Network Working Group V. Birk
Internet-Draft B. Hoeneisen
Intended status: Standards Track H. Marques
Expires: 8 December 2023 pEp Foundation
6 June 2023
pretty Easy privacy (pEp): Key Synchronization Protocol (KeySync)
draft-pep-keysync-03
Abstract
This document describes the pEp KeySync protocol, which is designed
to perform secure peer-to-peer synchronization of private keys across
devices belonging to the same user.
Modern users of messaging systems typically have multiple devices for
communicating, and attempting to use encryption on all of these
devices often leads to situations where messages cannot be decrypted
on a given device due to missing private key data. Current
approaches to resolve key synchronicity issues are cumbersome and
potentially insecure. The pEp KeySync protocol is designed to
facilitate this personal key synchronization in a user-friendly
manner.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-pep-keysync/.
Discussion of this document takes place on the medup non-WG mailing
list (mailto:medup@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/medup/.
Source for this draft and an issue tracker can be found at
https://gitea.pep.foundation/pEp.foundation/internet-drafts.
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/.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 6
1.4. Main Challenge . . . . . . . . . . . . . . . . . . . . . 6
1.5. Approach . . . . . . . . . . . . . . . . . . . . . . . . 7
2. General Description . . . . . . . . . . . . . . . . . . . . . 7
2.1. Use Cases for pEp KeySync . . . . . . . . . . . . . . . . 7
2.1.1. Form Device Group . . . . . . . . . . . . . . . . . . 8
2.1.2. Add New Device to Existing Device Group . . . . . . . 8
2.1.3. Exchange Private Keys . . . . . . . . . . . . . . . . 9
2.1.4. Leave Device Group . . . . . . . . . . . . . . . . . 9
2.1.5. Remove other Device from Device Group . . . . . . . . 9
2.2. Interaction Diagrams . . . . . . . . . . . . . . . . . . 10
2.2.1. Form Device Group . . . . . . . . . . . . . . . . . . 10
2.2.2. Add New Device to Existing Device Group . . . . . . . 17
2.2.3. Exchange Private Keys . . . . . . . . . . . . . . . . 25
2.2.4. Leave Device Group . . . . . . . . . . . . . . . . . 25
2.2.5. Remove other Device from Device Group . . . . . . . . 25
3. Security Considerations . . . . . . . . . . . . . . . . . . . 25
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Normative References . . . . . . . . . . . . . . . . . . 26
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7.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Reference Implementation . . . . . . . . . . . . . . 27
A.1. Description of Finite State Machine . . . . . . . . . . . 28
A.1.1. States . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.2. Conditions . . . . . . . . . . . . . . . . . . . . . 39
A.1.3. Actions . . . . . . . . . . . . . . . . . . . . . . . 41
A.1.4. Transitions . . . . . . . . . . . . . . . . . . . . . 47
A.1.5. Events . . . . . . . . . . . . . . . . . . . . . . . 47
A.1.6. Messages . . . . . . . . . . . . . . . . . . . . . . 49
Appendix B. Code excerpts . . . . . . . . . . . . . . . . . . . 56
B.1. Finite State Machine . . . . . . . . . . . . . . . . . . 56
B.2. ASN.1 Type Definitions . . . . . . . . . . . . . . . . . 71
Appendix C. Document Changelog . . . . . . . . . . . . . . . . . 73
Appendix D. Open Issues . . . . . . . . . . . . . . . . . . . . 74
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 74
1. Introduction
The pretty Easy privacy (pEp) [I-D.pep-general] protocols describe a
set of conventions for the automation of operations traditionally
seen as barriers to the use and deployment of secure end-to-end
interpersonal messaging. These include, but are not limited to, key
management, key discovery, and private key handling.
This document specifies the pEp KeySync protocol, a means for secure,
decentralized, peer-to-peer synchronization of private keys across
devices belonging to the same user, allowing that user to send and
receive encrypted communications from any of their devices.
For pEp implementations, pEp KeySync is a critical part of the
broader pEp Sync protocol, which is designed to be extensible to
allow for the synchronization of additional user data, such as
configuration settings and peer trust status information across a
single user's devices.
This document will provide a general description of pEp KeySync,
including idealized use cases, diagrams, and examples of messages
that may be generated during the KeySync process.
1.1. 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.
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1.2. 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]
Note: In pEp KeySync, the Handshake is used to authenticate own
devices (the User normally compares the Trustwords directly by
looking at the screens of the devices involved).
* 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]
* Transport: A general description of what Transport means in pEp
context can be found in [I-D.pep-general].
For pEp Sync there are special requirements to the Transport,
i.e., that messages can be sent to ones own devices and that every
own device (including the sender) receives the messages in the
same order.
* 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'.
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* User: An individual entity using pEp. A User may have one or more
Identities.
* User-ID: A unique identifier for a given User.
* 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.
* Identity: A binding between a User (unique User-ID) and an Address
(email, network ID, URI, etc.). Each Identity is uniquely
identified by this binding. Identities contain a number of
different pieces of information, often including, but not limited
to:
- User-ID
- Address
- Default Key
- Username
- Preferred encryption format
- Information about whether this is an Own Identity
- ...
A single User may have multiple identities. See also [RFC4949].
* Own Identity: An Identity corresponding to one of the Own User's
Addresses.
* Device Group: A set of devices controlled by one pEp User that
have successfully completed the KeySync setup process and
synchronize Identity information, such as cryptographic keys.
This data is synchronized through a common channel for a given
Identity. For example, if a User's Identity is tied to a specific
email address, the common channel for this Identity could be an
inbox.
* Sole Device: A device which is not part of a Device Group.
* Grouped Device: A device which is already part of a Device Group.
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* Beacon (message): A technical text message that is broadcast by
Sole Devices and transmitted through a message sent to the channel
of an Identity. Other Sole Devices, or a Grouped Device of the
same unique Identity and using that Identity's channel, can
interpret this Beacon in order to initiate negotiation for the
formation of a Device Group.
* Transaction ID (TID): A UUID version 4, variant 1 number generated
by each device during the pEp KeySync process in order to identify
the respective devices involved.
* Default Key: The Identity's or User's key that is marked as the
key to be used to encrypt to (in the case of communications
partners) or to sign with (in the case of the Own User).
* Own Key: A public/private keypair corresponding to a User's Own
Identity.
1.3. Problem Statement
Secure and private digital communication is becoming a necessity for
many people. Encryption protocols which utilize key pairs are the
most popular and easily implemented methods to ensure a message is
authentic and can be trusted.
However, most modern users have multiple devices for communicating,
and attempting to use encryption on all of these devices often leads
to situations where messages cannot be decrypted on a given device
due to missing private key data. For example, Alice sends an
encrypted message to Bob, using the public key of a key pair that Bob
generated on his laptop. When Bob attempts to decrypt the message on
his mobile phone, the private key that he generated on his laptop is
not available. As a result, Bob must either use his laptop to
decrypt the message, or attempt to copy the correct private key to
his mobile device, which may expose his private key to potential
leaks or theft. Using, in turn, centralized solutions to share the
missing private key data has the disadvantage to be prone to
infrastructure attacks and also leads to availability issues.
1.4. Main Challenge
The main challenge that pEp KeySync is designed to overcome is to
perform the synchronization in a secure manner so that private keys
are not leaked or exposed to theft.
Note: The case of an adversary getting physical access to the device
itself is beyond the scope of this document.
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1.5. Approach
The basic approach to solving the multiple-device decryption problem
is to synchronize private keys among the devices of a User in a
secure manner. pEp achieves this by giving Users the option to form a
Device Group with their devices. When the User initiates this
process, a Handshake occurs, and the User is presented with a
Trustwords dialog for pairing purposes (cf. [I-D.pep-trustwords]).
Simply put, the User MUST complete this Trustwords dialog (to confirm
for the authenticity of the transport channel) before the automatic
and security-sensitive transfer of private key information can occur.
2. General Description
The pEp KeySync protocol allows a User to securely synchronize
private key data for multiple Identities across their various
devices. This synchronization process is decentralized and performed
as a two-phase commit (2PC) protocol structure. This structure
ensures consensus among the devices at all stages of the KeySync
process.
KeySync's 2PC transaction is accomplished through the implementation
of a Finite State Machine (FSM) on each pEp-enabled device. This FSM
not only sends and receives network traffic, which allows devices to
communicate with each other throughout the KeySync process, but also
interacts with the core pEp implementation itself.
Once activated by the User, pEp KeySync initiates the formation of a
Device Group, and the User is guided through a Handshake process on
its respective devices. A User can choose to reject or cancel this
process at any time, from either device, and private key data is not
exchanged until the group formation process is verified on both
devices.
Once a Device Group is formed, a User can add additional devices to
its group through the same joining procedure. Upon adding the new
device to the existing Device Group, key data is synchronized among
all Grouped Devices, allowing a User to communicate privately from
any of its secure Identities.
2.1. Use Cases for pEp KeySync
This section describes ideal-condition use cases for pEp KeySync.
The focus is on the core procedures and on the scenarios where
everything works. Unexpected user behavior, error handling, race
conditions, etc., are generally omitted from this section in order to
focus on the general concepts of pEp KeySync. Additional use cases
will be discussed in further detail throughout Appendix A.
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2.1.1. Form Device Group
Our User, Alice, has two devices that are configured with pEp-
implementing messaging clients and share the same Identity for her
preferred communication channel. In our example, this communication
channel is the inbox for a specific email address, alice@example.org,
which Alice has configured on each device. Let us call these devices
Alice_Mobile and Alice_Tablet. Each device already has its own
dedicated key pair, which was automatically generated by the pEp
protocol when Alice configured her email inbox on her respective
devices.
When Alice sends an email from Alice_Mobile, it is encrypted by the
key for that specific device, as are any replies she might receive.
If she wishes to read that email (or replies to it) on Alice_Tablet,
she is unable to do so because the key pair for Alice_Tablet is
different. Alice wants to read all of her encrypted communications
on both of her devices, but currently cannot do so, as the devices do
not have any authenticated and secure established connection to each
other and thus cannot share key pair data without compromising her
privacy.
Alice will use pEp KeySync to form a Device Group and add her devices
to it. pEp KeySync provides an authenticated and secure connection
for Alice to exchange private key data among her devices, which will
allow her to have full access to all of her encrypted messages on
both devices.
2.1.2. Add New Device to Existing Device Group
Sometime after devices Alice_Mobile and Alice_Tablet have formed a
Device Group (cf. Section 2.1.1), Alice buys another device,
Alice_Laptop, which is also configured with pEp-implementing
messaging clients and shares the same Identity for her preferred
communication channel (the aforementioned email address).
Alice_Laptop also has a key pair, which was automatically generated
by the pEp protocol, just as the Grouped Devices Alice_Mobile and
Alice_Tablet have. But while the Grouped Devices know each other and
have exchanged private keys, Alice_Laptop and the Grouped Devices
don't have any connection to each other. Thus, Alice does not have
full, encrypted communication capability across the three devices.
As before with devices Alice_Mobile and Alice_Tablet, Alice will use
pEp KeySync to add device Alice_Laptop to the existing Device Group,
allowing all three devices to exchange private key information, and
Alice to have full access to her messages from any of them.
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2.1.3. Exchange Private Keys
All devices from Alice are part of a Device Group (cf. Section 2.1.1
and Section 2.1.2). However, as keys may expire or get reset (cf.
[I-D.pep-keyreset]), it is inevitable that new key pairs will be
generated. For Alice to maintain her ability to read all encrypted
messages on all devices, any new private key needs to be shared with
the other devices in the Device Group.
All devices in Alice's Device Group will share the latest private
keys as they are generated, keeping all of her devices up to date and
functioning as desired.
