Network Working Group H. Marques
Internet-Draft C. Luck
Intended status: Standards Track pEp Foundation
Expires: January 9, 2020 B. Hoeneisen
July 08, 2019

pretty Easy privacy (pEp): Privacy by Default


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

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Table of Contents

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) 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.

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:

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.

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:

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. [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 take special care not to overburden users with technical terms, especially those specific to cryptography, like “keys”, “certificates”, or “fingerprints”. Users may explicitly opt for exposure to these terms; i.e., advanced settings MAY be available, and in some cases, these options may be required. However, these options SHALL NOT be exposed to users of pEp implementations unless necessary or opted-for.”

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 an unambiguous privacy status is clearly visible to the user, both on a per-contact as well as per-message level. This allows users to see at a glance 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.

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. 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.

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

For this reason, 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).

[[ 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 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.

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:

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

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

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.

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. ]]

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” 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.

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:

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:

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:

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:

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 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, Athena Schumacher, Ben Campbell, Brian Trammell, Bron Gondwana, Daniel Kahn Gillmor, Enrico Tomae, Eric Rescorla, Gabriele Lenzini, Hans-Peter Dittler, Iraklis Symeonidis, Kelly Bristol, Mirja Kuehlewind, Nana Kerlstetter, 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.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014.

15.2. Informative References

[I-D.birk-pep-trustwords] Hoeneisen, B. and H. Marques, "IANA Registration of Trustword Lists: Guide, Template and IANA Considerations", Internet-Draft draft-birk-pep-trustwords-04, July 2019.
[I-D.hoeneisen-pep-keysync] Hoeneisen, B. and H. Marques, "pretty Easy privacy (pEp): Key Synchronization Protocol", Internet-Draft draft-hoeneisen-pep-keysync-00, July 2019.
[I-D.marques-pep-email] Marques, H., "pretty Easy privacy (pEp): Email Formats and Protocols", Internet-Draft draft-marques-pep-email-02, October 2018.
[I-D.marques-pep-handshake] Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp): Contact and Channel Authentication through Handshake", Internet-Draft draft-marques-pep-handshake-03, July 2019.
[I-D.marques-pep-rating] Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp): Mapping of Privacy Rating", Internet-Draft draft-marques-pep-rating-02, 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.
[OpenPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D. and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[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.
[RFC8280] ten Oever, N. and C. Cath, "Research into Human Rights Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280, October 2017.
[SRC.enigmailpep] "Source code for Enigmail/pEp", July 2019.
[SRC.pepcore] "Core source code and reference implementation of pEp (engine and adapters)", July 2018.
[SRC.pepforandroid] "Source code for pEp for Android", July 2019.
[SRC.pepforios] "Source code for pEp for iOS", July 2019.
[SRC.pepforoutlook] "Source code for pEp for Outlook", July 2019.

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:

   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 (

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

    // 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]:

// 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
//  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

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 ]]

Appendix C. Open Issues

[[ RFC Editor: This section should be empty and is to be removed before publication ]]

Authors' Addresses

Hernani Marques pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: URI:
Claudio Luck pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: URI:
Bernie Hoeneisen Ucom Standards Track Solutions GmbH CH-8046 Zuerich, Switzerland Phone: +41 44 500 52 40 EMail: (bernhard.hoeneisen AT URI: