack (MITM): cf. [RFC4949]
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3. Protocol's Core Design Principles
3.1. Privacy by Default
The pEp protocols are to intended specifically to ensure privacy.
There exist cases in the secure communications ecosystem, however,
where achieving privacy is in direct contradiction to security
though. For instance, in PGP's Web of Trust, relations between
people and trust levels are exposed to the public. Additionally, the
privacy of queries is not ensured in such a model when obtaining keys
from remote locations. Within the pEp protocols, when security and
privacy goals are not in conflict, then the protocols are designed to
maximize both security and privacy. However, where they contradict
each other, privacy goals are chosen as the default over security
considerations. However, in implementing these protocols, it is
always the case that users SHOULD have the choice to override the
default by corresponding options.
In pEp messaging (e.g., when using HTML) content SHALL NOT be
obtained from remote locations as this constitutes a privacy breach.
3.2. Data Minimization
Another important design goal is data minimization, which includes
data spareness and hiding all technically concealable information
when possible.
3.3. Interoperability
The pEp propositions seek to be interoperable with already-widespread
message formats and cryptographic protocols and implementations.
Seamless communication between users of software which implements pEp
and and users of other messaging tools for end-to-end encryption is a
design goal.
Therefore, pEp abides by the following guidelines:
o pEp is conservative (strict) in requirements for pEp
implementations and how they interact with pEp or other
(messaging) implementations.
o pEp is liberal in accepting input from non-pEp implementations
(e.g., it will not produce, but will support the decryption of,
PGP/INLINE formats).
o Where pEp requires divergence from an RFC for privacy reasons
(e.g., from OpenPGP propositions as defined in [RFC4880], options
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SHOULD be implemented to empower the user to override pEp's
defaults.
3.4. Peer-to-Peer (P2P)
Messaging and verification processes in pEp are designed to work in a
peer-to-peer (P2P) manner, without the involvement of intermediaries.
This means there MUST NOT be any pEp-specific central services
whatsoever needed for pEp implementations, both in the case of
verification of peers and for the actual encryption.
However, implementers of pEp MAY provide options for interoperation
with providers of centralized infrastructures (e.g., to enable users
to communicate with their peers on platforms with vendor lock-in).
Trust provided by global Certificate Authorities (e.g., commercial
X.509 CAs) SHALL NOT be signaled as trustworthy (cf.
[I-D.marques-pep-rating]) to users of pEp (e.g., when interoperating
with peers using S/MIME) by default.
3.5. User Experience (UX)
Implementers of pEp MUST take special care not to confuse users with
technical terms, especially those of cryptography (e.g., "keys",
"certificates" or "fingerprints"), unless users explicitly ask for
such terms; i.e., advanced settings MAY be available, and in some
cases further options may even be required. However, those SHALL NOT
be unnecessarily exposed to users of pEp implementations at first
glance.
The authors believe widespread adoption of end-to-end cryptography is
much less of a problem if the users are not confronted with the need
to understand cryptography; that is to say, a central goal of pEp of
the pEp protocol is that users can just rely on the principles of
Privacy by Default.
As a consequence, this means that users must not wait for
cryptographic tasks (e.g., key generation or public key retrieval) to
finish before being able to have their respective message clients
ready to communicate. The end result of this is that pEp
implementers MUST ensure that the ability to draft, send and receive
messages is always preserved - even if that means a message is sent
out unencrypted, thus being in accordance with the Opportunistic
Security approach outlined in [RFC7435].
In turn, pEp implementers MUST ensure that a discernible privacy
status is clearly visible to the user - on a per-contact as well as
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per-message level - so that users easily understand which level of
privacy messages are about to be sent with or were received with,
respectively.
[[Note: We are aware of the fact that usually UX requirements are not
part of RFCs. However, in order to encourage massive adoption of
secure end-to-end encryption while at the same time avoiding putting
users at risk, we believe certain straightforward signaling
requirements for users to be a good idea, just as is currently done
for already-popular instant messaging services.]]
4. Specific Elements in pEp
4.1. pEp identity system
In pEp, users MUST have the ability to have multiple different
identities.
pEp users MUST have the option to choose different identities. This
allows an Internet user to decide how to reveal himself/herself to
the world and is an important element in order to achieve privacy.
