pretty Easy privacy (pEp): Privacy by Default
draft-birk-pep-05

Document Type Active Internet-Draft (individual)
Last updated 2019-11-04
Stream (None)
Intended RFC status (None)
Formats plain text xml pdf htmlized bibtex
Stream Stream state (No stream defined)
Consensus Boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date
Responsible AD (None)
Send notices to (None)
Network Working Group                                            V. Birk
Internet-Draft                                                H. Marques
Intended status: Standards Track                            B. Hoeneisen
Expires: May 7, 2020                                      pEp Foundation
                                                       November 04, 2019

             pretty Easy privacy (pEp): Privacy by Default
                           draft-birk-pep-05

Abstract

   The pretty Easy privacy (pEp) model and protocols describe a set of
   conventions for the automation of operations traditionally seen as
   barriers to the use and deployment of secure, privacy-preserving 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).  Human Rights-enabling principles like
   Data Minimization, End-to-End and Interoperability are explicit
   design goals.  For the goal of usable privacy, pEp introduces means
   to verify communication between 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 with email), and are
   written with the intent to be interoperable with already widely-
   deployed systems in order to 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 7, 2020.

Birk, et al.               Expires May 7, 2020                  [Page 1]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

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
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.3.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Protocol's Core Design Principles . . . . . . . . . . . . . .   6
     2.1.  Privacy by Default  . . . . . . . . . . . . . . . . . . .   6
     2.2.  Data Minimization . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Interoperability  . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Peer-to-Peer  . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  User Interaction  . . . . . . . . . . . . . . . . . . . .   8
   3.  Identity System . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  User  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Address . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Key . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Identity  . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Key Management  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Key Generation  . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Private Keys  . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Storage . . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.2.  Passphrase  . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Key Reset . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Public Key Distribution . . . . . . . . . . . . . . . . .  11
       4.4.1.  UX Considerations . . . . . . . . . . . . . . . . . .  12
       4.4.2.  No addition of unnecessary metadata . . . . . . . . .  12
       4.4.3.  No centralized public key storage or retrieval by
               default . . . . . . . . . . . . . . . . . . . . . . .  12
       4.4.4.  Example message flow  . . . . . . . . . . . . . . . .  12
     4.5.  Key Reset . . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Trust Management  . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Privacy Status  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Handshake . . . . . . . . . . . . . . . . . . . . . . . .  15

Birk, et al.               Expires May 7, 2020                  [Page 2]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

     5.3.  Trust Rating  . . . . . . . . . . . . . . . . . . . . . .  16
     5.4.  Trust Revoke  . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Synchronization . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Private Key Synchronization . . . . . . . . . . . . . . .  16
     6.2.  Trust Synchronization . . . . . . . . . . . . . . . . . .  16
   7.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Options in pEp  . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Option "Passive Mode" . . . . . . . . . . . . . . . . . .  17
     8.2.  Option "Disable Protection" . . . . . . . . . . . . . . .  17
     8.3.  Option "Extra Keys" . . . . . . . . . . . . . . . . . . .  17
       8.3.1.  Use Case for Organizations  . . . . . . . . . . . . .  17
       8.3.2.  Use Case for Key Synchronization  . . . . . . . . . .  18
     8.4.  Option "Blacklist Keys" . . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  18
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Introduction . . . . . . . . . . . . . . . . . . . . . .  19
     12.2.  Current software implementing pEp  . . . . . . . . . . .  19
     12.3.  Reference implementation of pEp's core . . . . . . . . .  20
     12.4.  Abstract Crypto API examples . . . . . . . . . . . . . .  21
   13. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Code Excerpts  . . . . . . . . . . . . . . . . . . .  24
     A.1.  pEp Identity  . . . . . . . . . . . . . . . . . . . . . .  24
       A.1.1.  Corresponding SQL . . . . . . . . . . . . . . . . . .  24
     A.2.  pEp Communication Type  . . . . . . . . . . . . . . . . .  25
     A.3.  Abstract Crypto API examples  . . . . . . . . . . . . . .  27
       A.3.1.  Encrypting a Message  . . . . . . . . . . . . . . . .  27
       A.3.2.  Decrypting a Message  . . . . . . . . . . . . . . . .  28
       A.3.3.  Obtain Common Trustwords  . . . . . . . . . . . . . .  30
   Appendix B.  Document Changelog . . . . . . . . . . . . . . . . .  30
   Appendix C.  Open Issues  . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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 which enable access to Human
   Rights.  Today's applications widely lack privacy support that
   ordinary users can easily adapt.  The pretty Easy privacy (pEp)