2.1.4. Leave Device Group
Alice decides that her mobile phone, Alice_Mobile, should no longer
have access to private keys of the Device Group.
Alice will use pEp KeySync on her mobile phone to leave the Device
Group. This also initiates the pEp KeyReset protocol, which resets
keys for all Own Identities (cf. [I-D.pep-keyreset]) on the
remaining devices. Furthermore, Sync is deactivated on Alice_Mobile.
In the future, if Alice desires, she can re-add Alice_Mobile to a
Device Group. If Alice wants to do this, she will first have to re-
enable Sync on Alice_Mobile and then initiate the joining procedure
(cf. Section 2.1.2) again. If there was only one device left, no
Device Group exists anymore. In this case Alice will have to
initiate the Form Device Group (cf. Section 2.1.1) instead of the
joining procedure.
2.1.5. Remove other Device from Device Group
Let's assume one of Alice's devices, Alice_Tablet, was stolen or
became otherwise compromised. To limit the damage, she needs to
ensure that Alice_Tablet no longer receives updates to private keys
from other Device Group members. Furthermore, she needs to reset the
keys for all Own Identities (cf. [I-D.pep-keyreset]) on the
remaining devices, which includes informing communication partners to
no longer use the (potentially) compromised keys.
Note: In order to prevent any reset (new) keys to reach Alice_Tablet,
the channel credentials (e.g., IMAP password) should be changed
before this step.
On all of her remaining Grouped Devices, Alice needs to initiate the
Leave Device Group procedure as described in Section 2.1.4. As a
result, the Device Group will be dissolved.
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For KeySync to work again on her remaining devices, a new Device
Group needs to be formed. Therefore, Alice will first have to re-
enable Sync on her remaining devices and then initiate the Form
Device Group procedure (cf. Section 2.1.1) again. For every
additional remaining device (if any), she will have to initiate the
joining procedure (cf. Section 2.1.2) again.
2.2. Interaction Diagrams
The following interaction diagrams depict what happens during Alice's
KeySync scenarios in a simplified manner. For each scenario, we
first present a successful case, then an unsuccessful case and,
finally, a case that has been interrupted, or discontinued. Some
details are skipped here for the sake of readability. Descriptions
of the interactions are included after each diagram.
Each pEp-enabled device runs its own Finite State Machine (FSM),
which interact with each other throughout the KeySync process, and
drive the UI options presented to Alice (the 'User' in all diagrams,
unless otherwise noted). All Messages are 'broadcast' between
devices. The TIDs added to each Message allow the identification of
received Messages which pertain to the ongoing transaction and the
device which sent it.
For events requiring Alice's interaction in order to proceed, it does
not matter which device has the specified option chosen first unless
otherwise indicated. For example, if an event states that Alice must
choose 'Accept' on the 'Offerer' device in order to continue, the
process will be unaffected if she does so on the 'Requester' device
first. The only difference is that the order of the roles for the
remainder of the given scenario will be swapped.
2.2.1. Form Device Group
2.2.1.1. Successful Case
,-.
`-'
/|\
|
,----------------. / \ ,------------------.
|'Offerer' device| User |'Requester' device|
`-------+--------' | `--------+---------'
| | |
| | |
| 1(r). Beacon (challenge TID) |
|<--------------------------------------------|
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| | |
| 1(o). Beacon (challenge TID) |
|-------------------------------------------->|
| | |
| 2. NegotiationRequest |
|<--------------------------------------------|
| | |
| | 3. Display Trustwords|
| |<- - - - - - - - - - -|
| | |
| 4. NegotiationOpen |
|-------------------------------------------->|
| | |
| 5. Display Trustwords| |
| - - - - - - - - - - >| |
| | |
| ,-----------------------------. |
| |Handshake (user comparison | |
| |of Trustwords) successful | |
| `-----------------------------' |
| | |
,-----------------------------------. |
|User presses 'Accept' button | |
|on 'Requester' device | |
`-----------------------------------' |
| | 6. Accept |
| | - - - - - - - - - - >|
| | |
| 7. CommitAcceptRequester |
|<--------------------------------------------|
| | |
| | |
,-----------------------------------. |
|User presses 'Accept' button | |
|on 'Offerer' device | |
`-----------------------------------' |
| 8. Accept | |
|<- - - - - - - - - - -| |
| | |
| 9. CommitAcceptOfferer |
|-------------------------------------------->|
| | |
| 10. OwnKeysRequester + keys |
|<--------------------------------------------|
| | |
,--------------------. | |
|Offerer is Grouped | | |
`--------------------' | |
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| | |
| 11. OwnKeysOfferer + keys |
|-------------------------------------------->|
| | |
| | ,----------------------.
| | |Requester is Grouped |
| | `----------------------'
| | |
| | |
,-------+--------. | ,--------+---------.
|'Offerer' device| User |'Requester' device|
`----------------' ,-. `------------------'
`-'
/|\
|
/ \
As depicted above, our User, Alice, intends to form a Device Group in
order to share key material between her devices in an authenticated
and secure manner. The group is formed by an 'Offerer' device and a
'Requester' device. The names 'Offerer' and 'Requester' are derived
from the FSM (cf. Appendix A.1), in which the device roles are
defined during the start sequence, which is necessary for the FSM to
work as intended.
During initialization of pEp KeySync, each device generates a
Transaction-ID (TID). These TIDs are sent as a Challenge in a Beacon
over the mutual channel, and the device roles of 'Offerer' and
'Requester' are determined by the numeric value of each device's
unique TID.
1. Every device sends a Beacon Message containing a Challenge TID.
Upon receipt of a Beacon Message from another device, the
received Challenge TID is compared with the device's own
Challenge TID. The device which has a TID with a lower
numerical value is assigned as the 'Requester', and the other
device is automatically assigned as the 'Offerer'.
Note: The 'Offerer' device MUST NOT start a negotiation. In the
event the earlier Beacon Message is lost, the 'Offerer' device
re-sends its own Beacon and waits for a response. Message 1(r)
depicts the Beacon Message sent by the 'Requester' device and is
not required for the process to continue.
2. After determination of the role, the 'Requester' device sends a
NegotiationRequest Message.
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3. The 'Requester' device displays the Trustwords to Alice.
4. Upon receipt of the NegotiationRequest Message, the 'Offerer'
device sends a NegotiationOpen Message.
5. The 'Offerer' device displays the Trustwords to Alice.
6. Alice compares the Trustwords of both devices. As the
Trustwords are the same on both devices, she chooses the
'Accept' option on the 'Requester' device.
Note: Alice may choose 'Accept' on the 'Offerer' device first,
in which case the sequence of the messages is slightly different
(i.e. message 8 is sent before message 6). However, the result
will be exactly the same.
7. On receipt of Alice's 'Accept' from the 'Offerer' device, the
'Requester' device sends a CommitAcceptRequester Message.
The 'Offerer' device receives this Message and waits for Alice
to choose 'Accept'.
8. Alice compares the Trustwords of both devices and chooses the
'Accept' option on the 'Offerer' device.
9. Once Alice chooses 'Accept', the 'Offerer' device sends a
CommitAcceptOfferer Message.
10. Upon receipt of the CommitAcceptOfferer Message, the 'Requester'
device sends an OwnKeysRequester Message along with Alice's
local key pairs (private and public keys) to be synchronized.
11. Upon receipt of the OwnKeysRequester Message, the 'Offerer'
device saves the 'Requester' device keys and combines them with
the existing 'Offerer' device keys. This means that the
'Offerer' device is grouped.
The 'Offerer' device sends an OwnKeysOfferer Message along with
its own existing local key pairs (private and public keys) to be
synchronized.
Upon receipt of the OwnKeysOfferer Message, the 'Requester'
device saves the 'Offerer' keys combined with the 'Requester'
keys. This means that the 'Requester' device is also grouped.
The formation of the Device Group has been successful.
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2.2.1.2. Unsuccessful Case
,-.
`-'
/|\
|
,----------------. / \ ,------------------.
|'Offerer' device| User |'Requester' device|
`-------+--------' | `--------+---------'
| | |
| | |
,--------------------------------------------------------------!.
|Messages (1-5) are same as in the successful case (see above) |_\
`----------------------------------------------------------------'
| | |
| | |
| ,-----------------------------. |
| |Handshake (user comparison | |
| |of Trustwords) unsuccessful | |
| `-----------------------------' |
| ,------------------------------------.
| |User presses 'Reject' button |
| |on 'Requester' device |
| `------------------------------------'
| | R6. Reject |
| | - - - - - - - - - - >|
| | |
| R7. CommitReject |
|<-------------------------------------------|
| | |
,--------------------------------------------------------------!.
| Devices (still not grouped) will not try again |_\
`----------------------------------------------------------------'
| | |
,-------+--------. | ,--------+---------.
|'Offerer' device| User |'Requester' device|
`----------------' ,-. `------------------'
`-'
/|\
|
/ \
For unsuccessful KeySync attempts, messages 1-5 are the same as in a
successful attempt (see above), but once the Trustwords are shown,
events are as follows:
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R6. Our User, Alice, compares the Trustwords of both devices. As the
Trustwords do not match, she chooses the 'Reject' option on the
'Requester' device.
Note: The User may choose 'Reject' on the 'Offerer' device, in
which case the origin and/or destination of the messages
change. However, the result will be exactly the same.
R7. Once Alice chooses the 'Reject' option, the 'Requester' device
sends a CommitReject Message to the 'Offerer' device.
Once the CommitReject Message is sent and received by the respective
devices, they cannot form a Device Group, and pEp KeySync is disabled
on both devices. As a result, there are no further attempts to form
a Device Group involving either of these two devices. KeySync may be
re-enabled in the pEp settings on the affected device(s).
2.2.1.3. Discontinuation Case
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,-.
`-'
/|\
|
,----------------. / \ ,------------------.
|'Offerer' device| User |'Requester' device|
`-------+--------' | `--------+---------'
| | |
| | |
,--------------------------------------------------------------!.
|Messages (1-5) are same as in the successful case (see above) |_\
`----------------------------------------------------------------'
| | |
| | |
| ,-----------------------------. |
| |Handshake (user comparison | |
| |of Trustwords) discontinued | |
| `-----------------------------' |
| ,------------------------------------.
| |User presses 'Cancel' button |
| |on 'Requester' device |
| `------------------------------------'
| | C6. Cancel |
| | - - - - - - - - - - >|
| | |
| C7. Rollback |
|<-------------------------------------------|
| | |
,--------------------------------------------------------------!.
| Devices (still not grouped) will try again |_\
`----------------------------------------------------------------'
| | |
,-------+--------. | ,--------+---------.
|'Offerer' device| User |'Requester' device|
`----------------' ,-. `------------------'
`-'
/|\
|
/ \
For discontinued (canceled) KeySync attempts, messages 1-5 are the
same as in a successful attempt (see above), but once the Trustwords
are shown, events are as follows:
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C6. Our User, Alice, decides to discontinue the process and chooses
the 'Cancel' option on the 'Requester' device.
Note: The User may choose 'Cancel' on the 'Offerer' device, in
which case the origin and/or destination of the messages
change. However, the result will be exactly the same.
C7. Once Alice chooses the 'Cancel' option, the 'Requester' device
sends a rollback Message to the 'Offerer' device.
The devices do not form a Device Group. KeySync remains enabled on
both devices, and Alice can attempt to form a Device Group again.