These different identities MUST NOT be externally correlatable with
each other by default. On the other hand, combining different
identities when such information is known MUST be supported (alias
support).
4.2. Identifiers
4.2.1. Key
A key is an OpenPGP-compatible asymmetric key pair. Other formats
and temporary symmetrical keys can be generated by Key Mapping.
Keys in pEp are identified by the full fingerprint (fpr) of the
public key.
4.2.2. User
A user is a real world human being or a group of human beings. If it
is a single human being, it can be called person.
A user is identified by a user ID (user_id). The user_id SHOULD be a
UUID, it MAY be an arbitrary unique string.
The own user can have a user_id like all other users. If it doesn't,
then it has PEP_OWN_USERID "pEp_own_userId" as user_id.
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A user can have a default key. (cf. Section 4.2.1)
4.2.3. Identity
An identity is a (possibly pseudonymous) representation of a user
encapsulating how this user appears in the network.
An identity is defined by the mapping of user_id to address. If no
user_id is known, it is guessed by mapping of username and address.
An identity can have a temporary user_id as a placeholder until a
real user_id is known.
An identity can have a default key. (cf. Section 4.2.1)
[[ Note: This is the reason why in current pEp implementations for
each (email) account a different key pair is created, which allows a
user to retain different identities. ]]
In Appendix A.1 you can find how a pEp identity is defined in the
reference implementation of the pEp Engine.
4.2.4. Address
An address is a network address, e.g., an SMTP address or another
URI.
[[ Note: It might be necessary to introduce further addressing
schemes through IETF contributions or IANA registrations, e.g.,
implementing pEp to bridge to popular messaging services with no URIs
defined. ]]
4.3. Example: Difference between pEp and OpenPGP
+--------------------+--------------+-------------------------------+
| pEp | OpenPGP | Comments |
+--------------------+--------------+-------------------------------+
| user_id | (no concept) | ID for a person, i.e. a |
| | | contact |
| | | |
| username + address | uid | comparable only for email |
| | | |
| fpr | fingerprint | used as key ID in pEp |
| | | |
| (no concept) | Key ID | does not exist in pEp |
+--------------------+--------------+-------------------------------+
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5. Key Management
In order to achieve the goal of widespread adoption of secure
communications, key management in pEp MUST be automated.
5.1. Key Generation
A pEp implementation MUST ensure that cryptographic keys for every
identity configured are available. If a corresponding key pair for
the identity of a user is found and said identity fulfills the
requirements (e.g., for email, as set out in
[I-D.marques-pep-email]), said key pair MUST be reused. Otherwise a
new key pair MUST be generated. This may be carried out instantly
upon its configuration.
On devices with limited processing power (e.g., mobile devices) the
key generation may take more time than a user is willing to wait. If
this is the case, users SHOULD NOT be stopped from communicating,
i.e., the key generation process SHOULD be carried out in the
background.
5.2. Private Keys
5.2.1. Storage
Private keys in pEp implementations MUST always be held on the end
user's device(s): pEp implementers MUST NOT rely on private keys
stored in centralized remote locations. This applies even for key
storages where the private keys are protected with sufficiently long
passphrases. It is considered a violation of pEp's P2P design
principle to rely on centralized infrastructures (cf. Section 3.4).
This also applies for pEp implementations created for applications
not residing on a user's device (e.g., web-based MUAs). In such
cases, pEp implementations MUST be done in a way such that the
locally-held private key can neither be directly accessed nor leaked
to the outside world.
[[ Note: It is particularly important that browser add-ons
implementing pEp functionality do not obtain their cryptographic code
from a centralized (cloud) service, as this must be considered a
centralized attack vector allowing for backdoors, negatively
impacting privacy. ]]
Cf. Section 7.1 for a means to synchronize private keys among
different devices of the same network address in a secure manner.
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5.2.2. Passphrase
Passphrases to protect a user's private key MUST be supported by pEp
implementations, but MUST NOT be enforced by default. That is, if a
pEp implementation finds a suitable (i.e., secure enough)
cryptographic setup, which uses passphrases, pEp implementations MUST
provide a way to unlock the key. However, if a new key pair is
generated for a given identity, no passphrase MUST be put in place.
