Network Working Group V. Birk
Internet-Draft H. Marques
Intended status: Standards Track S. Shelburn
Expires: December 29, 2018 pEp Foundation
June 27, 2018

pretty Easy privacy (pEp): Privacy by Default


Building on already available security formats and message transports (like PGP/MIME for email), and with the intention to stay interoperable to systems widespreadly deployed, pretty Easy privacy (pEp) describes protocols to automatize operations (key management, key discovery, private key handling including peer-to-peer synchronization of private keys and other user data across devices) that have been seen to be barriers to deployment of end-to-end secure interpersonal messaging. pEp also relies on “Trustwords” (as a word mapping of of fingerprints) to verify communication peers and proposes a trust rating system to denote secure types of communications and signal the privacy level available on a per-user and per-message level. In this document, the general design choices and principles of pEp are outlined.

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

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This Internet-Draft will expire on December 29, 2018.

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Table of Contents

1. Introduction

The pretty Easy privacy (pEp) protocols are propositions to the Internet community to create software for peers to automatically encrypt, anonymize (where possible, depending on the message transport used) and verify their daily written digital communications – this is done by building upon already existing standards and tools and automatizing all steps a user would need to carry out to engage in secure end-to-end encrypted communications without depending on centralized infrastructures.

Particularly, pEp proposes to automatize key management, key discovery and also synchronization of secret key material by an in-band peer-to-peer approach.

To mitigate Man-In-The-Middle Attacks (MITM) and as the only manual step users may carry out, Trustwords [I-D.birk-pep-trustwords] as natural language representations of two peers’ fingerprints are proposed, for peers to put trust on their communication channel.

[[ Note: The pEp initiators had to learn from the CryptoParty movement, from which the project emerged, that step-by-step guides can be helpful to a particular set of users to engage in secure end-to-end communications, but that for a much major fraction of users it would be more convenient to have the step-by-step procedures put into actual code (as such, following a protocol) and thus automatizing the initial configuration and whole usage of cryptographic tools.]]

The Privacy by Default principles that pretty Easy privacy (pEp) introduces, are in accordance with the perspective outlined in [RFC7435] to bring Opportunistic Security in the sense of “some protection most of the time”, with the subtle, but important difference that when privacy is weighted against security, the choice falls to privacy. Therefore, data minimization is a primary goal in pEp (e.g., omitting unnecessary email headers and encrypting the subject line).

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 the different standardized and/or widely spread communications channels, such that users can reach their peers in the most privacy-enhancing way possible.

[[At this stage it is not year clear to us how many of our implementation details should be part of new RFCs and at which places we can safely refer to already existing RFCs to make clear on which RFCs we are already relying.]]

2. Terms

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

3. Protocol’s Core Design Principles

3.1. Privacy by Default

The pEp protocols are about to ensure privacy. However, there are cases where privacy and security are contradicting, e.g., in PGP’s Web of Trust, because relations between people and trust levels are leaked. Also query privacy is not ensured in such a model when obtaining keys from remote locations. Whenever security and privacy fit together, highest security and privacy is to be reached. However, where they contradict each other, privacy is always to be chosen over security. Though, users SHOULD have the choice to override the default by corresponding options.

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

3.2. Interoperability

The pEp propositions seek to be interoperable with message formats and cryptography already widespread. Seamless communication between users of software, which implements pEp and other messaging tools for end-to-end encryption, is a design goal.


3.3. Peer-to-Peer (P2P)

Messaging and verification processes in pEp are designed to work Peer-to-Peer (P2P) without intermediaries in between.

This means, there MUST NOT be any pEp-specific central services whatsoever needed for implementers of pEp to rely on, neither for verification of peers nor for the actual encryption.

Still, implementers of pEp MAY provide options to interoperate 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. [E-D.birk-pep-trust-rating]) to users of pEp (e.g., when interoperating with peers using S​/​MIME) by default.