2.2.2. Add New Device to Existing Device Group
2.2.2.1. Successful Case
,-------. ,-.
|New | `-' ,--------. ,--------.
|device | /|\ |Active | |Passive |
|to join| | |device | |devices |
|group | / \ |in group| |in group|
`---+---' User `---+----' `---+----'
| | | |
| | | |
| 1. Beacon | |
|--------------------------------->|------------------>|
| | | |
| 2(w). NegotiationRequestGrouped | |
|<---------------------------------| |
| | | |
| 2(l). NegotiationRequestGrouped (discarded) |
|<-----------------------------------------------------|
| | | |
| 3. NegotiationOpen | |
|--------------------------------->| |
| 4. Display | | |
| Trustwords | | |
| - - - - - - - >| | |
| | | 5. GroupHandshake |
| | 6(w). Display |------------------>|
| | Trustwords | |
| |<- - - - - - - - | |
| | | |
| | 6(l) Display Trustwords |
| |<- - - - - - - - - - - - - - - - - - |
| | | |
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| ,-----------------------------. | |
| |Handshake (user comparison | | |
| |of Trustwords) successful | | |
| `-----------------------------' | |
| | | |
| ,------------------------------. |
| |User presses 'Accept' button | |
| |on a device in group | |
| `------------------------------' |
| | 7. Accept | |
| | - - - - - - - ->| |
| | | 8. GroupTrust |
| | | ThisKey |
| | |------------------>|
| | | |
| 9. CommitAcceptForGroup | |
|<---------------------------------| |
| | | |
,------------------------------. | |
|User presses 'Accept' button | | |
|on new device | | |
`------------------------------' | |
| 10. Accept | | |
|<- - - - - - - -| | |
| | | |
| 11. CommitAccept | |
|--------------------------------->| |
| | | |
| 12. GroupKeysForNewMember (key data) |
|<---------------------------------| |
| | | |
,------------. | | |
|New device | | | |
|is grouped | | | |
`------------' | | |
| | | |
| 13. GroupKeysAndClose (key data) | |
|--------------------------------->| |
| | | |
| | ,------------. |
| | |New device | |
| | |is accepted | |
| | `------------' |
| | | |
| 13. GroupKeysAndClose (key data) |
|----------------------------------------------------->|
| | | |
| | | ,------------.
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| | | |New device |
| | | |is accepted |
| | | `------------'
| | | |
,---+---. User ,---+----. ,---+----.
|New | ,-. |Active | |Passive |
|device | `-' |device | |devices |
|to join| /|\ |in group| |in group|
|group | | `--------' `--------'
`-------' / \
As depicted above, our User, Alice, intends to add a new device to
her existing Device Group.
1. When Alice initializes the pEp KeySync process, the new device
sends a Beacon Message.
2. Upon receipt of a Beacon Message from this new, Ungrouped
Device, all Grouped Devices in Alice's existing Device Group
send a NegotiationRequestGrouped Message to the New Device.
Note: Messages 2(w) and 2(l) are instances of the same
(NegotiationRequestGrouped) Message type sent from the Grouped
Devices. Only the first NegotiationRequestGrouped Message
received by the New Device is acknowledged. In this example,
2(w) (the "winner") is processed, while message 2(l) (the
"loser") will be ignored and discarded. The result will be the
same, no matter which NegotiationRequestGrouped Message is
processed first.
3. Upon receipt of the NegotiationRequestGrouped Message 2(w), the
New Device answers with a NegotiationOpen Message to the device
that issued the "winning" NegotiationRequestGrouped Message.
4. The New Device displays the Trustwords to Alice.
5. Upon receipt of the NegotiationOpen Message, the "winner" device
sends a GroupHandshake Message to the "loser" device(s), in
order to activate the Trustwords dialog on all Grouped Devices.
6. All Grouped Devices display the Trustwords to the User.
Note: Messages 6(w) and 6(l) are instances of the same Action on
different devices.
7. Alice compares the Trustwords of all devices and chooses the
'Accept' option on any of the Grouped Devices.
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Note 1: The Grouped Device that Alice chooses the 'Accept'
option on assumes the role of the Active Device for the
remainder of the KeySync process, while the other device(s) in
the Device Group are assigned the passive role.
Note 2: Alice may choose 'Accept' on the new device first, in
which case sequence of the messages is slightly different (i.e.,
message 10 is sent before message 7). However, the result will
be exactly the same.
8. Once Alice chooses the 'Accept' option, the Active Device sends
a GroupTrustThisKey Message to the Passive Device(s) in the
existing Device Group.
9. The Active Device also sends a CommitAcceptForGroup Message to
the new device. Upon receipt, the New Device waits for Alice to
choose 'Accept'.
10. Alice compares the Trustwords on both the New Device and the
Active Device, then chooses the 'Accept' option on the new
device.
11. Once Alice chooses 'Accept', the New Device sends a CommitAccept
Message to the Active Device.
12. Upon receipt of the CommitAccept Message, the Active Device
device sends a GroupKeysForNewMember Message to the New Device,
along with Alice's local key pairs (private and public keys) for
synchronization.
13. The New Device receives the GroupKeysForNewMember Message and
saves the received keys combined with its Own Keys. The new
device has successfully joined the Device Group.
The New Device sends a GroupKeysAndClose Message to all devices
in the Device Group, along with its own original local key pairs
(private and public keys) for synchronization.
Note: In the diagram, all messages marked "13.
GroupKeysAndClose (key data)" are a single message, but drawn
separately in order to convey that the message is sent to all
devices in the Device Group.
Upon receipt of the GroupKeysAndClose Message from the New
Device, the Active and Passive Devices save the New Device keys
and combine them with their Own Keys. All keys are now
synchronized among the devices.
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Note: There is no Event Handler to process the GroupKeysAndClose
Message explicitly, as all decryptable Messages containing keys
are implicitly processed and the received keys saved.
[[ TODO: Decide whether the implicit importing keys should
rather be replaced by explicit Actions in Event Handlers. ]]
2.2.2.2. Unsuccessful Case
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,-------. ,-.
|New | `-' ,--------. ,--------.
|device | /|\ |Active | |Passive |
|to join| | |device | |devices |
|group | / \ |in group| |in group|
`---+---' User `---+----' `---+----'
| | | |
| | | |
,---------------------------------------------------------------!.
|Messages (1-6) are same as in the successful case (see above) |_\
`-----------------------------------------------------------------'
| | | |
| | | |
| ,-----------------------------. | |
| |Handshake (user comparison | | |
| |of Trustwords) unsuccessful | | |
| `-----------------------------' | |
| ,------------------------------. |
| |User presses 'Reject' button | |
| |on a device in group | |
| `------------------------------' |
| | R7. Reject | |
| | - - - - - - - ->| |
| | | |
| R8. CommitReject | |
|<---------------------------------|------------------>|
| | | |
,----------!. | | |
|New device|_\ | | |
|(still not | | | |
|grouped) | | | |
|will not | | | |
|try again | | | |
`------------' | | |
| | | |
,---+---. User ,---+----. ,---+----.
|New | ,-. |Active | |Passive |
|device | `-' |device | |devices |
|to join| /|\ |in group| |in group|
|group | | `--------' `--------'
`-------' / \
For unsuccessful KeySync attempts, messages 1-6 are the same as in a
successful attempt (see above), but once the Trustwords are shown,
events are as follows:
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R7. Our User, Alice, compares the Trustwords displayed on both
devices. If the Trustwords do not match, she chooses the
'Reject' option on one of the Grouped Devices (which becomes
the Active Device).
Note: Alice may choose 'Reject' on the new device, in which
case the origin and/or destination of the messages
change. However, the result will be exactly the same.
R8. Upon receipt of the 'Reject' Event, the Active Device sends
a CommitReject Message to both the New Device which attempted to
join, and the Passive Device(s) in the Device Group.
Note: In the diagram, "R8. CommitReject" represents the message
that is sent to all devices participating in the handshake.
Once the CommitReject Message is sent and received by the respective
devices, they cannot form a Device Group, and pEp KeySync is disabled
on the New Device. pEp KeySync may be re-enabled in the pEp settings
on the affected device.
2.2.2.3. Discontinuation Case
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,-------. ,-.
|New | `-' ,--------. ,--------.
|device | /|\ |Active | |Passive |
|to join| | |device | |devices |
|group | / \ |in group| |in group|
`---+---' User `---+----' `---+----'
| | | |
| | | |
,---------------------------------------------------------------!.
|Messages (1-6) are same as in the successful case (see above) |_\
`-----------------------------------------------------------------'
| | | |
| | | |
| ,-----------------------------. | |
| |Handshake (user comparison | | |
| |of Trustwords) discontinued | | |
| `-----------------------------' | |
| ,------------------------------. |
| |User presses 'Cancel' button | |
| |on a device in group | |
| `------------------------------' |
| | C7. Cancel | |
| | - - - - - - - ->| |
| | | |
| C8. Rollback | |
|<---------------------------------|------------------>|
| | | |
,----------!. | | |
|New device|_\ | | |
|(still not | | | |
|grouped) | | | |
|will try | | | |
|again | | | |
`------------' | | |
| | | |
,---+---. User ,---+----. ,---+----.
|New | ,-. |Active | |Passive |
|device | `-' |device | |devices |
|to join| /|\ |in group| |in group|
|group | | `--------' `--------'
`-------' / \
For discontinued (canceled) KeySync attempts, messages 1-6 are the
same as in a successful attempt (see above), but once the Trustwords
are shown, events are as follows:
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C7. Our User, Alice, decides to discontinue the process and chooses
the 'Cancel' option on one of the Grouped Devices (which becomes
the Active Device).
Note: Alice may choose 'Cancel' on the New Device, in which
case the origin and/or destination of the messages
change. However, the result will be the same.
C8. When Alice chooses 'Cancel', the Active Device sends a Rollback
Message to both the New Device and any Passive Devices in the
Device Group.
Note: In the diagram, all messages marked "C8. Rollback"
represents the message that is sent to all devices participating
in the handshake.
The new device does not join the Device Group. KeySync remains
enabled and joining a Device Group can start again at any time.
2.2.3. Exchange Private Keys
[[ TODO ]]
2.2.4. Leave Device Group
[[ TODO ]]
2.2.5. Remove other Device from Device Group
[[ TODO ]]
3. Security Considerations
[[ TODO ]]
4. Privacy Considerations
[[ TODO ]]
5. IANA Considerations
This document has no actions for IANA.
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6. Acknowledgments
The authors would like to thank the following people who provided
substantial contributions, helpful comments or suggestions for this
document: Berna Alp, Claudio Luck, Damian Rutz, Damiano Boppart,
Itzel Vazquez Sandoval, Kelly Bristol Krista Bennett, Nana
Karlstetter, and Sofia Balicka.
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]
7. References
7.1. Normative References
[I-D.pep-general]
Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy
privacy (pEp): Privacy by Default", Work in Progress,
Internet-Draft, draft-pep-general-02, 16 December 2022,
<https://datatracker.ietf.org/doc/html/draft-pep-general-
02>.
[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://datatracker.ietf.org/doc/html/
draft-pep-keyreset-00>.
[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>.
[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>.
7.2. Informative References
[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://datatracker.ietf.org/doc/html/
draft-pep-handshake-00>.
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[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-01,
23 December 2022, <https://datatracker.ietf.org/doc/html/
draft-pep-trustwords-01>.
[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/>.
[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>.
Appendix A. Reference Implementation
[[ Note: The full Finite State Machine code can be found in
Appendix B.1. This section is not a complete reference at this time.
The authors intend to refine this section in future revisions of this
document. ]]
The pEp KeySync Finite State Machine (FSM) is based on a two-phase
commit protocol (2PC) structure. This section describes the States,
Conditions, Actions, Events, and Messages which comprise the pEp
KeySync FSM, and are intended to allow readers to understand the
general functionality and Message flow of the FSM.
States are used to direct Actions, Events, and Messages. States may
have timeouts. If a timeout is triggered certain functionality is
executed.
Conditions are checks performed to determine a specify different
behaviors of the FSM depending on the environment, for example, the
content of I/O buffers.
Actions describe internal FSM functionality, and fall into two
general types. The first Action type directs the State transitions
within the FSM, and the second type drives UI functionality. Actions
may call internal functions, which are not further described here.