The authors assume that the enforcement of secure (i.e., unique and
long enough) passphrases would massively reduce the number of pEp
users (by hassling them), while providing little to no additional
privacy for the common cases of passive monitoring being carried out
by corporations or state-level actors.
5.2.3. Private Key Export / Import
A private key can be exported from one device for import onto another
device. When pEp's Key Sync (cf. Section 7.1) is not available or
not desired to be used, this is largely a manual process.
5.3. Public Key Distribution
As the key is available (cf. Section 5.3) implementers of pEp are
REQUIRED to ensure that the identity's public key is attached to
every outgoing message. However, this MAY be omitted if the peer has
previously received a message encrypted with the public key of the
sender.
The sender's public key SHOULD be sent encrypted whenever possible,
i.e., when a public key of the receiving peer is available. If no
encryption key of the recipient is available, the sender's public key
MAY be sent unencrypted. In either case, this approach ensures that
messaging clients (e.g., MUAs that at least implement OpenPGP) do not
need to have pEp implemented to see a user's public key. Such peers
thus have the chance to (automatically) import the sender's public
key.
If there is already a known public key from the sender of a message
and it is still valid and not expired, new keys MUST not be used for
future communication unless they are signed by the previous key (to
avoid a MITM attack). Messages MUST always be encrypted with the
receiving peer's oldest public key, as long as it is valid and not
expired.
Implementers of pEp SHALL prevent the display of public keys attached
to messages (e.g, in email) to the user in order to prevent user
confusion by files they are potentially unaware of how to handle.
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Metadata (e.g., email headers) MUST NOT be added to announce a user's
public key. This is considered unnecessary information leakage as it
may affect user privacy, which depends also on a country's data
retention laws. Furthermore, this affects interoperability to
existing users (e.g., in the OpenPGP field) that have no notion of
such header fields and thus lose the ability to import any such keys
distributed this way. It SHOULD, though, be supported to obtain
other users' public keys by extracting them from respective header
fields of received messages (in case such approaches get widespread).
Keyservers or generally intermediate approaches to obtain a peer's
public key SHALL NOT be used by default. On the other hand, the user
MAY be provided with the option to opt-in for remote locations to
obtain keys, considering the widespread adoption of such approaches
for key distribution.
Keys generated or obtained by pEp clients MUST NOT be uploaded to any
(intermediate) keystore locations without the user's explicit
consent.
5.4. Key Reset
[[ TODO: This section will explain how to deal with keys no longer
valid, e.g. if leaked ]]
6. Trust Management
The following example roughly describes a pEp scenario with a typical
initial message flow to demonstrate key exchange and basic trust
management:
1. Alice - knowing nothing of Bob - sends a message to Bob. As Alice
has no public key from Bob, this message 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 messaging client is now able to encrypt the message. At this
point the rating for Alice changes to "encrypted" in Bob's
messaging client, which (UX-wise) can be displayed using yellow
color (cf. Section 6.3).
3. Alice receives Bob's key. As of now Alice is also able to send
secure messages to Bob. The rating for Bob changes to "encrypted"
(with yellow color) in Alice's messaging client (cf.
Section 6.3).
4. If Alice and Bob want to prevent man-in-the-middle (MITM)
attacks, they can engage in a pEp Handshake comparing their so-
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called Trustwords (cf. Section 6.2) and confirm this process if
those match. After doing so, their identity rating changes to
"encrypted and authenticated" (cf. Section 6.3), which (UX-wise)
can be displayed using a green color.
As color code changes for an identity, it is also applied to future
messages to/from this identity.
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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 sucessfully compare their |
| Trustwords over an alternative channel |
| (e.g., phone line) |
|<-- -- -- -- -- -- -- -- -- -- -- -- -- -->|
| |
+-----------------------+ +-----------------------+
| Privacy Status for B: | | Privacy Status for A: |
| *Trusted* | | *Trusted* |
+-----------------------+ +-----------------------+
| |
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6.1. Privacy Status
For end-users, the most important component of pEp, which MUST be
made visible on a per-recipient and per-message level, is the Privacy
Status.
