Internet DRAFT - draft-pep-general
draft-pep-general
Network Working Group V. Birk
Internet-Draft H. Marques
Intended status: Standards Track B. Hoeneisen
Expires: 19 June 2023 pEp Foundation
16 December 2022
pretty Easy privacy (pEp): Privacy by Default
draft-pep-general-02
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 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 19 June 2023.
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Copyright Notice
Copyright (c) 2022 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
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provided without warranty as described in the Revised 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. End-to-End . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Peer-to-Peer . . . . . . . . . . . . . . . . . . . . . . 8
2.6. User Interaction . . . . . . . . . . . . . . . . . . . . 8
3. pEp Identity System . . . . . . . . . . . . . . . . . . . . . 9
3.1. User . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Own User . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Identity . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Key Management . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 11
4.2. Private Keys . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Storage . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Passphrase . . . . . . . . . . . . . . . . . . . . . 12
4.3. Public Key Distribution . . . . . . . . . . . . . . . . . 13
4.3.1. UX Considerations . . . . . . . . . . . . . . . . . . 13
4.3.2. No centralized public key storage or retrieval by
default . . . . . . . . . . . . . . . . . . . . . . . 13
4.3.3. Example message flow . . . . . . . . . . . . . . . . 14
4.4. Key Reset . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Trust Management . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Privacy Status . . . . . . . . . . . . . . . . . . . . . 16
5.2. Trust Rating . . . . . . . . . . . . . . . . . . . . . . 16
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5.3. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Synchronization . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Private Key Synchronization . . . . . . . . . . . . . . . 17
7. Options in pEp . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Option "Passive Mode" . . . . . . . . . . . . . . . . . . 17
7.2. Option "Disable Protection" . . . . . . . . . . . . . . . 17
7.3. Option "Extra Keys" . . . . . . . . . . . . . . . . . . . 18
7.3.1. Use Case for Organizations . . . . . . . . . . . . . 18
7.3.2. Use Case for Key Synchronization . . . . . . . . . . 18
7.4. Option "Blacklist Keys" . . . . . . . . . . . . . . . . . 18
7.5. Option "Trusted Server" . . . . . . . . . . . . . . . . . 19
7.5.1. Changing Server Trust . . . . . . . . . . . . . . . . 19
8. Interoperability . . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 20
11.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 20
11.2. Current software implementations of pEp . . . . . . . . 20
11.3. Reference implementation of pEp's core . . . . . . . . . 21
11.4. Abstract Crypto API examples . . . . . . . . . . . . . . 22
12. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
14.1. Normative References . . . . . . . . . . . . . . . . . . 23
14.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Code Excerpts . . . . . . . . . . . . . . . . . . . 26
A.1. pEp Identity . . . . . . . . . . . . . . . . . . . . . . 26
A.1.1. Corresponding SQL . . . . . . . . . . . . . . . . . . 26
A.2. pEp Communication Type . . . . . . . . . . . . . . . . . 27
A.3. Abstract Crypto API examples . . . . . . . . . . . . . . 29
A.3.1. Encrypting a Message . . . . . . . . . . . . . . . . 29
A.3.2. Decrypting a Message . . . . . . . . . . . . . . . . 30
A.3.3. Obtain Common Trustwords . . . . . . . . . . . . . . 32
Appendix B. Document Changelog . . . . . . . . . . . . . . . . . 33
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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],
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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 to automate 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 to
achieve 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 to 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 that follows the end-to-end principle.
To mitigate man-in-the-middle (MITM) attacks by an active adversary,
and as the only manual step users carry out in the course of the
protocols, pEp proposes a Trustword [I-D.pep-trustwords] mechanism.
Trustwords are natural language representations of two peers'
fingerprints. Users can verify their trust in a paired communication
channel by comparing the corresponding Trustwords.
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
communication channels such that users can reach communication
partners in the most privacy-enhancing way possible.
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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.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.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.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.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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.3. Terms
The following terms are defined for the scope of this document:
* 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.pep-handshake]
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* Trustwords: A representation of 16-bit natural numbers (0 to
65535) as natural language words: For each natural language a
fixed number-to-word map can be defined as convention and
registered with IANA. Trustwords are generated from the combined
public key fingerprints of a both communication partners.