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Events are exchanged both between negotiation partners as well as the
pEp core implementation itself to trigger Actions and send Messages.
Messages contain information to ensure the integrity of the KeySync
session as well as additional data, depending on the type of Message
(cf. Appendix A.1.6).
A.1. Description of Finite State Machine
A full diagram of the implemented pEp KeySync FSM can be found at the
following URL:
https://gitea.pep.foundation/pEp.foundation/internet-
drafts/raw/branch/master/pep-keysync/figures/keysync_fsm_full.svg
For convenience (better readability), there is also a simplified
diagram of the implemented pEp KeySync FSM, which does not contain
the transitions that occur when choosing the 'Cancel' or 'Reject'
options. The simplified diagram can be found at the following URL:
https://gitea.pep.foundation/pEp.foundation/internet-
drafts/raw/branch/master/pep-keysync/figures/
keysync_fsm_simplified.svg
The first letter of the terms State, Condition, Action, Event and
Message is capitalized, whenever it said term refers to the FSM.
A.1.1. States
The FSM has two types of States:
1. Stable States:
The FSM of KeySync has two Stable States that do not time out:
* Sole (cf. Appendix A.1.1.2)
* Grouped (cf. Appendix A.1.1.10)
2. Transitional States:
All other States (cf. below) are Transitional States that time
out.
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A.1.1.1. InitState
On initialization, the FSM enters InitState, which evaluates and
determines a device's group status. If the device is detected to
belong to a Device Group, it issues a SynchronizeGroupKeys Message to
the Grouped Devices (to request an update on the Group Keys), and the
FSM transitions to State Grouped (cf. Appendix A.1.2.1). Otherwise,
a new Challenge TID is created and sent out inside of a Beacon
Message, and the FSM transitions to State Sole.
A.1.1.2. Sole
This is the default FSM State for an Ungrouped Device.
On initialization, this State shows the device as being in the Sole
State.
The FSM also listens for Beacons from other devices. Upon receipt of
a Beacon Message from another device, the received Challenge TID is
compared with the own Challenge. The device with the lower Challenge
TID is assigned the 'Requester' role, and the other device is
automatically assigned the 'Offerer' role. If a device is determined
to be the 'Offerer', it resends the Beacon. If a device is
determined to be the 'Requester', it issues a NegotiationRequest
Event to the 'Offerer'.
When the 'Offerer' device receives this NegotiationRequest Message,
it responds with a NegotiationOpen Message, and the 'Offerer' FSM
transitions to State HandshakingOfferer where it awaits the
'Requester' device response.
On receipt of a Grouped device's NegotiationRequestGrouped Message,
it responds with a NegotiationOpen Message, and the 'Requester' FSM
transitions to State HandshakingToJoin.
On receipt of the 'Offerer' device's NegotiationOpen Message, the
'Requester' FSM transitions to State HandshakingRequester.
In this State, other Events may also be processed, but these Events
do not result in a transition to another State.
A.1.1.3. HandshakingOfferer
This State can only be entered by the 'Offerer' device from Sole
State.
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On initialization, it drives user interface options, including the
Trustwords dialog. The User is prompted to compare Trustwords and
choose from the following options:
* Accept: The 'Requester' public key used in the Handshake is
trusted, and the FSM transitions to State
HandshakingPhase1Offerer.
* Reject: A CommitReject Message is sent to the 'Requester' device,
pEp KeySync is disabled, and the FSM transitions to State End.
* Cancel: A Rollback Message is sent to the 'Requester' device, and
the FSM transitions to State Sole.
If the User selects one of the above options on the 'Requester'
device, the 'Requester' FSM sends a response to the 'Offerer' device.
When this response is received, the 'Offerer' FSM performs a
sameNegotiation Condition on the current negotiation session to
verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM proceeds as
follows, depending on the Message received:
* CommitAcceptRequester: The 'Requester' FSM transitions to State
HandshakingPhase2Offerer.
* CommitReject: pEp KeySync is disabled, and the FSM transitions to
State End.
* Rollback: The FSM transitions to State Sole.
A.1.1.4. HandshakingRequester
This State can only be entered by the 'Requester' device from Sole
State.
On initialization, it drives user interface options, including the
Trustwords dialog. The User is prompted to compare Trustwords, and
choose from the following options:
* Accept: The 'Offerer' public key is trusted, a
CommitAcceptRequester Message is sent to the 'Offerer' device, and
the FSM transitions to State HandshakingPhase1Requester.
* Reject: A CommitReject Message is sent to the 'Offerer' device,
pEp KeySync is disabled, and the FSM transitions to State End.
* Cancel: A Rollback Message is sent to the 'Offerer' device, and
the FSM transitions to State Sole.
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If the User selects the 'Cancel' or the 'Reject' options on the
'Offerer' device, the 'Offerer' FSM sends a response to the
'Requester' device. When this response is received, the 'Requester'
FSM performs a sameNegotiation Condition on the current negotiation
session to verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM proceeds as
follows, depending on the Message received:
* CommitReject: pEp KeySync is disabled, and the FSM transitions to
State End.
* Rollback: The FSM transitions to State Sole.
A.1.1.5. HandshakingPhase1Offerer
This State can only be entered by the 'Offerer' device from
HandshakingOfferer State.
In this State the FSM awaits and processes the response from a
'Requester' device in State HandshakingRequester. When this response
is received, the 'Offerer' FSM performs a sameNegotiation Condition
on the current negotiation session to verify that the current session
has not been disrupted or compromised. If this Condition returns
'true', the FSM proceeds as follows, depending on the Message
received:
* CommitAcceptRequester: A CommitAcceptOfferer Message is sent to
the 'Requester' device, and the FSM transitions to State
FormingGroupOfferer.
* CommitReject: The 'Requester' public key is mistrusted, pEp
KeySync is disabled, and the FSM transitions to State End.
* Rollback: The 'Requester' public key is mistrusted, and the FSM
transitions to State Sole.
A.1.1.6. HandshakingPhase1Requester
This State can only be entered by the 'Requester' device from
HandshakingRequester State.
In this State the FSM awaits and processes the response from an
'Offerer' device in State HandshakingOfferer or
HandshakingPhase2Offerer. When this response is received, the
'Requester' FSM performs a sameNegotiation Condition on the current
negotiation session to verify that the current session has not been
disrupted or compromised. If this Condition returns 'true', the FSM
proceeds as follows, depending on the Message received:
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* CommitAcceptOfferer: The FSM prepares the Own Keys on the
'Requester' device for synchronization. The FSM then issues an
OwnKeysRequester Message to the 'Offerer', which contains these
keys, and transitions to State FormingGroupRequester.
* CommitReject: The 'Offerer' public key is mistrusted, pEp KeySync
is disabled, and the FSM transitions to State End.
* Rollback: The 'Offerer' public key is mistrusted, and the FSM
transitions to State Sole.
A.1.1.7. HandshakingPhase2Offerer
This State can only be entered by the 'Offerer' device from a
HandshakingOfferer State.
In this State the FSM waits for the User's response on the 'Offerer'
device. The User is still prompted to compare Trustwords and choose
from the following options:
* Accept: The 'Requester' public key used in the Handshake is
trusted, a CommitAcceptOfferer Message is issued to the
'Requester', and the FSM transitions to State FormingGroupOfferer.
* Reject: A CommitReject Message is issued to the 'Requester'
device, pEp KeySync is disabled, and the FSM transitions to State
End.
* Cancel: A Rollback Message is issued to the 'Requester' device,
and the FSM transitions to State Sole.
A.1.1.8. FormingGroupOfferer
This State can only be entered by the 'Offerer' device from
HandshakingPhase1Offerer or HandshakingPhase2Offerer State.
On initialization, the FSM prepares the Own Keys on the 'Offerer'
device for synchronization and makes a backup of these Own Keys.
Then it waits for the OwnKeysRequester Message from the 'Requester',
which contains the Own Keys and the information about all Own
Identities of the 'Requester'.
When this Message is received, the 'Offerer' FSM performs a
sameNegotiation Condition on the current negotiation session to
verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM saves the
'Requester' keys combined with the 'Offerer' keys in a shared
GroupKeys array (saveGroupKeys) and the 'Requester' device keys are
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marked as default for those respective Identities
(receivedKeysAreDefaultKeys). Then, the FSM prepares the Own Keys on
the 'Offerer' device for synchronization. Because the Keys are
already set to those of the 'Requester' device, it is taking its
former Own Keys and Own Identities from the backup (cf. above). The
Offerer sends the OwnKeysOfferer Message (with key material of its
Own Keys and Own Identities) to the 'Requester', a UI Event
(showGroupCreated) indicates that the Device Group process is
complete, and the FSM transitions to State Grouped.
Note: In case the 'Requester' device has transitioned to Sole State
due to a Cancel, this OwnKeysOfferer Message will not processed by
the 'Requester' device.
In case a (delayed) Cancel arrives (which normally cannot happen), a
Rollback Message is issued to the 'Requester' device, and the FSM
transitions to State Sole.
In case a (delayed) Rollback Message is received (which normally
cannot happen), the FSM transitions to State Sole.
A.1.1.9. FormingGroupRequester
This State can only be entered by the 'Requester' device from a
HandshakingPhase1Requester State.
In this State the FSM awaits and processes the Message OwnKeysOfferer
from an 'Offerer' device in State HandshakingPhase1Offerer or
HandshakingPhase2Offerer.
When this Message is received, the 'Requester' FSM performs a
sameNegotiation Condition on the current negotiation session to
verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM saves the
'Offerer' keys in a shared GroupKeys array (saveGroupKeys), and
prepares the device's Own Keys for synchronization. The 'Requester'
device keys are marked as default for those respective Identities
(ownKeysAreDefaultKeys). A UI Event (showGroupCreated) indicates
that the Device Group process is complete, and the FSM transitions to
State Grouped.
In case a (delayed) Cancel arrives (which normally cannot happen), a
Rollback Message is issued to the 'Offerer' device, and the FSM
transitions to State Sole.
Note: In case the 'Offerer' device has already transitioned to
Grouped State, this Rollback Message will not be processed by the
'Offerer' device.
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In case a (delayed) Rollback Message is received (which normally
cannot happen), the FSM transitions to State Sole.
A.1.1.10. Grouped
This is the default State for any Grouped Device.
On initialization, this State generates a new Challenge TID and shows
the device as being in the Grouped State. A UI Event
(showBeingInGroup) indicates that the Device is part of a Device
Group.
In this State the FSM also listens for Beacons from other devices
that are not yet part of the Device Group.
Upon receipt of a Beacon Message from Sole Device, the device sends a
NegotiationRequestGrouped Message and waits for the Sole Device to
respond with a NegotiationOpen Message.
On receipt of the NegotiationOpen Message from the Sole Device, the
FSM of the Grouped Device stores the negotiation information and
transitions to State HandshakingGrouped.
If the User requests to leave the device group, LeaveDeviceGroup is
triggered, i.e., an InitUnledGroupKeyReset Message is issued to the
other Device Group members, Sync is disabled locally, and a
resetOwnKeysUngrouped is performed (KeyReset on all Own Keys) .
Upon receipt of an InitUnledGroupKeyReset Message from another member
of the Device Group, useOwnResponse is performed (save the response
into the I/O Buffer), an ElectGroupKeyResetLeader Message is issued,
and the FSM transitions to State GroupKeyResetElection.
In this State, other Events may also be processed, but these Events
do not result in a transition to another State, e.g.,
GroupKeysUpdate.
A.1.1.11. HandshakingToJoin
This State can only be entered by a device in the Sole State that is
attempting to join an existing Device Group.