By colors, symbols and texts a user SHALL immediately understand how
private
o a communication channel with a given peer was or ought to be and
o a given message was or ought to be.
6.2. Handshake
To establishing trust between peers and to upgrade Privacy Status,
pEp defines a handshake, which is specified in
[I-D.marques-pep-handshake].
In pEp, Trustwords [I-D.birk-pep-trustwords] are used for users to
compare the authenticity of peers in order to mitigate MITM attacks.
By default, Trustwords MUST be used to represent two peers'
fingerprints of their public keys in pEp implementations.
In order to retain compatibility with peers not using pEp
implementations (e.g., Mail User Agents (MUAs) with OpenPGP
implementations without Trustwords), it is REQUIRED that pEp
implementers give the user the choice to show both peers'
fingerprints instead of just their common Trustwords.
6.3. Trust Rating
pEp includes a Trust Rating system defining Rating and Color Codes to
express the Privacy Status of a peer or message
[I-D.marques-pep-rating]. The ratings are labeled, e.g., as
"Unencrypted", "Encrypted", "Trusted", "Under Attack", etc. The
Privacy Status in its most general form is expressed with traffic
lights semantics (and respective symbols and texts), whereas the
three colors yellow, green and red can be applied for any peer or
message - like this immediately indicating how secure and trustworthy
(and thus private) a communication was or ought to be considered.
The pEp Trust Rating system with all its states and respective
representations to be followed is outlined in
[I-D.marques-pep-rating].
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Note: An example for the rating of communication types, the
definition of the data structure by the pEp Engine reference
implementation is provided in Appendix A.2.
6.4. Trust Revoke
[[ TODO: This section will explain how to deal with the situation
when a peer can no longer be trusted, e.g., if a peer's device is
compromised. ]]
7. Synchronization
An important feature of pEp is to assist the user to run pEp
applications on different devices, such as computer, mobile phone or
tables, at the same time. Therefore state needs to be synchronized
among the different devices.
7.1. Private Key Synchronization
A decentralized proposition - the pEp Key Synchronization protocol.
[E-D.birk-pep-keysync] - defines how pEp users can distribute their
private keys among different devices in a secure and trusted manner:
this allows Internet users to read their messages across their
different devices, when sharing a common address (e.g., the same
email account).
7.2. Trust Synchronization
[[ TODO: This section will explain how trust and other related state
is synchronized among different devices in a secure manner. ]]
8. Interoperability
pEp aims to be interoperable with existing applications designed to
enable privacy, e.g., OpenPGP and S/MIME in email.
9. Options in pEp
In this section a non-exhaustive selection of options is provided.
9.1. Option "Passive Mode"
By default the sender attaches its public key to any outgoing message
(cf. Section 5.3). For situations where a sender wants to ensure
that it only attaches a public key to an Internet user which has a
pEp implementation, a Passive Mode MUST be available.
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9.2. Option "Disable Protection"
Using this option, protection can be disabled generally or
selectively. Implementers of pEp MUST provide an option "Disable
Protection" to allow a user to disable encryption and signing for:
1. all communication
2. specific contacts
3. specific messages
The public key still attached, unless the option "Passive Mode" (cf.
Section 9.1) is activated at the same time.
9.3. Option "Extra Keys"
For internal environments there may be a need to centrally decrypt
persons' messages for archiving or other legal purposes (e.g., in the
contexts of public offices and enterprises) by authorized personnel.
Therefore, pEp implementers MAY provide an "Extra Keys" option where
a message gets encrypted with at least one additional public key.
The corresponding secret key(s) are intended to be held (safely and
securely), e.g., by CISO staff or other authorized personnel for such
an organization.
The Extra Keys feature MUST NOT be activated by default for any
network address and is intended to be an option only for
organizational identities and their corresponding network addresses
and accounts - not for addresses used for private purposes. That is,
the Extra Keys feature is a feature which SHOULD NOT apply to all
identities a user might posses, even if activated.
9.4. Option "Blacklist Keys"
An option "Blacklist Keys" MUST be provided for an advanced user to
be able to disable keys which the user does not want to be used
anymore for any new communications. However, the keys SHALL NOT be
deleted. It MUST still be possible to verify and decipher past
communications.