Trustwords are used for verification and establishment of trust
(for the respective keys and communication partners).
[I-D.pep-trustwords]
* 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."
* Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
form of active wiretapping attack in which the attacker intercepts
and selectively modifies communicated data to masquerade as one or
more of the entities involved in a communication association."
Note: Historically, MITM has stood for '_Man_-in-the-middle'.
However, to indicate that the entity in the middle is not always a
human attacker, MITM can also stand for 'Machine-in-the-middle' or
'Meddler-in-the-middle'.
2. 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 PKIX
[RFC5280] used e.g., for S/MIME [RFC8551].
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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 follows 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:
* pEp is conservative (strict) in requirements for pEp
implementations and how they interact with pEp or other compatible
implementations.
* 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.
* Finally, where pEp requires divergence from established RFCs due
to privacy concerns (e.g., from OpenPGP propositions as defined in
[RFC4880], options SHOULD be implemented which empower the user to
override pEp's defaults.
2.4. End-to-End
Because of the inherent privacy risks in using remote or centralized
infrastructures, implementations of pEp messaging, by default, MUST
NOT obtain content and information from remote or centralized
locations, as this constitutes a privacy breach. In email this issue
exists with HTML mails.
Encryption and decryption of messages MUST be executed on a user's
end-device and MUST NOT depend on any third-party network
infrastructure (i.e., any infrastructure outside a user's direct
control).
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2.5. 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) MUST NOT be signaled as trustworthy (cf.
[I-D.pep-rating]) to users of pEp (e.g., when interoperating with
peers using S/MIME) by default.
2.6. User Interaction
Implementers of pEp SHOULD NOT expose users to technical terms and
views, especially those specific to cryptography, like "keys",
"certificates", or "fingerprints". However, certain (advanced) users
MAY explicitly opt-in to see such terms, e.g., seeking for more
options. Advanced settings may be available. In some cases, such
settings or 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 SHOULD 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.
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[[ *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. pEp 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 pEp 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 pEp User ID (user_id) identifies a pEp User. The variable user_id
SHOULD be a Universally Unique IDentifier (UUID), however it MAY also
be an arbitrary unique string.
A pEp User may have a default pEp Key (cf. Section 3.3).
In the pEp reference implementation (cf. Appendix A), the 'id'
(text) field in table 'person' contains the pEp User ID.
Table 'identity' (that references to table 'person') the 'user_id'
(text) field contains the pEp User ID, too. The 'main_key_id' (text)
field in table 'person' contains the default pEp Key (that references
to field 'fpr' in table 'pgp_keypair'; cf. Section 3.3).
3.1.1. Own User
The Own User in pEp is a special case of a pEp User (cf.
Section 3.1), i.e. the (local) pEp User that utilizes the pEp client
in question. As all pEp Users also the Own User needs a User ID,
which is assigned at creation of the Own User entry.
In the pEp reference implementation (cf. Appendix A), the 'is_own'
(integer) field in table 'identity' is set to 1, if the entry belongs
to an Own User.
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3.2. Address
An address in pEp means the designator of a destination where
messages can be routed to and accessed from, e.g., email address,
Uniform Resource Identifier (URI), Network Access Identifier (NAI),
phone number, etc.
An address may belong to one or more users. A user may have multiple
Addresses.
[[ 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. ]]
In the pEp reference implementation (cf. Appendix A), the 'address'
(text) field in table 'identity' contains the address.
3.3. Key
A pEp Key is an asymmetric cryptographic key pair compatible to
OpenPGP [RFC4880].
pEp Keys are identified by the full fingerprint (fpr) of the public
key. The fingerprint is obtained from the specific cryptographic
application used to handle the keys. The canonical representation of
the fingerprint in pEp is upper-case hexadecimal with zero-padding
and no separators or spaces.
In the pEp reference implementation (cf. Appendix A), the 'fpr'
(text) field in table 'pgp_keypair' contains the fingerprint of the
pEp Key. The 'main_key_id' (text) field in tables 'person' and
'identity' (containing the default pEp Key) links to field 'fpr' in
table 'pgp_keypair').