On initialization, this State drives user interface options,
including the Trustwords dialog for joining a Device Group. The User
on the new device is prompted to compare Trustwords and choose from
the following options:
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* Accept: The existing Device Group's public key used in the
Handshake is trusted, and the FSM transitions to State
HandshakingToJoinPhase1.
* Reject: A CommitReject Message is sent to the existing Device
Group, pEp KeySync is disabled (on new device), and the FSM
transitions to State End.
* Cancel: A Rollback Message is sent to the existing Device Group,
and the FSM transitions to State Sole.
If the User selects one of the above options on a device that is part
of the existing Device Group, its FSM sends a response to the new
device. When this response is received, the FSM of the new device
performs a sameNegotiation Condition on the current negotiation
session to verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM proceeds as
follows, depending on the Message received:
* CommitAcceptForGroup: The FSM of the new device transitions to
State HandshakingToJoinPhase2.
* CommitReject: pEp KeySync is disabled (on the new device), and the
FSM transitions to State End.
* Rollback: The FSM transitions to State Sole.
A.1.1.12. HandshakingToJoinPhase1
This State is entered by a new device only, i.e., a device that is
not yet part of a Device Group.
In this State the FSM awaits and processes the response from a device
that is part of the existing Device Group. When this response is
received, the FSM of the new device performs a sameNegotiation
Condition on the current negotiation session to verify that the
current session has not been disrupted or compromised. If this
Condition returns 'true', the FSM proceeds as follows, depending on
the Message received:
* CommitAcceptForGroup: A CommitAccept Message is sent to the
existing Device Group, and the The FSM transitions to State
JoiningGroup.
* CommitReject: The existing Device Group's public key is
mistrusted, pEp KeySync is disabled (on the new device), and the
FSM transitions to State End.
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* Rollback: The existing Device Group's public key is mistrusted,
and the FSM transitions to State Sole.
A.1.1.13. HandshakingToJoinPhase2
This State is entered by a new device only, i.e., a device that is
not yet part of a Device Group.
In this State the FSM waits for the User's response on the new
device. The User is still prompted to compare Trustwords and choose
from the following options:
* Accept: The existing Device Groups's public key used in the
Handshake is trusted, a CommitAccept Message is issued to the
'Requester', and the FSM transitions to State JoiningGroup.
* Reject: A CommitReject Message is issued to the exiting Device
Group, pEp KeySync is disabled (on the new device), and the FSM
transitions to State End.
* Cancel: A Rollback Message is issued to the existing Device Group,
and the FSM transitions to State Sole.
A.1.1.14. JoiningGroup
This State is entered by a new device only, i.e., a device that is
not yet part of a Device Group.
On initialization, the FSM prepares the Own Keys on the new device
for synchronization and makes a backup of these Own Keys. Then it
waits for the OwnKeysForNewMember Message from the exiting Device
Group, which contains the Own Keys and the information about all Own
Identities of the existing Device Group.
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When this Message is received, the FSM of the new device performs a
sameNegotiationAndPartner Condition on the current negotiation
session to verify that both the current session and negotiation
partner have not been disrupted or compromised. If this Condition
returns 'true', the FSM saves the 'Requester' keys combined with the
keys of the existing group in a shared GroupKeys array
(saveGroupKeys) and the Device Group's keys are marked as default for
those respective Identities (receivedKeysAreDefaultKeys). Then, the
FSM prepares the Own Keys on the new device for synchronization.
Because the Keys are already set to the ones of the existing Device
Group, it is taking its former Own Keys and Own Identities from the
backup (cf. above). The new device sends the GroupKeysAndClose
Message (with key material of its Own Keys and Own Identities) to the
Device Group, a UI Event (showDeviceAdded) indicates that the join
Device Group process is complete, and the FSM transitions to State
Grouped.
A.1.1.15. HandshakingGrouped
This State is entered by Grouped Devices only, i.e., devices that are
part of a Device Group.
On initialization, this State drives UI options, including the
Trustwords dialog. The User is prompted to compare Trustwords, and
choose from the following options on any device belonging to the
existing Device Group:
* Accept: The new device's public key is trusted, and the FSM
transitions to State HandshakingGroupedPhase1.
* Reject: A CommitReject Message is sent to the new device and the
FSM transitions to State Grouped.
* Cancel: A Rollback Message is sent to the new device, and the FSM
transitions to State Grouped.
If the User selects the 'Cancel' or the 'Reject' options on the new
device, the new device's FSM sends a response to the existing Device
Group. Whenever this response is received by a Grouped Device, the
FSM performs a sameNegotiation Condition on the current negotiation
session to verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM proceeds as
follows, depending on the Message received:
* CommitReject: The FSM transitions to State Grouped.
* Rollback: The FSM transitions to State Grouped.
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When a GroupTrustThisKey Message is received from another device
group member, the key received along with this Message is trusted.
If the sameNegotiation Condition returns 'true', the FSM transitions
to State Grouped. This latter causes any device in a Device Group,
which is not actively taking part in the joining process, to abort
the User prompt to compare the Trustwords.
Note: In this State, other Events are processed, but these Events do
not result in a transition to another State and are not discussed
here.
A.1.1.16. HandshakingGroupedPhase1
This State is entered by Grouped Devices only, i.e., devices that are
already part of a Device Group.
On initialization, a Message GroupTrustThisKey is sent to the other
members of the Device Group and a Message CommitAcceptForGroup is
sent to the new device.
In this State the FSM awaits and processes the response from an new
device in State HandshakingToJoin or HandshakingToJoinPhase2. When
this response is received, the Grouped Device's FSM performs a
sameNegotiation Condition on the current negotiation session to
verify that the current session has not been disrupted or
compromised. If this Condition returns 'true', the FSM proceeds as
follows, depending on the Message received:
* CommitAccept: The FSM prepares the Own Keys on the Grouped Device
for synchronization. The FSM then issues a
SendGroupKeysForNewMember Message to the new device, which
contains these keys. Then a UI Event (showDeviceAccepted)
indicates that the new device has been successfully added to the
Device Group, and the FSM transitions to State Grouped. [[ TODO:
Check whether 'go Grouped' should be removed in this Event Handler
]]
* CommitReject: The 'Offerer' public key is mistrusted and the FSM
transitions to State Grouped.
* Rollback: The 'Offerer' public key is mistrusted, and the FSM
transitions to State Grouped.
In case a GroupKeysAndClose Message arrives from another group
member, the FSM transitions to State Grouped.
In this State also various other Events are processed, which do not
result in a transition to another State.
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A.1.1.17. GroupKeyResetElection
This State is entered by Grouped Devices only, i.e., devices that are
already part of a Device Group (normally at reception of an
InitUnledGroupKeyReset Message). It is used to determine the
"leader" for a KeyReset, to avoid multiple executions of KeyReset.
Whichever device from a Device Group sends this Message first, will
be the "leader" of the KeyReset.
The device waits for the ElectGroupKeyResetLeader Message from any
member of the Device Group (that is, including its own
ElectGroupKeyResetLeader Message).
If this Message is received, the FSM of the new device performs a
sameResponse Condition to determine whether or not the Message was
sent by its own device or another Device Group member. If the
Message was sent by its own device, a resetOwnGroupedKeys Action is
triggered, and the FSM transitions to State Grouped. If the Message
was sent by another Device Group member, the FSM just transitions to
State Grouped.
Note: All other ElectGroupKeyResetLeader Messages will be ignored,
once the FSM is back to State Grouped.
A.1.2. Conditions
Conditions are implemented with the keyword 'condition'. The code of
their implementations can contain all elements, which can be
contained by the code of Event Handlers (cf. Appendix A.1.5.1), too.
All Conditions can either yield 'true' or 'false' on successful
execution, or, if the Condition fails, the FSM is brought into an
error state and reinitialized.
A.1.2.1. deviceGrouped
The 'deviceGrouped' Condition evaluates 'true' if a device is already
in a Device Group. This is determined by checking if there are Group
Keys already. This boolean value is available and eventually altered
locally on every KeySync-enabled device. For example, in the
reference implementation, this boolean value is stored in a local SQL
database.
The 'deviceGrouped' value is what the KeySync FSM uses upon
initialization (in InitState) to determine whether a device should
transition to State Sole or State Grouped.
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A.1.2.2. fromGroupMember
The 'fromGroupMember' Condition evaluates 'true' if the incoming Sync
Message is coming from a Device Group member. This is used for
"double checking".
A.1.2.3. keyElectionWon
The 'keyElectionWon' Condition evaluates 'true', if the fingerprint
(FPR) of the Sender Key of the partner is greater than the FPR of our
Default Key for the Account, which is being used as Active Transport.
In this case our Own Keys are going to be used as Group Keys.
Otherwise, it evaluates 'false' and the Own Keys of the partner will
be the Group Keys.
A.1.2.4. sameChallenge
The 'sameChallenge' Condition evaluates 'true' if the Challenge of
the incoming Sync Message is identical to the Challenge of the
Device, i.e., this is a Sync Message that was sent by the device
itself.
A.1.2.5. sameNegotiation
The 'sameNegotiation' Condition is dependent upon the
'storeNegotiation' Action, which stores the active negotiation
session while the KeySync process is performed. This Condition
evaluates 'true' if the 'storeNegotiation' value of the incoming Sync
Message is identical to that of the 'storeNegotiation' value that the
device is in, i.e. the incoming Sync Message is part of the same
Negotiation.
This serves as a session fidelity check. If this boolean evaluates
'true', it confirms that the pEp KeySync session in progress is the
same throughout.
A.1.2.6. sameNegotiationAndPartner
Similar to the 'sameNegotiation' Condition, the
'sameNegotiationAndPartner' Condition is dependent upon the
'storeNegotiation' Action, which stores the active negotiation
session while the KeySync process is performed. The
'sameNegotiation' Condition evaluates 'true' if both
'storeNegotiation' value of the incoming Sync Message is identical to
that of the 'storeNegotiation' value that the Device is in, AND the
negotiation partner did not change.
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This Condition also serves as a session fidelity check. If this
boolean evaluates 'true', it confirms that the pEp KeySync session in
progress is the same throughout, and that the negotiation partner has
not changed.
A.1.2.7. sameResponse
The 'sameResponse' Condition evaluates 'true' if the Response of the
incoming Sync Message is identical to the Response of the device. In
this case the Response was correctly echoed.
A.1.2.8. weAreOfferer
The 'weAreOfferer' Condition evaluates 'true' if the Challenge of the
incoming Sync Message is greater than the Challenge of the device.
Otherwise we are the Requester and the Condition evaluates 'false'.
A.1.3. Actions
Actions are implemented with the keyword 'action'. Actions are
unconditionally executing the code of their implementation. Any or
all Actions may fail. In the event of failure, Actions bring the FSM
into an error state, and the FSM will be reinitialized.
A.1.3.1. backupOwnKeys
The 'backupOwnKeys' Action is to make a backup of all Own Keys, and
allows for restoration of the Own Keys.
A.1.3.2. disable
The 'disable' Action does as it implies. This Action shuts down the
FSM and disables KeySync functionality on the impacted device. It is
most commonly called in 'Reject' scenarios. For example, if a User
rejects a pEp Handshake on a device involved in a pEp Handshake, the
'disable' Action is called. Invoking the 'disable' Action results in
the FSM transitioning to State End, which automatically disables the
KeySync feature.
Note: pEp KeySync can be manually re-enabled in the pEp settings on
the disabled device.
A.1.3.3. newChallengeAndNegotiationBase
The 'newChallengeAndNegotiationBase' Action is to randomly compute a
new Challenge and a new Response (Negotiation Base). Both are copied
into the I/O Buffer.
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The 'newChallengeAndNegotiationBase' Action is invoked by a device
during an Init Event in either the Sole or Grouped State, and serves
to clear and generate a new Challenge TID and negotiation state.