10. Security Considerations
By attaching the sender's public key to outgoing messages, Trust on
First Use (TOFU) is established. However, this is prone to MITM
attacks. Cryptographic key subversion is considered Pervasive
Monitoring (PM) according to [RFC7258]. Those attacks can be
mitigated, if the involved users compare their common Trustwords.
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This possibility MUST be made easily accessible to pEp users in the
user interface implementation. If for compatibility reasons (e.g.,
with OpenPGP users) no Trustwords can be used, then a comparatively
easy way to verify the respective public key fingerprints MUST be
implemented.
As the use of passphrases for private keys is not advised, devices
themselves SHOULD use encryption.
11. Privacy Considerations
[[ TODO ]]
12. Implementation Status
12.1. 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."
12.2. 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]
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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.
12.3. Reference implementation of pEp's core
The pEp Foundation provides a reference implementation of pEp's core
principles and functionalities, which go beyond the documentation
status of this Internet-Draft. [SRC.pepcore]
pEp's reference implementation is composed of pEp Engine and pEp
Adapters (or bindings), alongside with some libraries which pEp
Engine relies on to function on certain platforms (like a NetPGP fork
we maintain for the iOS platform).
The pEp engine is a Free Software library encapsulating
implementations of:
o Key Management
Key Management in pEp engine is based on GnuPG key chains (NetPGP
on iOS). Keys are stored in an OpenPGP compatible format and can
be used for different crypto implementations.
o Trust Rating
pEp engine is sporting a two phase trust rating system. In phase
one there is a rating based on channel, crypto and key security
named "comm_types". In phase 2 these are mapped to user
representable values which have attached colors to present them in
traffic light semantics.
o Abstract Crypto API
The Abstract Crypto API is providing functions to encrypt and
decrypt data or full messages without requiring an application
programmer to understand the different formats and standards.
o Message Transports
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pEp engine will support a growing list of Message Transports to
support any widespread text messaging system including email, SMS,
XMPP and many more.
pEp engine is written in C99 programming language. It is not meant
to be used in application code directly. Instead, pEp engine is
coming together with a list of software adapters for a variety of
programming languages and development environments, which are:
o pEp COM Server Adapter
o pEp JNI Adapter
o pEp JSON Adapter
o pEp ObjC (and Swift) Adapter
o pEp Python Adapter
o pEp Qt Adapter
12.4. Abstract Crypto API examples
A selection of code excerpts from the pEp Engine reference
implementation (encrypt message, decrypt message, and obtain
trustwords) can be found in Appendix A.3.
13. Notes
The pEp logo and "pretty Easy privacy" are registered trademarks
owned by the non-profit pEp Foundation based in Switzerland.
Primarily, we want to ensure the following:
o Software using the trademarks MUST be backdoor-free.
o Software using the trademarks MUST be accompanied by a serious
(detailed) code audit carried out by a reputable third-party, for
any proper release.
The pEp Foundation will help to support any community-run (non-
commercial) project with the latter, be it organizationally or
financially.
Through this, the foundation wants to make sure that software using
the pEp trademarks is as safe as possible from a security and privacy
point of view.
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14. Acknowledgments
The authors would like to thank the following people who have
provided significant contributions to the development of this
document: Volker Birk, Krista Bennett, and S. Shelburn.
Furthermore, the authors would like to thank the following people who
who provided helpful comments and suggestions for this document:
Alexey Melnikov, Ben Campbell, Brian Trammell, Bron Gondwana, Daniel
Kahn Gillmor, Enrico Tomae, Eric Rescorla, Gabriele Lenzini, Hans-
Peter Dittler, Iraklis Symeonidis, Mirja Kuehlewind, Neal Walfield,
Pete Resnick, Russ Housley, and Stephen Farrel.
This work was initially created by pEp Foundation, and then reviewed
and extended with funding by the Internet Society's Beyond the Net
Programme on standardizing pEp. [ISOC.bnet]
15. References
15.1. Normative References
[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>.
[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>.
[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>.