3.4. Identity
A pEp Identity is a representation of a pEp User (cf. Section 3.1),
defining how this pEp User appears within the network of a messaging
system. This representation may or may not be pseudonymous in
nature.
A pEp Identity is defined by mapping a pEp User ID (cf. Section 3.1)
to an address (cf. Section 3.2).
A pEp Identity can have a temporary pEp User ID (user_id) as a
placeholder until a real pEp User ID is known.
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In case different accounts are utilized by the same pEp User, a
different pEp Key (cf. Section 3.3) for each account MUST be
created. This allows a pEp User to retain different identities that
cannot be correlated by sharing the same key for those identities
(improved privacy).
A pEp User as well as each pEp Identity MAY have a default pEp Key.
When both - a pEp Identity and the related pEp User - have a default
pEp Key assigned, the pEp Identity's default pEp Key MUST take
precedence over the pEp User's default pEp Key.
In the pEp reference implementation (cf. Appendix A), the above
described mapping is done by adding an entry to table 'identity'
linking its field 'user_id' to the 'id' field in table 'person'.
3.5. Alias
pEp Aliases share the same key and identity.
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 (cf.
Section 3.3) for every configured pEp Identity (cf. Section 3.4) are
available. If a corresponding key pair for the identity of a pEp
User is found and said pEp Identity fulfills the requirements (e.g.,
for email, as set out in [I-D.pep-email]), said key pair SHOULD 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
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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 2.5).
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 designed in a way such that the
locally-held private key can neither be directly accessed by the
server application nor transfered to any server, i.e. the private key
MUST remain local and MUST NOT be leaked anywhere else.
Furthermore, it is important that browser add-ons implementing pEp
functionality do not dynamically obtain their cryptographic code from
a centralized (e.g., cloud) service. Cryptographic code MUST always
be stored locally, installed e.g., as part of a certified and signed
installation package, containing the add-on and additional code to
execute the cryptographic functions. Dynamically obtained code is
considered a centralized attack vector allowing for backdoors,
negatively impacting privacy and security.
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 SHOULD 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 SHOULD 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.
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4.3. Public Key Distribution
As the pEp Key is available (cf. Section 4.1) implementers of pEp
MUST ensure that by default the public key of the pEp Identity (cf.
Section 3.4) in use (Own User; cf. Section 3.1.1) is attached to
every outgoing message. However, this MAY be omitted if the pEp
Identity in use (Own User) has previously received a message from the
(remote) pEp Identity in question and that message was encrypted with
public key that the would otherwise be attached.
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.
4.3.1. UX Considerations
Implementers of pEp MUST 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.
4.3.2. No centralized public key storage or retrieval by default
Keyservers or generally intermediate approaches to obtain a peer's
public key MUST NOT be used by default. Though, the user MAY be
provided with an option (opt-in) to obtain keys from remote
locations, in order to regard 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.
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4.3.3. 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 5.2).
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 5.2).
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 5.2.
5. 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 5.3) and confirm if those 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.
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----- -----
| A | | B |
----- -----
| |
+------------------------+ +------------------------+
| auto-generate key pair | | auto-generate key pair |
| (if no key yet) | | (if no key yet) |
+------------------------+ +------------------------+
| |
+-----------------------+ +-----------------------+
| Privacy Status for B: | | Privacy Status for A: |
| *Unencrypted* | | *Unencrypted* |
+-----------------------+ +-----------------------+
| |
| A sends message to B (Public Key |
| attached) / optionally signed, but |
| NOT ENCRYPTED |
+------------------------------------------>|
| |
| +-----------------------+
| | Privacy Status for A: |
| | *Encrypted* |
| +-----------------------+
| |
| B sends message to A (Public Key |
| attached) / signed and ENCRYPTED |
|<------------------------------------------+
| |
+-----------------------+ |
| Privacy Status for B: | |
| *Encrypted* | |
+-----------------------+ |
| |
| A and B successfully compare their |
| Trustwords over an alternative channel |
| (e.g., phone line) |
|<-- -- -- -- -- -- -- -- -- -- -- -- -- -->|
| |
+-----------------------+ +-----------------------+
| Privacy Status for B: | | Privacy Status for A: |
| *Trusted* | | *Trusted* |
+-----------------------+ +-----------------------+
| |
4.4. Key Reset
Key Reset is specified in [I-D.pep-keyreset].