A.1.3.4. openNegotiation
The 'openNegotiation' Action clears Key and Identity of the partner
and calculates the Negotiation ID from the Negotiation Base and the
Challenge of the partner (by XOR).
An 'openNegotiation' Action is carried out either by a Sole Device in
the 'Requester' role, or a Grouped device upon receipt of a Beacon
Message from another Sole Device. Most importantly, this Action
ensures that the own TID and the Challenge TID of the Sole Device get
combined by the mathematical XOR function. In this way, a common TID
exists which can be used by both devices a User wishes to pair. This
TID is crucial in allowing the devices to recognize themselves in a
particular pairing process, as multiple pairing processes can occur
simultaneously.
A.1.3.5. ownKeysAreDefaultKeys
The 'ownKeysAreDefaultKeys' Action is to flag Default Keys of Own
Identities as Group Keys.
The ownKeysAreDefaultKeys Action is invoked by the 'Requester' device
during the final step of Device Group formation between two Sole
devices, and ensures that the Own Keys for the Identities on the
'Requester' device are set as the default for those respective
Identities.
A.1.3.6. prepareOwnKeys
The 'prepareOwnKeys' Action is to write a list of Own Identities into
the I/O Buffer and load the list of Own Keys into the device state.
The prepareOwnKeys Action is invoked during the latter phases of the
KeySync protocol for both new and existing Device Group joining
processes. This Action indicates to a device that all key
information that has been selected for synchronization should be
prepared for sending to the other negotiation partner.
A.1.3.7. prepareOwnKeysFromBackup
The 'prepareOwnKeysFromBackup' Action is to restore the formerly
backed up Own Keys (cf. Appendix A.1.3.1) into the I/O Buffer. This
Action is similar to prepareOwnKeys (cf. Appendix A.1.3.6).
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A.1.3.8. receivedKeysAreDefaultKeys
The 'receivedKeysAreDefaultKeys' Action is to set the received Own
Keys as Default Keys for the Own Identities.
A.1.3.9. resetOwnGroupedKeys
The 'resetOwnGroupedKeys' Action is to carry out a KeyReset on Own
Group Keys (cf. [I-D.pep-keyreset]).
A.1.3.10. resetOwnKeysUngrouped
The 'resetOwnKeysUngrouped' Action is to carry out a KeyReset on all
Own Keys (cf. [I-D.pep-keyreset]).
A.1.3.11. saveGroupKeys
The 'saveGroupKeys' Action is to load Own Identities from the I/O
Buffer and store them as Own Identities.
The 'saveGroupKeys' Action directs the addition of any keys received
during a KeySync process to a GroupKeys array, along with any
existing Own or Grouped Device Keys.
A.1.3.12. showBeingInGroup
The 'showBeingInGroup' Action is to signal to the application that
the device is member of a Device Group.
The showBeingInGroup Action in State Grouped drives a UI Event that
can be used to notify a pEp User that their device belongs to a
Device Group.
A.1.3.13. showBeingSole
The 'showBeingSole' Action is to signal to the application that the
device is not member of a Device Group.
The 'showBeingSole' Action in State Sole drives a UI Event that can
be used to notify a pEp User that its device is Sole (ungrouped).
A.1.3.14. showDeviceAccepted
The 'showDeviceAccepted' Action is to signal to the application that
the device has been accepted as member of the Device Group.
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The 'showDeviceAccepted' Action drives a UI Event that is used to
notify a pEp User that a Sole Device was accepted as member of an
existing Device Group.
A.1.3.15. showDeviceAdded
The 'showDeviceAdded' Action is to signal to the application that the
device has been added as member of the Device Group.
The 'showDeviceAdded' Action drives a UI Event that is used to notify
a pEp User that a Sole Device was added to an already existing Device
Group.
A.1.3.16. showGroupCreated
The 'showGroupCreated' Action is to signal to the application that
the Device Group has been created.
In either role that a Sole Device can assume ('Requester' or
'Offerer'), the Action 'showGroupCreated' drives a UI Event which
notifies a User that a new Device Group was formed from two Sole
Devices.
A.1.3.17. showGroupedHandshake
The 'showGroupedHandshake' Action is to signal to the application of
a Grouped Device that a new device is about to join that Device
Group.
The 'showGroupedHandshake' Action drives a UI Event on a Grouped
device, which a pEp implementer uses to display a pEp Handshake
dialog. This dialog indicates that there is a new Sole Device that
is requesting to join the Device Group that this Grouped Device
belongs to.
A.1.3.18. showJoinGroupHandshake
The 'showJoinGroupHandshake' Action is to signal to the application
of an Ungrouped Device that it is about to join an existing Device
Group.
The 'showJoinGroupHandshake' Action drives a UI Event on a Sole
Device attempting to join an existing Device Group, and is used by
pEp implementers to show a Handshake dialog on the Sole Device.
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A.1.3.19. showSoleHandshake
The 'showSoleHandshake' Action is to signal to the application of an
Ungrouped Device that it is about to from a new Device Group.
For cases where two Sole Devices are attempting to form a new Device
Group, the showSoleHandshake Action drives a UI Event, which a pEp
implementer uses to display a pEp Handshake dialog to each of the
devices in negotiation.
A.1.3.20. storeNegotiation
The 'storeNegotiation' Action is to store the Negotiation for the
device in the I/O Buffer. Both, the Sender FPR and partner's
Identity are stored for later comparison.
The storeNegotiation Action saves the received non-own negotiation
information, which is used, e.g., by the sameNegotiation Condition to
perform a session fidelity check (cf. Appendix A.1.2.5).
A.1.3.21. storeThisKey
The 'storeThisKey' Action is to load the Sender Key of the partner
from the I/O Buffer and store it for later use.
A.1.3.22. tellWeAreGrouped
The 'tellWeAreGrouped' Action is to set the is_grouped Field in the
I/O Buffer to 'true'.
The tellWeAreGrouped Action is used by devices already in the Grouped
State. It is sent in a Beacon and indicates to Sole Devices that
they are entering a negotiation with a Grouped Device. For the Sole
Device, receiving this Action determines which State the FSM will
transition to next.
A.1.3.23. tellWeAreNotGrouped
The 'tellWeAreNotGrouped' Action is to set the is_grouped Field in
the I/O Buffer to 'false'.
The 'tellWeAreNotGrouped' Action is used by Sole Devices which are
assigned the role of 'Requester' upon Challenge TID comparison, and
is sent along with a NegotiationRequest Event to indicate to the
'Offerer' device that a negotiation request with a Sole Device is
being entered.
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A.1.3.24. trustThisKey
The 'trustThisKey' Action applies trust to the stored Key of the
negotiation partner and loads this Key into the I/O Buffer.
The trustThisKey Action is executed in all States when a User chooses
'Accept' on the Handshake dialog. Trust for the public key from the
negotiation partner is granted so the rest of the KeySync process can
be conducted securely and authenticated. The trust also extends to
the private key portion of the key pair at a later stage in the
KeySync process, i.e., after the User has chosen 'Accept' on both
devices. If the process is canceled or rejected at any point after
the public key trust has been granted, that trust will be removed
(cf. Appendix A.1.3.25).
A.1.3.25. untrustThisKey
The 'untrustThisKey' Action is to revoke trust from the formerly
stored Key of the partner and clear the Key in the I/O Buffer.
If the 'Cancel' or 'Reject' options are chosen at any point during
the KeySync process after a negotiation partner's public key has been
trusted, trust on that public key is removed (cf.
Appendix A.1.3.24). The 'untrustThisKey' Action ensures that the
negotiation partner's public key can never be attached to Messages
sent to any outside peers from the recipient's device.
A.1.3.26. useOwnChallenge
The 'useOwnChallenge' Action is to copy the Challenge of the device
into the I/O Buffer.
Once a Beacon is received by a device in either the Sole or Grouped
State, the 'useOwnChallenge' Action attaches the device's generated
Challenge TID to an outgoing Beacon or NegotiationRequest Event for
comparison and session verification purposes.
A.1.3.27. useOwnResponse
The 'useOwnResponse' Action is to copy the Response of the device
into the I/O Buffer.
A.1.3.28. useThisKey
The 'useThisKey' Action is to copy the stored Sender Key of the
partner into the I/O Buffer.
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A.1.4. Transitions
Transitions are changes between States within the FSM, and are
indicated by the 'go' command in an Event Handler.
Example:
on Init {
if deviceGrouped {
send SynchronizeGroupKeys;
go Grouped;
}
do newChallengeAndNegotiationBase;
debug > initial Beacon
send Beacon;
go Sole;
}
In this example there are two Transitions, one to State Grouped and
one to State Sole:
Please see the desired State (Appendix A.1.1 and Appendix B.1) for
additional information on why and when these changes are triggered.
A.1.5. Events
While in a State, Events receive incoming Messages and prompt the
execution of any Event Handlers contained within (cf.
Appendix A.1.5.1).
A.1.5.1. Event Handlers
Event Handlers are code sections (containing Conditions, Actions,
Messages, or transitions) executed on receiving an Event. Please
refer to the desired State (Appendix B.1) for additional information
on specific Event Handlers.
A.1.5.2. Init Event
When the FSM transitions to a new State for the first time, the Init
Event (if present) is called. Init Events typically drive UI actions
and Event Handlers associated with core functionality of the
protocol. All States may have a handler for an Init Event (including
the InitState).
Example of an Init Event Handler:
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on Init {
if deviceGrouped {
send SynchronizeGroupKeys;
go Grouped;
}
do newChallengeAndNegotiationBase;
debug > initial Beacon
send Beacon;
go Sole;
}
A.1.5.3. Message Event
If a Sync Message (cf. Appendix A.1.6) arrives through the network
then the Event with the name of the Message occurs.
Example of a Message Event Handler:
In this example an Event Handler is defined, which is executed when a
Beacon Message arrives:
on Beacon {
do openNegotiation;
do tellWeAreGrouped;
do useOwnResponse;
send NegotiationRequestGrouped;
do useOwnChallenge;
}
A.1.5.4. Signaled Events
Events that are signaled from the core pEp implementation, unless
they share their name with a Message.
Example of a Signaled Event Handler:
The KeyGen Event has no corresponding Message. Therefore, it does
not occur when a Sync Message arrives, but rather when it is signaled
from code:
on KeyGen {
do prepareOwnKeys;
send GroupKeysUpdate;
}
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A.1.5.5. External Events
External Event may be used to signal a User Interaction to the FSM.
Example:
on Accept {
do trustThisKey;
send CommitAcceptRequester;
go HandshakingPhase1Requester;
}
If Events are part of an API then their IDs must be well-defined.
Therefore, it is possible to define such IDs in the FSM.
Example:
external Accept 129;
A.1.6. Messages
KeySync is a network protocol, which is implemented using Sync
Messages. The Sync Messages for KeySync are defined at the end of
the FSM code in Appendix B.1.
Example:
message Beacon 2, type=broadcast, ratelimit=10, security=unencrypted {
field TID challenge;
auto Version version;
}
The wire format of Sync Messages is defined in Abstract Syntax
Notation One (ASN.1), cf. Appendix B.2, using Packed Encoding Rules
(PER).
Sync Messages are transported inside (e.g., as attachments of) pEp
Messages. Hence those are carried by the same Transports, which
transmit pEp messages (like, e.g., SMTP and IMAP for email). Some
Sync Messages must be sent in copy on all Transports. Others are
transported on the Active Transport only. The Active Transport is
the transport on which the last Sync Message was received.
A.1.6.1. Message Name and ID
Each Sync Message has a name and an ID.