Birk, et al.               Expires May 7, 2020                  [Page 3]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   protocols generally conform to the principles outlined in [RFC8280],
   and, as such, can facilitate the adoption and correct usage of secure
   and private communications technology.

   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.

   The pEp project emerged from the CryptoParty movement.  During that
   time, the initiators learned that while step-by-step guides can help
   some users engage in secure end-to-end communications, it is both
   more effective and convenient for the vast majority of users if these
   step-by-step guides are put into running code (following a protocol),
   which automates the initial configuration and general usage of
   cryptographic tools.  To facilitate this goal, pEp proposes the
   automation of key management, key discovery, and key synchronization
   through an in-band approach which follows the end-to-end principle.

   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.

   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.

Birk, et al.               Expires May 7, 2020                  [Page 4]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

1.1.  Relationship to other pEp documents

   While this document outlines the general design choices and
   principles of pEp, other related documents specialize in more
   particular aspects of the model, or the application of pEp on a
   specific protocol like as follows:

   1.  pEp-enabled applications (e.g., pEp email
       [I-D.marques-pep-email]).

   2.  Helper functions for peer interaction, which facilitate
       understanding and handling of the cryptographic aspects of pEp
       implementation for users (e.g., pEp Handshake
       [I-D.marques-pep-handshake]).

   3.  Helper functions for interactions between a user's own devices,
       which give the user the ability to run pEp applications on
       different devices at the same time, such as a computer, mobile
       phone, or tablets (e.g., pEp KeySync
       [I-D.hoeneisen-pep-keysync]).

   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].

   [[ Note: At this stage it is not yet clear to us how many of our
   implementation details should be part of new RFCs and where we can
   safely refer to already existing RFCs to clarify which RFCs we rely
   on. ]]

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].

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,

Birk, et al.               Expires May 7, 2020                  [Page 5]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

      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."

2.  Protocol's Core Design Principles

2.1.  Privacy by Default

   pEp's most important goal is to ensure privacy above all else.  To
   clarify, pEp's protocol defaults are designed to maximize both
   security and privacy, but in the few cases where achieving both more
   privacy and more security are in conflict, pEp chooses more privacy.

   In contrast to pEp's prioritization of user privacy, OpenPGP's Web-
   of-Trust (WoT) releases user and trust level relationships to the
   public.  In addition, queries to OpenPGP keyservers dynamically
   disclose the social graph, indicating a user's intent to communicate
   with specific peers.  Similar issues exist in other security
   protocols that rely upon a centralized trust model, such as the
   certificate revocation protocols used in XPKI (S/MIME).

   [[*TODO*: Fix the wording and reference to XPKI, S/MIME]].

   In pEp messaging (e.g., when using HTML) content and information
   SHALL NOT be obtained from remote locations as this constitutes a
   privacy breach.

   Because of the inherent privacy risks in using remote or centralized
   infrastructures, implementations of pEp messaging, by default, SHALL
   NOT obtain content and information from remote or centralized
   locations, as this constitutes a privacy breach.  In email this issue
   exists with HTML mails.

Birk, et al.               Expires May 7, 2020                  [Page 6]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

2.2.  Data Minimization

   Data minimization keeps data spare and hides all technically
   concealable information whenever possible.  It is an important design
   goal of pEp.

2.3.  Interoperability

   The proposed pEp protocols seek interoperability with established
   message formats, as well as cryptographic security protocols and
   their widespread implementations.

   To achieve this interoperability, pEp MUST follow Postel's Robustness
   Principle outlined in [RFC1122]: "Be liberal in what you accept, and
   conservative in what you send."

   Particularly, pEp applies Postel's principle as follows:

   o  pEp is conservative (strict) in requirements for pEp
      implementations and how they interact with pEp or other compatible
      implementations.

   o  pEp liberally accepts input from non-pEp implementations.  For
      example, in email, pEp will not produce outgoing messages, but
      will transparently support decryption of incoming PGP/INLINE
      messages.

   o  Finally, where pEp requires divergence from established RFCs due
      to privacy concerns (e.g., from OpenPGP propositions as defined in
      [OpenPGP], options SHOULD be implemented which empower the user to
      override pEp's defaults.

2.4.  Peer-to-Peer

   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.

Birk, et al.               Expires May 7, 2020                  [Page 7]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   [I-D.marques-pep-rating]) to users of pEp (e.g., when interoperating
   with peers using S/MIME) by default.

2.5.  User Interaction

   Implementers of pEp MUST NOT expose users to technical terms and
   views, especially those specific to cryptography, like "keys",
   "certificates", or "fingerprints".  Users may explicitly opt-in to
   see such terms wanting seeking for more options; i.e., advanced
   settings MAY be available, and in some cases, these options may be
   required.

   The authors believe that widespread adoption of end-to-end
   cryptography is possible if users are not required to understand
   cryptography and key management.  This belief forms the central goal
   of pEp, which is that users can simply rely on the principles of
   Privacy by Default.

   On the other hand, to preserve usability, users MUST NOT be required
   to wait for cryptographic tasks such as key generation to complete
   before being able to use their respective message client for its
   default purpose.  In short, pEp implementers MUST ensure that the
   ability to draft, send, and receive messages is always preserved,
   even if that means a message is sent unencrypted, in accordance with
   the Opportunistic Security approach outlined in [RFC7435].

   In turn, pEp implementers MUST ensure that a distinguishable privacy
   status is clearly visible to the user, both on a per-contact as well
   as per-message level.  This allows users to assess both the privacy
   level for the message and the trust level of its intended recipients
   before choosing to send it.

   [[ *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 it is currently
   done for already-popular instant messaging services. ]]

3.  Identity System

   Everyone has the right to choose how to reveal themselves to the
   world, both offline and online.  This is an important element to
   maintain psychological, physical, and digital privacy.  As such, pEp
   users MUST have the option to choose their identity, and they MUST
   have the ability to maintain multiple identities.

Birk, et al.               Expires May 7, 2020                  [Page 8]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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).

3.1.  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 the own
   user does not have a user_id, then it is assigned a "pEp_own_userId"
   instead.

   A user can have a default key.  [[ TODO: Provide ref explaining this.
   ]]

3.2.  Address

   A pEp address is a network address, e.g., an SMTP address or another
   Universal Resource Identifier (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. ]]

3.3.  Key

   A key is an OpenPGP-compatible asymmetric cryptographic key pair.

   Keys in pEp are identified by the full fingerprint (fpr) of the
   public key.  The fingerprint is to be obtained from the specific
   cryptographic service used to handle the keys.  The canonical
   representation in pEp is upper-case hexadecimal with zero-padding and
   no separators or spaces.

3.4.  Identity

   An identity is a representation of a user, encapsulating how this
   user appears within the network of a messaging system.  This
   representation may or may not be pseudonymous in nature.

   An identity is defined by mapping a user_id to an address.  If no
   user_id is known, it is guessed by mapping a username to an address.

Birk, et al.               Expires May 7, 2020                  [Page 9]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   An identity can have a temporary user_id as a placeholder until a
   real user_id is known.

   In pEp a different key pair for each (e.g., email) account MUST be
   created.  This allows a user to retain different identities, which
   are not correlated by the usage of the same key for all of those.
   This is beneficial in terms of privacy.

   A user MAY have a default key; each identity a user has MAY have a
   default key (of its own).  When both an Identity and the related User
   have a default key set, the Identity's default key MUST override the
   User's default key.