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5. Trust Management
5.1. Privacy Status
The Privacy Status for an identity can change due to a number of
factors. A change of privacy status will update the color code
assigned to this identity accordingly, and is applied to future
communications with this identity.
The Privacy Status is the most important component of pEp.
Implementers of pEp MUST ensure that he Privacy Status is clearly
visible to the user - on a per-recipient as well as on a per-message
level, so that the user immediately understands from colors, symbols
and texts how private
* a communication channel with a given peer was (or ought to be) and
* a given message was (or ought to be).
The Privacy Status is further specified in [I-D.pep-rating].
5.2. Trust Rating
pEp includes a Trust Rating system defining Rating and Color Codes to
display the Privacy Status of a peer or message (cf.
[I-D.pep-rating]):
* Ratings are labeled, e.g., as "Unencrypted", "Encrypted",
"Trusted", "Under Attack", etc.
* The Privacy Status in its most general form is represented with
traffic lights semantics (and respective symbols and words). The
three colors "yellow", "green" and "red" are assigned to
communication channels or messages (depending on the Privacy
Status). Those immediately indicate how secure and trustworthy
(and thus private) a communication was or ought to be considered.
Note that there the may be no color applied, e.g. for the case
that no public key is available to engage in private
communications with an identity.
The pEp Trust Rating system with all its states and respective
representations is specified in [I-D.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.
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5.3. Handshake
To establish trust between peers and to upgrade the Privacy Status
(cf. Section 5.1) of a peer, pEp defines a handshake, which is
specified in [I-D.pep-handshake] and illustrated in Section 4.3.3
above. During the handshake Trustwords [I-D.pep-trustwords] are
presented to pEp Users to compare and thus verify the authenticity of
peers in order to mitigate MITM attacks. Trustwords are normally
computed from the two peers' fingerprints of their public keys.
In order to retain compatibility with peers not using pEp
implementations (e.g., Mail User Agents (MUAs) with OpenPGP [RFC4880]
implementations without Trustwords), it is REQUIRED that pEp
implementers provide the user with the option to show both peers'
fingerprints instead of (or in addition to) the Trustwords.
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.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 channel, such as an email
account.
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 4.3). For situations where a sender wants to
ensure that it only attaches a public key to an Internet user which
has a pEp implementation, a Passive Mode MUST be made available.
7.2. 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:
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1. all communication
2. specific contacts
3. specific messages
Note that this option does not change the behavior on whether of not
to attach the public key. Whether or not to attach the public key
depends on the option "Passive Mode" (cf. Section 7.1).
7.3. Option "Extra Keys"
7.3.1. 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.
7.3.2. Use Case for Key Synchronization
The "Extra Keys" feature also plays a role during pEp's KeySync
protocols [I-D.pep-keysync], 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.
7.4. 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 MUST NOT be deleted.
It MUST still be possible to verify and decipher past communications
that used these keys.
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7.5. Option "Trusted Server"
By default messages are stored encrypted on a server. With this
option enabled messages will be stored unencrypted.
This may be useful in trusted environments (such as organizations)
e.g., to conform to legal requirements such as archiving regulations.
pEp implementations MUST provide a "Trusted Server" option. With the
user's explicit consent (opt-in), unencrypted copies of the Messages
MUST be held on the server regarded as trusted.
7.5.1. Changing Server Trust
Changing the server trust is possible at any time.
7.5.1.1. Changing from trusted to untrusted server
When changing from trusted server to untrusted server,
implementations MAY encrypt all existing messages on the server,
though this is NOT REQUIRED. However, new messages MUST be stored
encrypted and whenever an existing (unencrypted) message is re-
opened, it MUST be stored encrypted.
7.5.1.2. Changing from untrusted to trusted server
When changing from untrusted server to trusted server, implementation
MAY decrypt all existing messages on the server, though this is NOT
REQUIRED. However, new messages MUST be stored unencrypted and
whenever an existing (encrypted) message is re-opened, it MUST be
stored unencrypted.