3.4. User Experience (UX)

Implementers of pEp MUST take special care not to confuse users with technical terms, especially those of cryptography (e.g., “keys”, “certificates” or “fingerprints”), unless users explicitly ask for such terms; i.e., advanced settings MAY be available, in some cases further options may even be required. However, those SHALL NOT be unnecessarily exposed to users of pEp implementations at the first sight.

The authors believe widespread adoption of end-to-end cryptography is much less of an issue, if the users are not hassled and visibly forced in any way to use cryptography, i.e., a goal of pEp is that users can just rely on the principles of Privacy by Default.

By consequence, this means that users must not wait for cryptographic tasks (e.g., key generation or public key retrieval) to finish, before being able to have their respective message clients ready to communicate. This finally means, pEp implementers MUST ensure that the ability to draft, send and receive messages is always preserved – even if that means a message is sent out unencrypted, thus being in accordance with the Opportunistic Security approach outlined in [RFC7435].

In turn, pEp implementers MUST ensure a Privacy Status is clearly visible to the user – on contact as well as on message level – so that users easily understand, which level of privacy messages are about to be sent or were received with, respectively.

[[Note: We are aware of the fact that usually UX requirements are not part of RFCs. However, to have massively more people engaged in secure end-to-end encryption and at the same time to avoid putting users at risk, we believe requiring certain straightforward signaling for the users to be a good idea – in a similar way as this happens to be the case for already popular Instant Messaging services.]]

4. pEp identity system

In pEp, users MUST have the possibility to have different identities.

pEp users MUST have the option to choose different identities. This allows an Internet user to decide how to reveal oneself to the world and is an important element to achieve privacy.

The different identities MUST NOT correlate with other by default. On the other hand, combining different identities MUST be supported (to support aliases).

4.1. Terms

4.1.1. Key

A key is an OpenPGP compatible asymmetric key pair. Other formats and temporary symmetrical keys can be generated by Key Mapping.

Keys in pEp are identified by the full fingerprint (fpr) of its public key.

4.1.2. User

A user is a real world human being or a group of human beings. If it is a single human being, it can be called person.

A user is identified by a user ID (user_id). The user_id SHOULD be an UUID, it MAY be an arbitrary unique string.

The own user can have a user_id like all other users. If it doesn’t, then it has PEP_OWN_USERID “pEp_own_userId” as user_id.

A user can have a default key.

4.1.3. Address

An address is a network address, e.g., an SMTP address or another URI.

[[ Note: It might be necessary to introduce further addressing schemes through IETF contributions or IANA registrations, e.g., implementing pEp to bridge to popular messaging services with no URIs defined. ]]

4.1.4. Identity

An identity is a (possibly pseudonymous) representation of a user encapsulating how this user appears in the network.

An identity is defined by the mapping of user_id to address. If no user_id is known, it is guessed by mapping of username and address.

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

An identity can have a default key.

[[ Note: This is the reason why in current pEp implementations for each email account a different key pair is created, which allows a user to retain different identities. ]]

In Appendix A.1 you can find how a pEp identity is defined in the reference implementation of the pEp Engine.

4.2. Example: Difference between pEp and OpenPGP

pEp OpenPGP Comments
user_id (no concept) ID for a person, i.e. a contact
username + address uid comparable only for email
fpr fingerprint used as key ID in pEp
(no concept) Key ID does not exist in pEp

5. Key Management

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

A pEp implementation MUST ensure cryptographic keys for end-to-end cryptography are generated for every identity configured (or instantly upon its configuration [[ TODO: unclear/rewrite/simplify ]]), if no secure cryptographic setup can be found. Users SHALL NOT be stopped from communicating – this also applies for initial situations where cryptographic keys are not generated fast enough. This process MUST be carried out in the background so the user is not stopped from communicating. [[ TODO: rewrite/simplify ]]

pEp includes a Trust Rating system [E-D.birk-pep-trust-rating] defining what kind of encryption is considered reliable and is thus secure enough for usage in pEp implementations. This also applies for keys already available for the given identity. If the available keys are considered insecure (e.g, insufficient key length), pEp implementers are REQUIRED to generate new keys for use with the respective identity.