   [[ TODO: Provide ref explaining this. ]]

   In Appendix A.1, the definition of a pEp identity can be found
   according to the reference implementation by the pEp engine.

4.  Key Management

   In order to achieve the goal of widespread adoption of secure
   communications, key management in pEp MUST be automated.

4.1.  Key Generation

   A pEp implementation MUST ensure that cryptographic keys for every
   configured identity 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.

4.2.  Private Keys

4.2.1.  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

Birk, et al.               Expires May 7, 2020                 [Page 10]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   principle to rely on centralized infrastructures (cf.  Section 2.4).
   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 6.1 for a means to synchronize private keys among
   different devices of the same network address in a secure manner.

4.2.2.  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.

4.3.  Key Reset

4.4.  Public Key Distribution

   As the key is available (cf.  Section 4.1) 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.

Birk, et al.               Expires May 7, 2020                 [Page 11]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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.

4.4.1.  UX Considerations

   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.

4.4.2.  No addition of unnecessary metadata

   Metadata, such as email headers, MUST NOT be added in order to
   announce a user's public key.  This is considered unnecessary
   information leakage, may affect user privacy, and may be subject to a
   country's data retention laws (cf.  Section 2.2).  Furthermore, this
   may affect interoperability to existing users that have no knowledge
   of such header fields, such as users of OpenPGP in email, and lose
   the ability to import any keys distributed in this way as a result.
   The ability to extract and receive public keys from such metadata
   SHOULD be supported, however, in the event these approaches become
   widespread.

4.4.3.  No centralized public key storage or retrieval by default

   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.

4.4.4.  Example message flow

   The following example roughly describes a pEp scenario with a typical
   initial message flow to demonstrate key exchange and basic trust
   management:

   The following example describes a pEp scenario between two users -
   Alice and Bob - in order to demonstrate the message flow that occurs
   when exchanging keys and determining basic trust management for the
   first time:

Birk, et al.               Expires May 7, 2020                 [Page 12]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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 receives Alice's message and her public key.  He is able to
       reply to her and encrypt the message.  His public key is
       automatically attached to the message.  Because he has her public
       key now, Alice's rating in his message client changes to
       'encrypted'.  From a UX perspective, this status is displayed in
       yellow (cf.  Section 5.3).

   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 5.3).

   4.  Alice receives Bob's reply with his public key attached.  Now,
       Alice can send secure messages to Bob as well.  The rating for
       Bob changes to yellow, or 'encrypted', in Alice's messaging
       client Section 5.3.

   5.  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 5.2) and confirm this process if
       those match.  After doing so, their identity rating changes to
       "encrypted and authenticated" (cf.  Section 5.3), which (UX-wise)
       can be displayed using a green color.  See also Section 5.

   6.  Alice and Bob can encrypt now, but they are not yet
       authenticated, leaving them vulnerable to man-in-the-middle
       (MitM) attacks.  To prevent this from occurring, Alice and Bob
       can engage in a pEp Handshake to compare their Trustwords (cf.
       Section 5.2) and confirm if they match.  After this step is
       performed, their respective identity ratings change to "encrypted
       and authenticated", which is represented by a green color (cf.
       Section 5.

Birk, et al.               Expires May 7, 2020                 [Page 13]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

             -----                                       -----
             | 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*       |
   +-----------------------+                   +-----------------------+
               |                                           |

Birk, et al.               Expires May 7, 2020                 [Page 14]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

4.5.  Key Reset

   [[ TODO: This section will explain how to deal with invalid keys,
   e.g., if expired or (potentially) leaked. ]]

5.  Trust Management

   [[ TODO: Intro ]]

5.1.  Privacy Status

   The trust status for an identity can change due to a number of
   factors.  These shifts will cause the color code assigned to this
   identity to change accordingly, and is applied to future
   communications with this identity.

   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.

5.2.  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.

Birk, et al.               Expires May 7, 2020                 [Page 15]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

5.3.  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].