8. Interoperability
pEp aims to be interoperable with existing applications designed to
enable privacy, e.g., OpenPGP [RFC4880] and S/MIME [RFC8551] in
email.
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
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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. IANA Considerations
This document has no actions for IANA.
11. Implementation Status
11.1. Introduction
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "[...] this will allow reviewers and working
groups to assign due consideration to documents that have the benefit
of running code, which may serve as evidence of valuable
experimentation and feedback that have made the implemented protocols
more mature. It is up to the individual working groups to use this
information as they see fit."
11.2. Current software implementations of pEp
The following software implementations of the pEp protocols (to
varying degrees) already exists:
* pEp for Outlook as add-on for Microsoft Outlook, release
[SRC.pepforoutlook]
* pEp for iOS (implemented in a new MUA), release [SRC.pepforios]
* pEp for Android (based on a fork of the K9 MUA), release
[SRC.pepforandroid]
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* pEp for Thunderbird as an add-on for Thunderbird, release
[SRC.pepforthunderbird]
Note: The former community project Enigmail/pEp as add-on for
Thunderbird was discontinued and replaced by pEp's own add-on for
Thunderbird [SRC.pepforthunderbird] in 2021.
pEp for Android, iOS, Outlook and Thunderbird are provided by pEp
Security, a commercial entity specializing in end-user pEp
implementations.
All software is available as Free and Open Source Software and
published also in source form.
11.3. 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:
* 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.
* 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.
* 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.
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* 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:
* pEp COM Server Adapter
* pEp JNI Adapter
* pEp JSON Adapter
* pEp ObjC (and Swift) Adapter
* pEp Python Adapter
11.4. 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.
12. 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:
* Software using the trademarks MUST be backdoor-free.
* Software using the trademarks MUST be accompanied by a serious
(detailed) code audit carried out by a reputable third-party, for
any proper release.
The pEp Foundation will help to support any community-run (non-
commercial) project with the latter, be it organizationally or
financially.
Through this, the foundation wants to make sure that software using
the pEp trademarks is as safe as possible from a security and privacy
point of view.
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13. 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, Linda Carmen Schmid, Luca Saiu, Krista
Bennett, Mirja Kuehlewind, Nana Karlstetter, Neal Walfield, Pete
Resnick, Russ Housley, S. Shelburn, 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]
14. References
14.1. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14.2. Informative References
[I-D.pep-email]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Email Formats and Protocols", Work in Progress, Internet-
Draft, draft-pep-email-02, 16 December 2022,
<https://www.ietf.org/archive/id/draft-pep-email-02.txt>.
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[I-D.pep-handshake]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Contact and Channel Authentication through Handshake",
Work in Progress, Internet-Draft, draft-pep-handshake-00,
16 December 2022, <https://www.ietf.org/archive/id/draft-
pep-handshake-00.txt>.
[I-D.pep-keyreset]
Hoeneisen, B., "pretty Easy privacy (pEp): Key Reset",
Work in Progress, Internet-Draft, draft-pep-keyreset-00,
15 December 2022, <https://www.ietf.org/archive/id/draft-
pep-keyreset-00.txt>.
[I-D.pep-keysync]
Birk, V., Hoeneisen, B., and K. Bristol, "pretty Easy
privacy (pEp): Key Synchronization Protocol (KeySync)",
Work in Progress, Internet-Draft, draft-pep-keysync-02, 13
July 2020, <https://www.ietf.org/archive/id/draft-pep-
keysync-02.txt>.
[I-D.pep-rating]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Mapping of Privacy Rating", Work in Progress, Internet-
Draft, draft-pep-rating-00, 16 December 2022,
<https://www.ietf.org/archive/id/draft-pep-rating-00.txt>.
[I-D.pep-trustwords]
Hoeneisen, B. and H. Marques, "IANA Registration of
Trustword Lists: Guide, Template and IANA Considerations",
Work in Progress, Internet-Draft, draft-pep-trustwords-00,
16 December 2022, <https://www.ietf.org/archive/id/draft-
pep-trustwords-00.txt>.
[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/>.
[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>.