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.

5.1. Private Keys

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 MUST be considered a violation of pEp’s P2P design principle to rely on centralized infrastructures. 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 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. ]]

A decentralized proposition – the pEp Key Synchronization protocol. [E-D.birk-pep-keysync] – defines how pEp users can distribute their private keys among different devices in a secure and trusted manner: this allows Internet users to read their messages across their different devices, when sharing a common address (e.g., the same email account).

5.2. Public Key Distribution

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 previously received a message encrypted with the public key of the sender.

The sender’s public key SHOULD be sent encrypted whenever possible, i.e. when a public key of the receiving peer is available. If no encryption key of the recipient is available, the sender’s public key MAY be sent unencrypted. In either case, this approach ensures that messaging clients (e.g., MUAs that at least implement OpenPGP) do not need to have pEp implemented to see a user’s public key. Such peers thus have the chance to (automatically) import the sender’s public key.

If there is already a known public key from the sender of a message and it is still valid and not expired, new keys MUST not be used for future communication, unless they are signed by the previous key (to avoid a MITM attack). Messages MUST always be encrypted with the receiving peer’s oldest public key, as long as it is valid and not expired.

Implementers of pEp SHALL prevent that public keys attached to messages (e.g, in email) are displayed to the user, in order to avoid users getting confused by a file they cannot potentially deal with.

Metadata (e.g., email headers) MUST NOT be added to announce a user’s public key. This is considered unnecessary information leakage as it may affect users’ privacy, which depends also on a country’s data retention laws. Furthermore, this affects interoperability to existing users (e.g., in the OpenPGP field) that have no notion of such header fields and thus lose the ability to import any such keys distributed this way. It SHOULD, though, be supported to obtain other users’ public keys by extracting them from respective header fields of received messages (in case such approaches get widespread).

Keyservers or generally intermediate approaches to obtain a peer’s public key SHALL NOT be used by default. On the other hand, the user MAY be provided with the option to opt-in for remote locations to obtain keys, considering the widespread adoption of such approaches for key distribution.

Keys generated or obtained by pEp clients SHALL NOT be uploaded to any (intermediate) keystore locations without the user’s explicit consent.

5.3. Passphrases

Passphrases to protect a user’s private key MUST be supported by pEp implementations, but SHALL 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 SHALL 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.

6. Privacy Status

For end-users, the most important component of pEp, which MUST be made visible on a per-recipient and per-message level, is the Privacy Status.

By colors, symbols and texts a user SHALL immediately understand how private

The Privacy Status in its most general form MUST be 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. In cases no (special) Privacy Status can be inferred for peers or messages, no color (or the gray color) MUST be shown and respective texts – being “unknown” or “unreliable” – MUST be shown.

The detailed Privacy Status as an end-user element of the pEp Trust Rating system with all its states and respective representations to be followed is outlined in [E-D.birk-pep-trust-rating].

7. Options in pEp

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

7.1. Option “Passive Mode”

By default the sender attaches its public key to any outgoing message (cf. Section 5.2). For situations where a sender wants to ensure that it only attaches a public key to an Internet user which has a pEp implementation, a Passive Mode MUST be available.

7.2. Option “Disable Protection”

This option SHALL not affect the user’s ability to decipher already received or sent messages. [[ TODO: Public key added in these cases? ]]

Protection can be disabled generally or selectively.

7.2.1. For all communications

Implementers of pEp MUST provide an option “Disable Protection” for the user’s choice to disable any outgoing encryption and signing.

7.2.2. For some communications

Implementers of pEp MUST provide an option to allow users to disable protection (encryption and signing) for specific contacts or messages.