15.2. Informative References
[E-D.birk-pep-keysync]
Birk, V. and H. Marques, "pretty Easy privacy (pEp): Key
Synchronization Protocol", June 2018,
<https://pep.foundation/dev/repos/internet-
drafts/file/tip/pep-keysync/
draft-birk-pep-keysync-NN.txt>.
Early draft
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[I-D.birk-pep-trustwords]
Birk, V., Marques, H., and B. Hoeneisen, "IANA
Registration of Trustword Lists: Guide, Template and IANA
Considerations", draft-birk-pep-trustwords-02 (work in
progress), June 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.marques-pep-handshake]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Contact and Channel Authentication through Handshake",
draft-marques-pep-handshake-01 (work in progress), October
2018.
[I-D.marques-pep-rating]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Mapping of Privacy Rating", draft-marques-pep-rating-00
(work in progress), July 2018.
[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/>.
[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>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[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>.
[RFC8280] ten Oever, N. and C. Cath, "Research into Human Rights
Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280,
October 2017, <https://www.rfc-editor.org/info/rfc8280>.
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[SRC.enigmailpep]
"Source code for Enigmail/pEp", July 2018,
<https://enigmail.net/index.php/en/download/source-code>.
[SRC.pepcore]
"Core source code and reference implementation of pEp
(engine and adapters)", July 2018,
<https://pep.foundation/dev/>.
[SRC.pepforandroid]
"Source code for pEp for Android", July 2018,
<https://pep-security.lu/gitlab/android/pep>.
[SRC.pepforios]
"Source code for pEp for iOS", July 2018,
<https://pep-security.ch/dev/repos/pEp_for_iOS/>.
[SRC.pepforoutlook]
"Source code for pEp for Outlook", July 2018,
<https://pep-security.lu/dev/repos/pEp_for_Outlook/>.
Appendix A. Code Excerpts
This section provides excerpts of the running code from the pEp
reference implementation pEp engine (C99 programming language).
A.1. pEp Identity
How the pEp identity is defined in the data structure (cf. src/
pEpEngine.h):
typedef struct _pEp_identity {
char *address; // C string with address UTF-8 encoded
char *fpr; // C string with fingerprint UTF-8
// encoded
char *user_id; // C string with user ID UTF-8 encoded
char *username; // C string with user name UTF-8
// encoded
PEP_comm_type comm_type; // type of communication with this ID
char lang[3]; // language of conversation
// ISO 639-1 ALPHA-2, last byte is 0
bool me; // if this is the local user
// herself/himself
identity_flags_t flags; // identity_flag1 | identity_flag2
// | ...
} pEp_identity;
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A.1.1. Corresponding SQL
Relational table scheme excerpts (in SQL) used in current pEp
implementations, held locally for every pEp installation in a SQLite
database:
CREATE TABLE person (
id text primary key,
username text not null,
main_key_id text
references pgp_keypair (fpr)
on delete set null,
lang text,
comment text,
device_group text,
is_pep_user integer default 0
);
CREATE TABLE identity (
address text,
user_id text
references person (id)
on delete cascade on update cascade,
main_key_id text
references pgp_keypair (fpr)
on delete set null,
comment text,
flags integer default 0,
is_own integer default 0,
timestamp integer default (datetime('now')),
primary key (address, user_id)
);
CREATE TABLE pgp_keypair (
fpr text primary key,
created integer,
expires integer,
comment text,
flags integer default 0
);
CREATE INDEX pgp_keypair_expires on pgp_keypair (
expires
);
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A.2. pEp Communication Type
In the following, is an example for the rating of communication types
as defined by a data structure (cf. src/pEpEngine.h [SRC.pepcore]):
typedef enum _PEP_comm_type {
PEP_ct_unknown = 0,
// range 0x01 to 0x09: no encryption, 0x0a to 0x0e:
// nothing reasonable
PEP_ct_no_encryption = 0x01, // generic
PEP_ct_no_encrypted_channel = 0x02,
PEP_ct_key_not_found = 0x03,