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[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[SRC.pepcore]
"Core source code and reference implementation of pEp
(engine and adapters)", March 2022,
<https://gitea.pep.foundation/pEp.foundation/pEpEngine>.
[SRC.pepforandroid]
"Source code for pEp for Android", December 2022,
<https://pep-security.lu/gitlab/android/pep>.
[SRC.pepforios]
"Source code for pEp for iOS", December 2022,
<https://pep-security.lu/gitlab/iOS/pep4ios>.
[SRC.pepforoutlook]
"Source code for pEp for Outlook", December 2022,
<https://pep-security.lu/gitlab/win/pEpForOutlook>.
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[SRC.pepforthunderbird]
"Source code for pEp for Thunderbird", December 2022,
<https://pep-security.lu/gitlab/thunderbird/
pEpForThunderbird>.
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\n
///< user_id MIGHT be set to
///< "pEp_own_userId" (use PEP_OWN_USERID
///< preprocessor define)
///< if this is own user's identity.
///< But it is not REQUIRED to be.
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
unsigned int major_ver; ///< highest version of pEp message
///< received, if any
unsigned int minor_ver; ///< highest version of pEp message
///< received, if any
PEP_enc_format enc_format; ///< Last specified format we encrypted
///< to for this identity
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:
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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,
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,
pEp_version_major integer default 0,
pEp_version_minor integer default 0,
enc_format 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
);
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
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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_key_expired_but_confirmed = 0x07, // NOT with confirmed bit.
// Just retaining info here
// in case of renewal.
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_compromised = 0x0e, // known compromised connection
PEP_ct_compromized = 0x0e, // deprecated misspelling
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)
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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() -->
*
* @brief Encrypt message in memory
*
* @param[in] session session handle
* @param[in,out] src message to encrypt - usually in-only
* except for the rating field, but can be
* in-out for unencrypted messages;
* in that case, we may attach the key
* and decorate the message.
* In any case, reset the rating.
* @param[in] extra extra keys for encryption
* @param[out] dst pointer to new encrypted message or
* NULL if no
* encryption could take place
* @param[in] enc_format The desired format this message should
* be encrypted with
* @param[in] flags flags to set special encryption
* features
*
* @retval PEP_STATUS_OK on success
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* @retval PEP_KEY_HAS_AMBIG_NAME at least one of the receipient
* keys has an ambiguous name
* @retval PEP_UNENCRYPTED on demand or no recipients with
* usable key, is left unencrypted,
* and key is attached to it
* @retval PEP_ILLEGAL_VALUE illegal parameter values
* @retval PEP_OUT_OF_MEMORY out of memory
* @retval any other value on error
*
* @warning the ownership of src remains with the caller
* the ownership of dst goes to the caller
*
* @warning enc_format PEP_enc_inline_EA:
* internal format of the encrypted attachments is changing, see
* https://dev.pep.foundation/Engine/ElevatedAttachments\n
* Only use this for transports without support for attachments
* when attached data must be sent inline
*
*/
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
);
A.3.2. Decrypting a Message
Cf. src/message_api.h [SRC.pepcore]:
/**
* <!-- decrypt_message_2() -->
*
* @brief Decrypt message in memory
*
* @param[in] session session handle
* @param[in,out] src message to decrypt - see warning about
* identities below.
* the rating field of src (instead of dst)
* is updated in case encryption fails.
* @param[out] dst pointer to new decrypted message or NULL
* on failure
* @param[in,out] keylist in: stringlist with additional keyids
* for reencryption if needed (will be freed
* and replaced with output keylist)
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* out: stringlist with keyids used for
* signing and encryption. first key is
* signer, additional keys are the ones it was
* encrypted to. Only signer and whichever
* of the user's keys was used are reliable
* @param[in,out] flags flags to signal special decryption features
*
* @retval <ERROR> any error status
* @retval PEP_DECRYPTED if message decrypted but not verified
* @retval PEP_CANNOT_REENCRYPT if message was decrypted (and possibly
* verified) but a reencryption operation
* is expected by the caller and failed
* @retval PEP_STATUS_OK on success
*
* @note Flags above are as follows:
* @verbatim
* -------------------------------------------------------------------|
* Incoming flags |
* -------------------------------------------------------------------|
* Flag | Description |
* ----------------------------------|--------------------------------|
* 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. |
* -------------------------------------------------------------------|
* Outgoing flags |
* -------------------------------------------------------------------|
* 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 |
* | modified_src field should |
* | contain a modified version of |
* | the source, at the moment |
* | always as a result of the |
* | input flags. |
* | |
* PEP_decrypt_flag_consume | used by sync to indicate this |
* | was a pEp internal message and |
* | should be consumed externally |
* | withoutshowing it as a normal |
* | message to the user |
* | |
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* PEP_decrypt_flag_ignore | used by sync |
* -------------------------------------------------------------------|
* @endverbatim
*
* @ownership 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.