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A.1.6.2. Message Types
There are different types of Messages:
* type=broadcast: Messages, which are meant to be copied on all
Transports
* type=anycast: Messages, which are meant to be sent on the Active
Transport only (default)
A.1.6.3. Security Context
Each Sync Message has a Security Context. The available Security
Contexts are:
* security=unencrypted: send and receive as unencrypted but signed
Sync Message
* security=untrusted: only accept when encrypted and signed
* security=trusted (default): only accept when coming over a Trusted
Channel and when originating from the Device Group
* security=attach_own_keys_for_new_member: like 'security=trusted'
but attach all Own Keys for a new member of the Device Group
* security=attach_own_keys_for_group: like 'security=trusted' but
attach all Own Keys for other Device Group members
A.1.6.4. Rate Limit
A Sync Message can have a Rate Limit 'ratelimit=<numeric>'. That
means it is only possible to send out one Message each <numeric>
second(s). A Rate Limit of 0 means no Rate Limit checking (default).
A.1.6.5. Fields
A Sync Message can have Fields. There are two types of Fields:
1. automatically calculated Fields, defined with the 'auto' keyword,
and
2. Fields, which are copied in and out from the I/O Buffer, marked
with the 'fields' keyword
The wire format of the Fields is depending on their type. The types
are defined in Appendix B.2. Additionally, the two basic types bool
(ASN.1: BOOLEAN) and int (ASN.1: INTEGER) are supported.
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Example for an 'auto' Field:
auto Version version;
This Field will be filled with the pEp Sync Protocol version. The
Version type is the only automatically calculated type so far.
Example for a Field coming from I/O Buffer:
field TID challenge;
A.1.6.6. Example
Here an example Message named 'Beacon' with ID=2 (and further
attributes) containing 'field' TID and 'auto' Version:
message Beacon 2, type=broadcast, ratelimit=10, security=unencrypted {
field TID challenge;
auto Version version;
}
A.1.6.7. I/O Buffer
There is an I/O Buffer for all Fields which occur in Messages. All
Messages share this I/O Buffer. Fields with the same name share one
space in the I/O Buffer. Hence, the I/O Buffer is built as superset
of all Fields' buffers.
A.1.6.8. Sending
Sending is performed as follows:
1. Calculating all 'auto' Fields and copying the result into the I/O
Buffer
2. Loading all Fields of the Message from I/O Buffer
3. Creating a Sync Message
4. Creating a transporting pEp message by attaching the Sync Message
using Base Protocol
5. Calling 'messageToSend()' with this pEp message
Example
send SynchronizeGroupKeys;
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A.1.6.9. Receiving
When a Message is being received the field values are being copied
into the I/O Buffer and the corresponding Event is being signaled.
A.1.6.10. Messages used in KeySync
In the following, a list of Messages (including format) used by the
KeySync FSM as described in Appendix A.1.1 is shown.
A.1.6.10.1. Beacon (ID=2)
Send beacon to everyone on channel.
message Beacon 2, type=broadcast, ratelimit=10, security=unencrypted {
field TID challenge;
auto Version version;
}
A.1.6.10.2. NegotiationRequest (ID=3)
Request negotiation (normally sent after own role has been determined
to be the Requester).
message NegotiationRequest 3, security=untrusted {
field TID challenge;
field TID response;
auto Version version;
field TID negotiation;
field bool is_group;
}
A.1.6.10.3. NegotiationOpen (ID=4)
Open negotiation (normally sent by Offerer or the device joining a
group as a response to 'NegotiationRequest').
message NegotiationOpen 4, security=untrusted {
field TID response;
auto Version version;
field TID negotiation;
}
A.1.6.10.4. Rollback (ID=5)
Rollback the transaction (normally sent after the User has pressed
'Cancel').
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message Rollback 5, security=untrusted {
field TID negotiation;
}
A.1.6.10.5. CommitReject (ID=6)
Abort the transaction (normally sent after the User has pressed
'Reject').
message CommitReject 6, security=untrusted {
field TID negotiation;
}
A.1.6.10.6. CommitAcceptOfferer (ID=7)
Commit the transaction (normally sent by the Offerer after the User
has pressed 'Accept' as a response to 'CommitAcceptRequester').
message CommitAcceptOfferer 7, security=untrusted {
field TID negotiation;
}
A.1.6.10.7. CommitAcceptRequester (ID=8)
Commit the transaction (normally sent by the Requester after the User
has pressed 'Accept').
message CommitAcceptRequester 8, security=untrusted {
field TID negotiation;
}
A.1.6.10.8. CommitAccept (ID=9)
Commit the transaction (normally sent by the Sole Device joining a
Group, after the User has pressed 'Accept' as a response to
'CommitAcceptGroup').
message CommitAccept 9, security=untrusted {
field TID negotiation;
}
A.1.6.10.9. CommitAcceptForGroup (ID=10)
Commit the transaction for the group (normally sent by a Grouped
Device after the User has pressed 'Accept').
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message CommitAcceptForGroup 10, security=untrusted {
field TID negotiation;
}
A.1.6.10.10. GroupTrustThisKey (ID=11)
The whole Device Group can trust this key (normally sent by a Grouped
Device to transfer a new key to the other members of the Device
Group).
message GroupTrustThisKey 11 {
field Hash key;
field TID negotiation;
}
A.1.6.10.11. GroupKeysForNewMember (ID=12)
Transfer Group Keys and Identities (normally sent by a Grouped Device
in reply to a 'CommitAccept' Message from the Sole Device joining the
group).
message GroupKeysForNewMember 12,
security=attach_own_keys_for_new_member {
field IdentityList ownIdentities;
}
A.1.6.10.12. GroupKeysAndClose (ID=13)
Transfer Keys and Identities of the new group member (normally sent
by the new group member in reply to a 'GroupKeysForNewMember' Message
from a Grouped Device).
message GroupKeysAndClose 13, security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
A.1.6.10.13. OwnKeysOfferer (ID=14)
Transfer the Offerer's Keys and Identities (normally sent by the
Offerer in reply to a 'OwnKeysRequester' Message from the Requester).
message OwnKeysOfferer 14, security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
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A.1.6.10.14. OwnKeysRequester (ID=15)
Transfer the Requester's Keys and Identities (normally sent by the
Requester in reply to a 'CommitAcceptOfferer' Message from the
Offerer).
message OwnKeysRequester 15, security=attach_own_keys_for_new_member {
field IdentityList ownIdentities;
}
A.1.6.10.15. NegotiationRequestGrouped (ID=16)
Request negotiation to join the group (normally sent by a Grouped
Device after receiving a 'Beacon' Message from a Sole Device).
message NegotiationRequestGrouped 16, security=untrusted {
field TID challenge;
field TID response;
auto Version version;
field TID negotiation;
field bool is_group;
}
A.1.6.10.16. GroupHandshake (ID=17)
Inform other members of the Device group about a new handshake
(normally sent by a Grouped Device after receiving a
'NegotiationOpen' Message from a Sole Device).
message GroupHandshake 17 {
field TID negotiation;
field Hash key;
}
A.1.6.10.17. GroupKeysUpdate (ID=18)
Transfer the Group Keys and Identities (normally sent by a Grouped
Device to the other members of the Device Group).
message GroupKeysUpdate 18, security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
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A.1.6.10.18. InitUnledGroupKeyReset (ID=19)
Initiate unled group KeyReset, i.e., the initiating device does not
perform the KeyReset itself. (This Message is normally sent by a
Grouped Device after a 'LeaveDeviceGroup' has been requested by the
User.)
Further information on KeyReset can be found in [I-D.pep-keyreset].
message InitUnledGroupKeyReset 19 {
}
A.1.6.10.19. ElectGroupKeyResetLeader (ID=20)
Initiate determination of "leader" for a KeyReset (normally sent by
all Grouped Devices to the all other members of the Device Group in
response to an InitUnledGroupKeyReset Message).
Further information can be found in Appendix A.1.1.17 and
[I-D.pep-keyreset].
message ElectGroupKeyResetLeader 20 {
field TID response;
}
A.1.6.10.20. SynchronizeGroupKeys (ID=21)
Request synchronization of Group Keys (normally sent by a Grouped
Device to the other members of the Device Group to trigger a
'GroupKeysUpdate' Message).
message SynchronizeGroupKeys 21, ratelimit=60 {
}
Appendix B. Code excerpts
B.1. Finite State Machine
Below you can find the code excerpt for the pEp KeySync FSM,
including Messages and external Events:
// This file is under BSD License 2.0
// Sync protocol for pEp
// Copyright (c) 2016-2020, pEp foundation
// Written by Volker Birk
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include ./fsm.yml2
protocol Sync 1 {
// all messages have a timestamp,
// time out and are removed after timeout
fsm KeySync 1, threshold=300 {
version 1, 2;
state InitState {
on Init {
if deviceGrouped {
send SynchronizeGroupKeys;
go Grouped;
}
do newChallengeAndNegotiationBase;
debug > initial Beacon
send Beacon;
go Sole;
}
}
state Sole timeout=off {
on Init {
do showBeingSole;
}
on KeyGen {
debug > key generated
send Beacon;
}
on CannotDecrypt {
debug > cry, baby
send Beacon;
}
on Beacon {
if sameChallenge {
debug > this is our own Beacon; ignore
}
else {
if weAreOfferer {
do useOwnChallenge;
debug > we are Offerer
send Beacon;
}
else /* we are requester */ {
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do openNegotiation;
do tellWeAreNotGrouped;
// requester is sending NegotiationRequest
do useOwnResponse;
send NegotiationRequest;
do useOwnChallenge;
}
}
}
// we get this from another sole device
on NegotiationRequest {
if sameChallenge { // challenge accepted
do storeNegotiation;
// offerer is accepting by confirming NegotiationOpen
// repeating response is implicit
send NegotiationOpen;
go HandshakingOfferer;
}
}
// we get this from an existing device group
on NegotiationRequestGrouped {
if sameChallenge { // challenge accepted
do storeNegotiation;
// offerer is accepting by confirming NegotiationOpen
// repeating response is implicit
send NegotiationOpen;
go HandshakingToJoin;
}
}
on NegotiationOpen {
if sameResponse {
debug > Requester is receiving NegotiationOpen
do storeNegotiation;
go HandshakingRequester;
}
else {
debug > cannot approve NegotiationOpen
}
}
}
// handshaking without existing Device group
state HandshakingOfferer timeout=600 {
on Init
do showSoleHandshake;
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// Cancel is Rollback
on Cancel {
send Rollback;
go Sole;
}
on Rollback {
if sameNegotiation
go Sole;
}
// Reject is CommitReject
on Reject {
send CommitReject;
do disable;
go End;
}
on CommitReject {
if sameNegotiation {
do disable;
go End;
}
}
// Accept means init Phase1Commit
on Accept {
do trustThisKey;
go HandshakingPhase1Offerer;
}
// got a CommitAccept from requester
on CommitAcceptRequester {
if sameNegotiation
go HandshakingPhase2Offerer;
}
}
// handshaking without existing Device group
state HandshakingRequester timeout=600 {
on Init
do showSoleHandshake;
// Cancel is Rollback
on Cancel {
send Rollback;
go Sole;
}
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on Rollback {
