specs/status-spec.md

Status Client Specification

Version: 0.1 (Draft)

Authors: Adam Babik adam@status.im, Dean Eigenmann dean@status.im, Oskar Thorén oskar@status.im (alphabetical order)

Table of Contents

• Abstract
• P2P Overlay
• Node discovery and roles
  • Bootstrapping
  • Discovery
  • Mailservers
  • Mobile nodes
• Whisper adapter
  • Node configuration
  • Keys management and encryption
  • Topics
• Secure Transport
• Data Sync
• Payloads and clients
• Design Rationale
  • P2P Overlay
    • Why devp2p? Why not use libp2p?
    • What about other RLPx subprotocols like LES, and Swarm?
    • Why do you use Whisper?
    • I heard you were moving away from Whisper?
    • Why is PoW for Whisper set so low?
    • Why do you not use Discovery v5 for node discovery?
• Footnotes
• Acknowledgements

Abstract

In this specification, we describe how to write a Status client for communicating with other Status clients.

We present a reference implementation of the protocol ¹ that is used in a command line client ² and a mobile app ³.

This document consists of two parts. The first outlines the specifications that have to be implemented in order to be a full Status client. The second gives a design rationale and answers some common questions.

Protocol layers

Implementing a Status clients means implementing the following layers. Additionally, there are separate specifications for things like key management and account lifecycle.

Layer	Purpose	Technology
Data and payloads	End user functionality	1:1, group chat, public chat
Data sync	Data consistency	MVDS Ratchet
Secure transport	Confidentiality, PFS, etc	Double Ratchet
Transport privacy	Routing, Metadata protection	Whisper
P2P Overlay	Overlay routing, NAT traversal	devp2p

Terminology

• Client: a Whisper node implementing the protocol
• Whisper node: an Ethereum node with Whisper V6 enabled (in the case of geth, it's --shh option)
• Status Whisper node: an Ethereum node with Whisper V6 enabled and additional Whisper extensions described below
• Whisper network: a group of Whisper nodes connected together through the internet connection and forming a graph
• MailServer: an Ethereum node with Whisper V6 enabled and a mail server registered capable of storing and providing offline messages
• Message: decrypted Whisper message
• Envelope: encrypted message with some metadata like topic and TTL sent between Whisper nodes; a symmetric or asymmetric key is needed to decrypt it and read the payload
• Offline message: an expired envelope stored by a Whisper node permanently

P2P Overlay

Status clients run on the public Ethereum network, as specified by the devP2P network protocols. devP2P provides a protocol for node discovery which is in draft mode here. See more on node discovery and management in the next section.

To communicate between Ethereum nodes, the RLPx Transport Protocol, v5 is used, which allows for TCP-based communication between nodes.

On top of this we run the RLPx-based subprotocol Whisper v6 for privacy-preserving messaging.

There MUST be an Ethereum node that is capable of discovering peers and implements Whisper V6 specification.

Node discovery and roles

There are four types of node roles:

1. Bootstrap nodes
2. Whisper relayers
3. Mailservers (servers and clients)
4. Mobile nodes (Status Clients)

To implement a standard Status client you MUST implement the last node type. The other node types are optional, but we RECOMMEND you implement a mailserver client mode, otherwise the user experience is likely to be poor.

Bootstrapping

To connect to other Status nodes you need to connect to a bootstrap node. These nodes allow you to discover other nodes of the network.

Currently the main bootstrap nodes are provided by Status Gmbh, but anyone can run these provided they are connected to the rest of the Whisper network.

Discovery

To implement a Status client you need to discover peers to connect to. We use a light discovery mechanism based on a combination of Discovery v5 and Rendezvous Protocol, (with some modifications). Additionally, some static nodes MAY also be used.

This part of the system is currently underspecified and requires further detail.

Mailservers

Whisper Mailservers are special nodes that helps with offline inboxing. This means you can receive Whisper envelopes after their TTL have expired, which is useful if they are sent while a node is offline. They operate on a store-and-forward model.

As a Status node, you are most likely to want to implement a mailserver client. This is currently underspecified, but it amounts to marking a node as trusted using the Whisper API and then fetching envelopes that have expired.

To implement a mailserver as a server (i.e. one that stores expired envelopes), you need to implement the following MailServer interface with two methods:

• Archive(env *Envelope)
• DeliverMail(whisperPeer *Peer, request *Envelope)

If a mail server is registered for a given Whisper client, it will save all incoming messages on a local disk (this is the simplest implementation, it can store the messages wherever it wants, also using technologies like swarm and IPFS) in the background.

Each node is meant to be independent and SHOULD keep a copy of all historic messages. High Availability (HA) can be achieved by having multiple nodes in different locations. Additionally, each node is free to store messages in a way which provides storage HA as well.

Saved messages are delivered to a requester (another Whisper peer) asynchronously as a response to p2pMessageCode message code. This is not exposed as a JSON-RPC method in shh namespace but it's exposed in status-go as shhext_requestMessages and blocking shh_requestMessagesSync. Read more about Whisper V6 extensions.

In order to receive historic messages from a filter, p2p messages MUST be allowed when creating the filter. Receiving p2p messages is implemented in geth's Whisper V6 implementation.

Mobile nodes

This is a Whisper node which connects to part of the Whisper network. It MAY relay messages. See next section for more details on how to use Whisper to communicate with other Status nodes.

Whisper adapter

Once a Whisper node is up and running there are some specific settings required to commmunicate with other Status nodes.

Worth noting is that in Whisper time MUST be synced between all nodes participating in the given network. A clock drift between two peers larger than 20 seconds MAY result in discarding incoming messages.

Node configuration

Whisper's Proof Of Work algorithm is used to deter denial of service and various spam/flood attacks against the Whisper network. The sender of a message must perform some work which in this case means processing time. Because Status' main client is a mobile client, this easily leads to battery draining and poor erformance of the app itself. Hence, all clients MUST use the following Whisper node settings:

• proof-of-work not larger than 0.002
• time-to-live not lower than 10 (in seconds)

Keys management and encryption

The protocol requires a key (symmetric or asymmetric) for the following actions:

• signing a message (a private key)
• decrypting received messages (a private key or symmetric key).

As private keys and symmetric keys are required to process incoming messages, they must be available all the time and are stored in memory.

All encryption algorithms used by Whisper are described in the Whisper V6 specification.

Symmetric keys are created using shh_generateSymKeyFromPassword Whisper V6 RPC API method which accepts one param, a string.

Messages encrypted with asymmetric encryption should be encrypted using recipient's public key so that only the recipient can decrypt them.

Key management and encryption for conversational security is described in Conversational Security, which provides properties such as forward secrecy.

Topics

There are two types of Whisper topics the protocol uses:

• static topic for user-message message type (also called contact discovery topic)
• dynamic topic based on a chat name for public-group-user-message message type.

The static topic is always the same and its hex representation is 0xf8946aac. In fact, the contact discovery topic is calculated using a dynamic topic algorithm described below with a constant name contact-discovery.

Having only one topic for all private chats has an advantage as it's very hard to figure out who talks to who. A drawback is that everyone receives everyone's messages but they can decrypt only these they have private keys for.

A dynamic topic is derived from a string using the following algorithm:

var hash []byte

hash = keccak256(name)

// Whisper V6 specific
var topic [4]byte

topic_len = 4

if len(hash) < topic_len {
    topic_len = len(hash)
}

for i = 0; i < topic_len; i++ {
    topic[i] = hash[i]
}

Whisper v6 extensions

Outside of Whisper v6, there are some extensions, message codes and RPC methods that MAY be useful for client implementers. An implementation of this can be found in a fork of Whisper here.

Secure Transport

In order to provide confidentiality, integrity, authentication and forward secrecy of messages we implement a secure transport on top of Whisper. This is used in 1:1 chats and group chats, but not for public chats. See Status Secure Transport Spec for more.

Data Sync

MVDS is used for 1:1 and group chats, however it is currently not in use for public chats.

Payloads and clients

On top of secure transport, we have various types of data sync clients and payload formats for things like 1:1 chat, group chat and public chat. These have various degrees of standardization. Please refer to Initial Message Payload Specification for more details.

Design Rationale

P2P Overlay

Why devp2p? Why not use libp2p?

At the time the main Status clients were being developed, devp2p was the most mature. However, it is likely we'll move over to libp2p in the future, as it'll provide us with multiple transports, better protocol negotiation, NAT traversal, etc.

For very experimental bridge support, see the bridge between libp2p and devp2p in Murmur.

What about other RLPx subprotocols like LES, and Swarm?

Status is primarily optimized for resource restricted devices, and at present time light client support for these protocols are suboptimal. This is a work in progress.

For better Ethereum light client support, see Re-enable LES as option. For better Swarm support, see Swarm adaptive nodes.

For transaction support, Status clients currently have to rely on Infura.

Status clients currently do not offer native support for file storage.

Why do you use Whisper?

Whisper is one of the three parts of the vision of Ethereum as the world computer, Ethereum and Swarm being the other two. Status was started as an encapsulation of and a clear window to this world computer.

I heard you were moving away from Whisper?

Whisper is not currently under active development, and it has several drawbacks. Among others:

• It is very wasteful bandwidth-wise and it doesn't appear to be scalable
• Proof of work is a poor spam protection mechanism for heterogenerous devices
• The privacy guarantees provided are not rigorous
• There's no incentives to run a node

Finding a more suitable transport privacy is an ongoing research effort, together with Vac and other teams in the space.

Why is PoW for Whisper set so low?

A higher PoW would be desirable, but this kills the battery on mobilephones, which is a prime target for Status clients.

This means the network is currently vulnerable to DDoS attacks. Alternative methods of spam protection are currently being researched.

Why do you not use Discovery v5 for node discovery?

At the time we implemented dynamic node discovery, Discovery v5 wasn't completed yet. Additionally, running a DHT on a mobile leads to slow node discovery, bad battery and poor bandwidth usage. Instead, each client can choose to turn on Discovery v5 for a short period of time until their peer list is populated.

For some further investigation, see here.

I heard something about mailservers being trusted somehow?

In order to use a mail server, a given node needs to connect to it directly, i.e. add the mailserver as its peer and mark it as trusted. This means that the mail server is able to send direct p2p messages to the node instead of broadcasting them. Effectively, it knows which topics the node is interested in, when it is online as well as many metadata like IP address.

Footnotes

1. https://github.com/status-im/status-protocol-go/
2. https://github.com/status-im/status-console-client/
3. https://github.com/status-im/status-react/

Acknowledgements