   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.

5.4.  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. ]]

6.  Synchronization

   An important feature of pEp is to assist the user to run pEp
   applications on different devices, such as personal computers, mobile
   phones and tablets, at the same time.  Therefore, state needs to be
   synchronized among the different devices.

6.1.  Private Key Synchronization

   The pEp KeySync protocol (cf.  [I-D.hoeneisen-pep-keysync]) is a
   decentralized proposition which defines how pEp users can distribute
   their private keys among their different devices in a user-
   authenticated manner.  This allows users to read their messages
   across their various devices, as long as they share a common address,
   such as an email account.

6.2.  Trust Synchronization

   [[ TODO: This section will explain how trust and other related state
   is synchronized among different devices in a user-authenticated
   manner. ]]

Birk, et al.               Expires May 7, 2020                 [Page 16]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

7.  Interoperability

   pEp aims to be interoperable with existing applications designed to
   enable privacy, e.g., OpenPGP and S/MIME in email.

8.  Options in pEp

   In this section a non-exhaustive selection of options is provided.

8.1.  Option "Passive Mode"

   By default, the sender attaches its public key to any outgoing
   message (cf.  Section 4.4).  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 made available.

8.2.  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 8.1) is activated at the same time.

8.3.  Option "Extra Keys"

8.3.1.  Use Case for Organizations

   For internal or enterprise environments, authorized personnel may
   need to centrally decrypt user messages for archival or other legal
   purposes.  Therefore, pEp implementers MAY provide an "Extra Keys"
   option in which a message is encrypted with at least one additional
   public key.  The corresponding secret key(s) are intended to be
   secured by CISO staff or other authorized personnel for the
   organization.

   However, it is crucial that the "Extra Keys" feature MUST NOT be
   activated by default for any network address, and is intended to be
   an option used only for organization-specific identities, as well as
   their corresponding network addresses and accounts.  The "Extra Keys"

Birk, et al.               Expires May 7, 2020                 [Page 17]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   feature SHOULD NOT be applied to the private identities, addresses,
   or accounts a user might possess once it is activated.

8.3.2.  Use Case for Key Synchronization

   The "Extra Keys" feature also plays a role during pEp's KeySync
   protocols, where the additional keys are used to decipher message
   transactions by both parties involved during the negotiation process
   for private key synchronization.  During the encrypted (but
   untrusted) transactions, KeySync messages are not just encrypted with
   the sending device's default key, but also with the default keys of
   both parties involved in the synchronization process.

8.4.  Option "Blacklist Keys"

   A "Blacklist Keys" option MAY be provided for an advanced user,
   allowing them to disable keys of peers that they no longer want to
   use in new communications.  However, the keys SHALL NOT be deleted.
   It MUST still be possible to verify and decipher past communications
   that used these keys.

9.  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.
   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.

10.  Privacy Considerations

   [[ TODO ]]

11.  IANA Considerations

   This document has no actions for IANA.

Birk, et al.               Expires May 7, 2020                 [Page 18]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

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]

   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.

Birk, et al.               Expires May 7, 2020                 [Page 19]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

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

      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

Birk, et al.               Expires May 7, 2020                 [Page 20]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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.

14.  Acknowledgments

   The authors would like to thank the following people who provided
   substantial contributions, helpful comments or suggestions for this
   document: Alexey Melnikov, Athena Schumacher, Ben Campbell, Brian
   Trammell, Bron Gondwana, Claudio Luck, Daniel Kahn Gillmor, Enrico
   Tomae, Eric Rescorla, Gabriele Lenzini, Hans-Peter Dittler, Iraklis
   Symeonidis, Kelly Bristol, Krista Bennett, Mirja Kuehlewind, Nana
   Kerlstetter, Neal Walfield, Pete Resnick, Russ Housley, S.  Shelburn,
   and Stephen Farrel.

Birk, et al.               Expires May 7, 2020                 [Page 21]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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

   [I-D.birk-pep-trustwords]
              Hoeneisen, B. and H. Marques, "IANA Registration of
              Trustword Lists: Guide, Template and IANA Considerations",
              draft-birk-pep-trustwords-04 (work in progress), July
              2019.