PEP_ct_key_expired = 0x04,
PEP_ct_key_revoked = 0x05,
PEP_ct_key_b0rken = 0x06,
PEP_ct_my_key_not_included = 0x09,
PEP_ct_security_by_obscurity = 0x0a,
PEP_ct_b0rken_crypto = 0x0b,
PEP_ct_key_too_short = 0x0c,
PEP_ct_compromized = 0x0e, // known compromized connection
PEP_ct_mistrusted = 0x0f, // known mistrusted key
// range 0x10 to 0x3f: unconfirmed encryption
PEP_ct_unconfirmed_encryption = 0x10, // generic
PEP_ct_OpenPGP_weak_unconfirmed = 0x11, // RSA 1024 is weak
PEP_ct_to_be_checked = 0x20, // generic
PEP_ct_SMIME_unconfirmed = 0x21,
PEP_ct_CMS_unconfirmed = 0x22,
PEP_ct_strong_but_unconfirmed = 0x30, // generic
PEP_ct_OpenPGP_unconfirmed = 0x38, // key at least 2048 bit
// RSA or EC
PEP_ct_OTR_unconfirmed = 0x3a,
// range 0x40 to 0x7f: unconfirmed encryption and anonymization
PEP_ct_unconfirmed_enc_anon = 0x40, // generic
PEP_ct_pEp_unconfirmed = 0x7f,
PEP_ct_confirmed = 0x80, // this bit decides if trust
// is confirmed
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// range 0x81 to 0x8f: reserved
// range 0x90 to 0xbf: confirmed encryption
PEP_ct_confirmed_encryption = 0x90, // generic
PEP_ct_OpenPGP_weak = 0x91, // RSA 1024 is weak (unused)
PEP_ct_to_be_checked_confirmed = 0xa0, //generic
PEP_ct_SMIME = 0xa1,
PEP_ct_CMS = 0xa2,
PEP_ct_strong_encryption = 0xb0, // generic
PEP_ct_OpenPGP = 0xb8, // key at least 2048 bit RSA or EC
PEP_ct_OTR = 0xba,
// range 0xc0 to 0xff: confirmed encryption and anonymization
PEP_ct_confirmed_enc_anon = 0xc0, // generic
PEP_ct_pEp = 0xff
} PEP_comm_type;
A.3. Abstract Crypto API examples
The following code excerpts are from the pEp Engine reference
implementation, to be found in src/message_api.h.
[[ Note: Just a selection; more functionality is available. ]]
A.3.1. Encrypting a Message
Cf. src/message_api.h [SRC.pepcore]:
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// encrypt_message() - encrypt message in memory
//
// parameters:
// session (in) session handle
// src (in) message to encrypt
// extra (in) extra keys for encryption
// dst (out) pointer to new encrypted message or NULL if
// no encryption could take place
// enc_format (in) encrypted format
// flags (in) flags to set special encryption features
//
// return value:
// PEP_STATUS_OK on success
// PEP_KEY_HAS_AMBIG_NAME at least one of the recipient
// keys has an ambiguous name
// PEP_UNENCRYPTED no recipients with usable key,
// message is left unencrypted,
// and key is attached to it
//
// caveat:
// the ownership of src remains with the caller
// the ownership of dst goes to the caller
DYNAMIC_API PEP_STATUS encrypt_message(
PEP_SESSION session,
message *src,
stringlist_t *extra,
message **dst,
PEP_enc_format enc_format,
PEP_encrypt_flags_t flags
);
Cf. src/message_api.h [SRC.pepcore]:
A.3.2. Decrypting a Message
Cf. src/message_api.h [SRC.pepcore]:
// decrypt_message() - decrypt message in memory
//
// parameters:
// session (in) session handle
// src (in) message to decrypt
// dst (out) pointer to new decrypted message
// or NULL on failure
// keylist (out) stringlist with keyids
// rating (out) rating for the message
// flags (out) flags to signal special decryption features
//
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// return value:
// error status
// or PEP_DECRYPTED if message decrypted but not verified
// or PEP_CANNOT_REENCRYPT if message was decrypted (and
// possibly verified) but a reencryption operation is
// expected by the caller and failed
// or PEP_STATUS_OK on success
//
// flag values:
// in:
// PEP_decrypt_flag_untrusted_server
// used to signal that decrypt function should engage in
// behaviour specified for when the server storing the
// source is untrusted
// out:
// PEP_decrypt_flag_own_private_key
// private key was imported for one of our addresses
// (NOT trusted or set to be used - handshake/trust is
// required for that)
// PEP_decrypt_flag_src_modified
// indicates that the src object has been modified. At
// the moment, this is always as a direct result of the
// behaviour driven by the input flags. This flag is the
// ONLY value that should be relied upon to see if such
// changes have taken place.