if sameNegotiation
go Sole;
}
// Reject is CommitReject
on Reject {
send CommitReject;
do disable;
go End;
}
on CommitReject {
if sameNegotiation {
do disable;
go End;
}
}
// Accept means init Phase1Commit
on Accept {
do trustThisKey;
send CommitAcceptRequester;
go HandshakingPhase1Requester;
}
}
state HandshakingPhase1Offerer {
on Rollback {
if sameNegotiation {
do untrustThisKey;
go Sole;
}
}
on CommitReject {
if sameNegotiation {
do untrustThisKey;
do disable;
go End;
}
}
on CommitAcceptRequester {
if sameNegotiation {
send CommitAcceptOfferer;
go FormingGroupOfferer;
}
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}
}
state HandshakingPhase1Requester {
on Rollback {
if sameNegotiation {
do untrustThisKey;
go Sole;
}
}
on CommitReject {
if sameNegotiation {
do untrustThisKey;
do disable;
go End;
}
}
on CommitAcceptOfferer {
if sameNegotiation {
do prepareOwnKeys;
send OwnKeysRequester;
go FormingGroupRequester;
}
}
}
state HandshakingPhase2Offerer {
on Cancel {
send Rollback;
go Sole;
}
on Reject {
send CommitReject;
do disable;
go End;
}
on Accept {
do trustThisKey;
send CommitAcceptOfferer;
go FormingGroupOfferer;
}
}
state FormingGroupOfferer {
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on Init {
// we need to keep in memory which keys
// we have before forming a new group
do prepareOwnKeys;
do backupOwnKeys;
}
on Cancel {
send Rollback;
go Sole;
}
on Rollback
go Sole;
on OwnKeysRequester {
if sameNegotiationAndPartner {
do saveGroupKeys;
do receivedKeysAreDefaultKeys;
// send the keys we had before forming a new group
do prepareOwnKeysFromBackup;
send OwnKeysOfferer;
do showGroupCreated;
go Grouped;
}
}
}
state FormingGroupRequester {
on Cancel {
send Rollback;
go Sole;
}
on Rollback
go Sole;
on OwnKeysOfferer {
if sameNegotiation {
do saveGroupKeys;
do prepareOwnKeys;
do ownKeysAreDefaultKeys;
do showGroupCreated;
go Grouped;
}
}
}
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state Grouped timeout=off {
on Init {
do newChallengeAndNegotiationBase;
do showBeingInGroup;
}
on CannotDecrypt {
debug > cry, baby
send SynchronizeGroupKeys;
}
on SynchronizeGroupKeys {
do prepareOwnKeys;
send GroupKeysUpdate;
}
on GroupKeysUpdate {
if fromGroupMember // double check
do saveGroupKeys;
}
on KeyGen {
do prepareOwnKeys;
send GroupKeysUpdate;
}
on Beacon {
do openNegotiation;
do tellWeAreGrouped;
do useOwnResponse;
send NegotiationRequestGrouped;
do useOwnChallenge;
}
on NegotiationOpen {
if sameResponse {
do storeNegotiation;
do useThisKey;
send GroupHandshake;
go HandshakingGrouped;
}
else {
debug > cannot approve NegotiationOpen
}
}
on GroupHandshake {
do storeNegotiation;
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do storeThisKey;
go HandshakingGrouped;
}
on GroupTrustThisKey {
if fromGroupMember // double check
do trustThisKey;
}
on LeaveDeviceGroup {
send InitUnledGroupKeyReset;
do disable;
do resetOwnKeysUngrouped;
}
on InitUnledGroupKeyReset {
debug > unled group key reset; new group keys will be elected
do useOwnResponse;
send ElectGroupKeyResetLeader;
go GroupKeyResetElection;
}
}
state GroupKeyResetElection {
on ElectGroupKeyResetLeader {
if sameResponse {
// the first one is from us, we're leading this
do resetOwnGroupedKeys;
go Grouped;
}
else {
// the first one is not from us
go Grouped;
}
}
}
// sole device handshaking with group
state HandshakingToJoin {
on Init
do showJoinGroupHandshake;
// Cancel is Rollback
on Cancel {
send Rollback;
go Sole;
}
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on Rollback {
if sameNegotiation
go Sole;
}
// Reject is CommitReject
on Reject {
send CommitReject;
do disable;
go End;
}
on CommitAcceptForGroup {
if sameNegotiation
go HandshakingToJoinPhase2;
}
on CommitReject {
if sameNegotiation {
do disable;
go End;
}
}
// Accept is Phase1Commit
on Accept {
do trustThisKey;
go HandshakingToJoinPhase1;
}
}
state HandshakingToJoinPhase1 {
on Rollback {
if sameNegotiation {
do untrustThisKey;
go Sole;
}
}
on CommitReject {
if sameNegotiation {
do untrustThisKey;
do disable;
go End;
}
}
on CommitAcceptForGroup {
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if sameNegotiation {
send CommitAccept;
go JoiningGroup;
}
}
}
state HandshakingToJoinPhase2 {
on Cancel {
send Rollback;
go Sole;
}
on Reject {
send CommitReject;
do disable;
go End;
}
on Accept {
do trustThisKey;
send CommitAccept;
go JoiningGroup;
}
}
state JoiningGroup {
on Init {
// we need to keep in memory which keys
// we have before joining
do prepareOwnKeys;
do backupOwnKeys;
}
on GroupKeysForNewMember {
if sameNegotiationAndPartner {
do saveGroupKeys;
do receivedKeysAreDefaultKeys;
// send the keys we had before joining
do prepareOwnKeysFromBackup;
send GroupKeysAndClose;
do showDeviceAdded;
go Grouped;
}
}
}
state HandshakingGrouped {
on Init
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do showGroupedHandshake;
// Cancel is Rollback
on Cancel {
send Rollback;
go Grouped;
}
on Rollback {
if sameNegotiation
go Grouped;
}
// Reject is CommitReject
on Reject {
send CommitReject;
go Grouped;
}
on CommitReject {
if sameNegotiation
go Grouped;
}
// Accept is Phase1Commit
on Accept {
do trustThisKey;
go HandshakingGroupedPhase1;
}
on GroupTrustThisKey {
if fromGroupMember { // double check
do trustThisKey;
if sameNegotiation
go Grouped;
}
}
on GroupKeysUpdate {
if fromGroupMember // double check
do saveGroupKeys;
}
}
state HandshakingGroupedPhase1 {
on Init {
send GroupTrustThisKey;
send CommitAcceptForGroup;
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}
on Rollback {
if sameNegotiation {
do untrustThisKey;
go Grouped;
}
}
on CommitReject {
if sameNegotiation {
do untrustThisKey;
go Grouped;
}
}
on CommitAccept {
if sameNegotiation {
do prepareOwnKeys;
send GroupKeysForNewMember;
do showDeviceAccepted;
go Grouped;
}
}
on GroupTrustThisKey {
if fromGroupMember // double check
do trustThisKey;
}
on GroupKeysUpdate {
if fromGroupMember // double check
do saveGroupKeys;
}
on GroupKeysAndClose {
if fromGroupMember { // double check
// do not save GroupKeys as default keys; key data is
// already imported
go Grouped;
}
}
}
external Accept 129;
external Reject 130;
external Cancel 131;
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// beacons are always broadcasted
message Beacon 2, type=broadcast,
ratelimit=10, security=unencrypted {
field TID challenge;
auto Version version;
}
message NegotiationRequest 3, security=untrusted {
field TID challenge;
field TID response;
auto Version version;
field TID negotiation;
field bool is_group;
}
message NegotiationOpen 4, security=untrusted {
field TID response;
auto Version version;
field TID negotiation;
}
message Rollback 5, security=untrusted {
field TID negotiation;
}
message CommitReject 6, security=untrusted {
field TID negotiation;
}
message CommitAcceptOfferer 7, security=untrusted {
field TID negotiation;
}
message CommitAcceptRequester 8, security=untrusted {
field TID negotiation;
}
message CommitAccept 9, security=untrusted {
field TID negotiation;
}
message CommitAcceptForGroup 10, security=untrusted {
field TID negotiation;
}
// default: security=trusted
// messages are only accepted when coming from the device group
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message GroupTrustThisKey 11 {
field Hash key;
field TID negotiation;
}
// trust in future
message GroupKeysForNewMember 12,
security=attach_own_keys_for_new_member {
field IdentityList ownIdentities;
}
message GroupKeysAndClose 13,
security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
message OwnKeysOfferer 14, security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
message OwnKeysRequester 15,
security=attach_own_keys_for_new_member {
field IdentityList ownIdentities;
}
// grouped handshake
message NegotiationRequestGrouped 16, security=untrusted {
field TID challenge;
field TID response;
auto Version version;
field TID negotiation;
field bool is_group;
}
message GroupHandshake 17 {
field TID negotiation;
field Hash key;
}
// update group
message GroupKeysUpdate 18, security=attach_own_keys_for_group {
field IdentityList ownIdentities;
}
// initiate unled group key reset
message InitUnledGroupKeyReset 19 {
}
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message ElectGroupKeyResetLeader 20 {
field TID response;
}
message SynchronizeGroupKeys 21, ratelimit=60 {
}
[...]
}
[...]
}
B.2. ASN.1 Type Definitions
Below you can find the ASN.1 Type definitions for the Messages used
in pEp KeySync FSM:
-- This file is under BSD License 2.0
-- Sync protocol for pEp
-- Copyright (c) 2016-2021 pEp foundation
-- Written by Volker Birk
PEP
{ iso(1) org(3) dod(6) internet(1) private(4)
enterprise(1) pEp(47878) basic(0) }
DEFINITIONS AUTOMATIC TAGS EXTENSIBILITY IMPLIED ::=
BEGIN
EXPORTS Identity, IdentityList, TID, Hash, Version, Rating, PString,
PStringList, PStringPair, PStringPairList, ISO639-1;
ISO639-1 ::= PrintableString(FROM ("a".."z")) (SIZE(2))
Hex ::= PrintableString(FROM ("A".."F" | "0".."9"))
Hash ::= Hex(SIZE(16..128)) -- 32bit Key ID to SHA512 in hex
PString ::= UTF8String (SIZE(0..1024))
PStringList ::= SEQUENCE OF PString
TID ::= OCTET STRING (SIZE(16)) -- UUID version 4 variant 1
Identity ::= SEQUENCE {
address PString,
fpr Hash,
user-id PString,
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username PString,
comm-type INTEGER (0..255),
lang ISO639-1
}
IdentityList ::= SEQUENCE OF Identity
Version ::= SEQUENCE {
major INTEGER (0..255) DEFAULT 1,
minor INTEGER (0..255) DEFAULT 2
}
Rating ::= ENUMERATED {
-- no color
cannot-decrypt (1),
have-no-key (2),
unencrypted (3),
unreliable (5),
b0rken (-2),
-- yellow
reliable (6),
-- green
trusted (7),
trusted-and-anonymized (8),
fully-anonymous (9),
-- red
mistrust (-1),
under-attack (-3)
}
PStringPair ::= SEQUENCE {
key PString,
value PString
}
PStringPairList ::= SEQUENCE OF PStringPair
END
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Appendix C. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
* draft-pep-keysync-03:
- Updated Use Cases 'Leave Device Group' and 'Remove other Device
from Device Group'
- Updated States, Conditions, Actions, Transitions, Events,
Messages
- Updated/Added Term definitions
- Harmonized capitalization
- Updated to xml2rfc v3
- Added venue tags
- Several minor edits
- Updated authors' list
* draft-pep-keysync-02:
- Improve clarity and readability
- Updated Section 2.1.1
* draft-pep-keysync-01:
- Updated FSM States, Actions, Messages, Events and interaction
diagrams to reflect recent design changes
- added latest revision of code and ASN.1 Type definitions
* draft-pep-keysync-00:
- Updated docname and author's section
* draft-hoeneisen-pep-keysync-01:
- Major rewrite of upper sections
- Adjust to reflect code changes
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- Move Finite State Machine reference and code to Appendices A &
B
* draft-hoeneisen-pep-keysync-00:
- Initial version
Appendix D. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication ]]
* Resolve several TODOs / add missing text
Authors' Addresses
Volker Birk
pEp Foundation
Oberer Graben 4
CH- 8400 Winterthur
Switzerland
Email: volker.birk@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/
Hernani Marques
pEp Foundation
Oberer Graben 4
CH- 8400 Winterthur
Switzerland
Email: hernani.marques@pep.foundation
URI: https://pep.foundation/
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