   [I-D.hoeneisen-pep-keysync]
              Hoeneisen, B., Marques, H., and K. Bristol, "pretty Easy
              privacy (pEp): Key Synchronization Protocol (KeySync)",
              draft-hoeneisen-pep-keysync-01 (work in progress), October
              2019.

   [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-03 (work in progress), July
              2019.

Birk, et al.               Expires May 7, 2020                 [Page 22]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   [I-D.marques-pep-rating]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Mapping of Privacy Rating", draft-marques-pep-rating-02
              (work in progress), July 2019.

   [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/>.

   [OpenPGP]  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>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [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>.

   [SRC.enigmailpep]
              "Source code for Enigmail/pEp", July 2019,
              <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 2019,
              <https://pep-security.lu/gitlab/android/pep>.

Birk, et al.               Expires May 7, 2020                 [Page 23]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   [SRC.pepforios]
              "Source code for pEp for iOS", July 2019,
              <https://pep-security.ch/dev/repos/pEp_for_iOS/>.

   [SRC.pepforoutlook]
              "Source code for pEp for Outlook", July 2019,
              <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;

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:

Birk, et al.               Expires May 7, 2020                 [Page 24]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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
   );

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

Birk, et al.               Expires May 7, 2020                 [Page 25]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

       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

       // 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

Birk, et al.               Expires May 7, 2020                 [Page 26]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

       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]:

Birk, et al.               Expires May 7, 2020                 [Page 27]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   // 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
   //

Birk, et al.               Expires May 7, 2020                 [Page 28]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   //  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

Birk, et al.               Expires May 7, 2020                 [Page 29]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   );

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-05:

Birk, et al.               Expires May 7, 2020                 [Page 30]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

      *  Change authors

      *  Minor changes, especially in identity system

   o  draft-birk-pep-04:

      *  Fix internal reference

      *  Add IANA Considerations section

      *  Add other use case of Extra Keys

      *  Add Claudio Luck as author

      *  Incorporate review changes by Kelly Bristol and Nana
         Karlstetter

   o  draft-birk-pep-03:

      *  Major restructure of the document

      *  Adapt authors to the actual authors and extend Acknowledgments
         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

Birk, et al.               Expires May 7, 2020                 [Page 31]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

   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  Shorten Introduction and Abstract

   o  References to RFC6973 (Privacy Considerations)

   o  Add references to prior work, and what differs here - PEM (cf.
      RFC1421-1424)

   o  Better explain Passive Mode

   o  Better explain / illustrate pEp's identity system

   o  Explain Concept of Key Mapping (e.g. to S/MIME, which is to be
      refined in pEp application docs auch as pEp email
      [I-D.marques-pep-email])

   o  Add more information to deal with organizational requirements

   o  Add text to Key Reset (Section 4.3) as well as reference as soon
      as available

   o  Add text to Trust Revoke (Section 5.4) as well as reference as
      soon as available

   o  Add text to Trust Synchronization (Section 6.2) as well as
      reference as soon as available

   o  Add text to Privacy Considerations (Section 10)

   o  Scan for leftovers of email-specific stuff and move it to pEp
      email I-D [I-D.marques-pep-email], while replacing it herein with
      generic descriptions.

Birk, et al.               Expires May 7, 2020                 [Page 32]
Internet-Draft          pretty Easy privacy (pEp)          November 2019

Authors' Addresses

   Volker Birk
   pEp Foundation
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland

   Email: volker.birk@pep.foundation
   URI:   https://pep.foundation/

   Hernani Marques
   pEp Foundation
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland

   Email: hernani.marques@pep.foundation
   URI:   https://pep.foundation/

   Bernie Hoeneisen
   pEp Foundation
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland

   Email: bernie.hoeneisen@pep.foundation
   URI:   https://pep.foundation/

Birk, et al.               Expires May 7, 2020                 [Page 33]