// PEP_decrypt_flag_consume
// used by sync
// PEP_decrypt_flag_ignore
// used by sync
//
//
// caveat:
// the ownership of src remains with the caller - however, the
// contents might be modified (strings freed and allocated anew
// or set to NULL, etc) intentionally; when this happens,
// PEP_decrypt_flag_src_modified is set.
// the ownership of dst goes to the caller
// the ownership of keylist goes to the caller
// if src is unencrypted this function returns PEP_UNENCRYPTED
// and sets
// dst to NULL
DYNAMIC_API PEP_STATUS decrypt_message(
PEP_SESSION session,
message *src,
message **dst,
stringlist_t **keylist,
PEP_rating *rating,
PEP_decrypt_flags_t *flags
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);
A.3.3. Obtain Common Trustwords
Cf. src/message_api.h [SRC.pepcore]:
// get_trustwords() - get full trustwords string
// for a *pair* of identities
//
// parameters:
// session (in) session handle
// id1 (in) identity of first party in communication
// - fpr can't be NULL
// id2 (in) identity of second party in communication
// - fpr can't be NULL
// lang (in) C string with ISO 639-1 language code
// words (out) pointer to C string with all trustwords
// UTF-8 encoded, separated by a blank each
// NULL if language is not supported or
// trustword wordlist is damaged or unavailable
// wsize (out) length of full trustwords string
// full (in) if true, generate ALL trustwords for these
// identities.
// else, generate a fixed-size subset.
// (TODO: fixed-minimum-entropy subset
// in next version)
//
// return value:
// PEP_STATUS_OK trustwords retrieved
// PEP_OUT_OF_MEMORY out of memory
// PEP_TRUSTWORD_NOT_FOUND at least one trustword not found
//
// caveat:
// the word pointer goes to the ownership of the caller
// the caller is responsible to free() it
// (on Windoze use pEp_free())
//
DYNAMIC_API PEP_STATUS get_trustwords(
PEP_SESSION session, const pEp_identity* id1,
const pEp_identity* id2, const char* lang,
char **words, size_t *wsize, bool full
);
Appendix B. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
o draft-birk-pep-03:
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* Major restructure of the document
* Adapt authors to the actual authors and extend acknowledgement
section
* Added several new sections, e.g., Key Reset, Trust Revoke,
Trust Synchronization, Private Key Export / Import, Privacy
Considerations (content yet mostly TODO)
* Added reference to HRPC work / RFC8280
+ Added text and figure to better explain pEp's automated Key
Exchange and Trust management (basic message flow)
* Lots of improvement in text and editorial changes
o draft-birk-pep-02:
* Move (updated) code to Appendix
* Add Changelog to Appendix
* Add Open Issue section to Appendix
* Fix description of what Extra Keys are
* Fix Passive Mode description
* Better explain pEp's identity system
o draft-birk-pep-01:
* Mostly editorial
o draft-birk-pep-00:
* Initial version
Appendix C. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication ]]
o References to RFC6973 (Privacy Considerations)
o Add references to prior work, and what differs here - PEM (cf.
RFC1421-1424)
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Internet-Draft pretty Easy privacy (pEp) March 2019
o Better explain Passive Mode
o Better explain / illustrate pEp's identity system
o Explain Key Mapping (to S/MIME)
o Add more information to deal with organizational requirements
o Add text to Key Reset (Section 5.4) as well as reference as soon
as available
o Add text to Trust Revoke (Section 6.4) as well as reference as
soon as available
o Add text to Trust Synchronization (Section 7.2) as well as
reference as soon as available
o Add references to Private Key Export / Import (Section 5.2.3) as
soon as reference available
o Add text to Privacy Considerations (Section 11)
Authors' Addresses
Hernani Marques
pEp Foundation
Oberer Graben 4
CH-8400 Winterthur
Switzerland
Email: hernani.marques@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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