7.3. Option “Extra Keys”

For internal environments there may be a need to centrally decrypt persons’ messages for archiving or other legal purposes (e.g., in the contexts of public offices and enterprises) by authorized personnel. Therefore, pEp implementers MAY provide an “Extra Keys” option where a message gets encrypted with at least one additional public key. The corresponding (shared) secret(s) to decrypt are intended to be held – safely – by CISO staff and/or other authorized personnel for such an organization. [[ TODO: Shared secret? no private key? ]]

The Extra Keys feature MUST NOT be activated by default for any network address and is intended to be an option only for organizational identities and their corresponding network addresses and accounts – not for addresses used for private purposes. That is, the Extra Keys feature is a feature which SHOULD NOT apply to all identities a user might posses, even if activated.

7.4. Option “Blacklist Keys”

An option “Blacklist Keys” MUST be provided for an advanced user to be able to disable keys which the user does not want to be used anymore for any new communications. However, the keys SHALL NOT be deleted. It MUST still be possible to verify and decipher past communications.

7.5. Establishing trust between peers

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.

8. 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 an 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.

9. Implementation Status

9.1. 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.”

9.2. 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:

9.3. 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.

9.4. Current software implementing pEp

The following software implementing the pEp protocols (to varying degrees) already exists; it does not yet go beyond implementing pEp for email, which is described nearer in [E-D.birk-pep-email]:

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.

10. Notes

The pEp logo and “pretty Easy privacy” are registered trademarks owned by pEp Foundation in Switzerland, a tax-free, non-commercial entity.

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.

11. Acknowledgments

The authors would like to thank the following people who have provided feedback or significant contributions to the development of this document: Bernie Hoeneisen, Brian Trammell, Enrico Tomae, Eric Rescorla Neal Walfield, and Stephen Farrel.

[[ TODO: Add those who commented on mailing list (on this draft) as well as those who provided feedback in person or during BarBoFs. ]]

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]

12. References

12.1. 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.

12.2. Informative References

[E-D.birk-pep-email] Birk, V. and H. Marques, "pretty Easy privacy (pEp): Secure and Trusted Email Communication", June 2018.

Early draft

[E-D.birk-pep-handshake] Marques, H., "pretty Easy privacy (pEp): Contact Authentication through Handshake", June 2018.

Early draft

[E-D.birk-pep-keysync] Birk, V. and H. Marques, "pretty Easy privacy (pEp): Key Synchronization Protocol", June 2018.

Early draft

[E-D.birk-pep-trust-rating] Birk, V. and H. Marques, "pretty Easy privacy (pEp): Trust Rating System", June 2018.

Early draft

[I-D.birk-pep-trustwords] Birk, V., Marques, H. and B. Hoeneisen, "IANA Registration of Trustword Lists: Guide, Template and IANA Considerations", Internet-Draft draft-birk-pep-trustwords-02, June 2018.
[ISOC.bnet] Simao, I., "Beyond the Net. 12 Innovative Projects Selected for Beyond the Net Funding. Implementing Privacy via Mass Encryption: Standardizing pretty Easy privacy’s protocols", June 2017.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D. and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007.
[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.
[SRC.enigmailpep] "Source code for Enigmail/pEp", June 2018.
[SRC.pepcore] "Core source code and reference implementation of pEp (engine and adapters)", June 2018.
[SRC.pepforandroid] "Source code for pEp for Android", June 2018.
[SRC.pepforios] "Source code for pEp for iOS", June 2018.
[SRC.pepforoutlook] "Source code for pEp for Outlook", June 2018.

Appendix A. Excerpts from the pEp Reference Implementation

This section provides excerpts of the running code from the pEp reference implementation pEp engine (C99 programming language). [[ TODO: Maybe rewrite sentence a bit ]]

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, an example for the rating of communication types, the definition of the 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

Volker Birk pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: URI:
Hernani Marques pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: URI:
Shelburn pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: URI: