This document describes a method that can be used to provide a secure channel between two peers, and thus provide confidentiality, integrity, authenticity and forward secrecy.
It is transport-agnostic and works over asynchronous networks.
It builds on the [X3DH](https://signal.org/docs/specifications/x3dh/) and [Double Ratchet](https://signal.org/docs/specifications/doubleratchet/) specifications, with some adaptations to operate in a decentralized environment.
# Motivation
Nodes on a network may want to communicate with each other in a secure manner, without other nodes network being able to read their messages.
# Specification
## Definitions
- **Perfect Forward Secrecy** is a feature of specific key-agreement protocols which provide assurances that session keys will not be compromised even if the private keys of the participants are compromised.
Specifically, past messages cannot be decrypted by a third-party who manages to get a hold of a private key.
- **Secret channel** describes a communication channel where a Double Ratchet algorithm is in use.
## Design Requirements
- **Confidentiality**: The adversary should not be able to learn what data is being exchanged between two Status clients.
- **Authenticity**: The adversary should not be able to cause either endpoint to accept data from any third party as though it came from the other endpoint.
- **Forward Secrecy**: The adversary should not be able to learn what data was exchanged between two clients if, at some later time, the adversary compromises one or both of the endpoints.
- **Integrity**: The adversary should not be able to cause either endpoint to accept data that has been tampered with.
All of these properties are ensured by the use of [Signal's Double Ratchet](https://signal.org/docs/specifications/doubleratchet/)
## Conventions
Types used in this specification are defined using the [Protobuf](https://developers.google.com/protocol-buffers/) wire format.
# Specification
## End-to-End Encryption
End-to-end encryption (E2EE) takes place between two clients.
The main cryptographic protocol is a Double Ratchet protocol, which is derived from the [Off-the-Record protocol](https://otr.cypherpunks.ca/Protocol-v3-4.1.1.html), using a different ratchet.
[The Waku v2 protocol](/spec/10/) subsequently encrypts the message payload, using symmetric key encryption.
Furthermore, the concept of prekeys (through the use of [X3DH](https://signal.org/docs/specifications/x3dh/)) is used to allow the protocol to operate in an asynchronous environment.
It is not necessary for two parties to be online at the same time to initiate an encrypted conversation.
## Cryptographic Protocols
This protocol uses the following cryptographic primitives:
Waku SHOULD be used for retrieving prekey bundles.
Since bundles stored in QR codes or ENS records cannot be updated to delete already used keys, the bundle MAY be rotated every 24 hours, and distributed via Waku.
## Flow
The key exchange can be summarized as follows:
1. Initial key exchange: Two parties, Alice and Bob, exchange their prekey bundles, and derive a shared secret.
2. Double Ratchet: The two parties use the shared secret to derive a new encryption key for each message they send.
3. Chain key update: The two parties update their chain keys. The chain key is used to derive new encryption keys for future messages.
4. Message key derivation: The two parties derive a new message key from their chain key, and use it to encrypt a message.
### 1. Initial key exchange flow (X3DH)
[Section 3 of the X3DH protocol](https://signal.org/docs/specifications/x3dh/#sending-the-initial-message) describes the initial key exchange flow, with some additional context:
- The peers' identity keys `IK_A` and `IK_B` correspond to their public keys;
- Since it is not possible to guarantee that a prekey will be used only once in a decentralized world, the one-time prekey `OPK_B` is not used in this scenario;
- Nodes SHOULD not send Bundles to a centralized server, but instead provide them in a decentralized way as described in the [Pre-keys section](#pre-keys).
Alice retrieves Bob's prekey bundle, however it is not specific to Alice. It contains:
Having established the initial shared secret `SK` through X3DH, it SHOULD be used to seed a Double Ratchet exchange between Alice and Bob.
Refer to the [Double Ratchet spec](https://signal.org/docs/specifications/doubleratchet/) for more details.
The initial message sent by Alice to Bob is sent as a top-level `ProtocolMessage` ([reference wire format](https://github.com/status-im/status-go/blob/a904d9325e76f18f54d59efc099b63293d3dcad3/services/shhext/chat/encryption.proto#L65)) containing a map of `DirectMessageProtocol` indexed by `installation-id` ([reference wire format](https://github.com/status-im/status-go/blob/1ac9dd974415c3f6dee95145b6644aeadf02f02c/services/shhext/chat/encryption.proto#L56)):
``` protobuf
message ProtocolMessage {
// The installation id of the sender
string installation_id = 2;
// A sequence of bundles
repeated Bundle bundles = 3;
// One to one message, encrypted, indexed by installation_id
-`DR_header`: Double ratchet header ([reference wire format](https://github.com/status-im/status-go/blob/a904d9325e76f18f54d59efc099b63293d3dcad3/services/shhext/chat/encryption.proto#L31)). Used when Bob's public bundle is available:
``` protobuf
message DRHeader {
// Alice's current ratchet public key (as mentioned in [DR spec section 2.2](https://signal.org/docs/specifications/doubleratchet/#symmetric-key-ratchet))
bytes key = 1;
// number of the message in the sending chain
uint32 n = 2;
// length of the previous sending chain
uint32 pn = 3;
// Bob's bundle ID
bytes id = 4;
}
```
-`DH_header`: Diffie-Hellman header (used when Bob's bundle is not available):
The chain key MUST be updated according to the `DR_Header` received in the `EncryptedMessageProtocol` message, described in [2.Double Ratchet](#2-double-ratchet).
### 4. Message key derivation
The message key MUST be derived from a single ratchet step in the symmetric-key ratchet as described in [Symmetric key ratchet](https://signal.org/docs/specifications/doubleratchet/#symmetric-key-ratchet)
The message key MUST be used to encrypt the next message to be sent.
# Security Considerations
1. Inherits the security considerations of [X3DH](https://signal.org/docs/specifications/x3dh/#security-considerations) and [Double Ratchet](https://signal.org/docs/specifications/doubleratchet/#security-considerations).
2. Inherits the security considerations of the [Waku v2 protocol](/spec/10/).
3. The protocol is designed to be used in a decentralized manner, however, it is possible to use a centralized server to serve prekey bundles. In this case, the server is trusted.
# Privacy Considerations
1. This protocol does not provide message unlinkability. It is possible to link messages signed by the same keypair.
# Copyright
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