*
* @ownership dst goes to the caller
*
* @ownership contents of keylist goes to the caller
*
* @note if src is unencrypted this function returns PEP_UNENCRYPTED
* and sets dst to NULL
* @note if src->enc_format is PEP_enc_inline_EA on input then elevate
* attachments will be expected
*
*
* @warning decrypt_message RELIES on the fact that identity
* information provided in src for recips and sender is AS
* TAKEN FROM THE ORIGINAL PARSED MESSAGE. This means that if
* update_identity or myself is called on those identities by
* the caller before passing the message struct to
* decrypt_message, the caller will have to cache and restore
* those to their original state before sending them to this
* function.
* ADAPTERS AND APPLICATIONS PLEASE TAKE NOTE OF THIS.
* (Obviously, this doesn't include information like
* user_ids, but we very specifically need the incoming
* usernames preserved so that they can be handled by the
* internal algorithm appropriately)
*/
DYNAMIC_API PEP_STATUS decrypt_message_2(
PEP_SESSION session,
message *src,
message **dst,
stringlist_t **keylist,
PEP_decrypt_flags_t *flags
);
A.3.3. Obtain Common Trustwords
Cf. src/message_api.h [SRC.pepcore]:
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/**
* <!-- get_trustwords() -->
*
* @brief Get full trustwords string for a *pair* of identities
*
* @param[in] session session handle
* @param[in] id1 identity of first party in communication -
* fpr can't be NULL
* @param[in] id2 identity of second party in communication -
* fpr can't be NULL
* @param[in] lang C string with ISO 639-1 language code
* @param[out] words 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
* @param[out] wsize length of full trustwords string
* @param[in] full if true, generate ALL trustwords for these
* identities.
* else, generate a fixed-size subset.
* (TODO: fixed-minimum-entropy
* subset in next version)
*
* @retval PEP_STATUS_OK trustwords retrieved
* @retval PEP_OUT_OF_MEMORY out of memory
* @retval PEP_ILLEGAL_VALUE illegal parameter values
* @retval PEP_TRUSTWORD_NOT_FOUND at least one trustword not found
*
* @warning 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 ]]
* draft-pep-general-02:
- Reorganized chapter structure to align with [I-D.pep-email]
- Update Terms and References
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- Serveral corrections and enhancements
* draft-pep-general-01:
- Minor update in Section 2.1
- Rewrite Section 4.2.1
* draft-pep-general-00:
- Major Rewrite of "pEp Identity Concept" section
- Major Rewrite of "Key Management" section
- Update Code Excerpts to match current implementation state
- Lots of corrections and editorial changes
* draft-birk-pep-06:
- Minor changes
* draft-birk-pep-05:
- Minor changes, especially in identity system
* draft-birk-pep-04:
- Fix internal reference
- Add IANA Considerations section
- Add other use case of Extra Keys
- Incorporate review changes by Kelly Bristol and Nana
Karlstetter
* draft-birk-pep-03:
- Major restructure of the document
- 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
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o 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
* 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
* draft-birk-pep-01:
- Mostly editorial
* draft-birk-pep-00:
- Initial version
Appendix C. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication ]]
* Shorten Introduction and Abstract
Authors' Addresses
Volker Birk
pEp Foundation
Oberer Graben 4
CH- 8400 Winterthur
Switzerland
Email: volker.birk@pep.foundation
URI: https://pep.foundation/
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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/
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