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Status Whisper Usage Specification

Version: 0.1 (Draft)

Authors: Adam Babik adam@status.im, Corey Petty corey@status.im, Oskar Thorén oskar@status.im (alphabetical order)

Abstract

Status uses Whisper to provide privacy-preserving routing and messaging on top of DevP2P. Whisper uses topics to partition its messages, and these are leveraged for all chat capabilities. In the case of public chats, the channel name maps directly to its Whisper topic. This allows allows anyone to listen on a single channel.

Additionally, since anyone can receive Whisper envelopes, it relies on the ability to decrypt messages to decide who is the correct recipient. We do however not rely on this property, but instead implement another secure transport layer on top of Whisper.

Finally, we use an extension of Whisper to provide the ability to do offline messaging.

Table of Contents

Introduction

In this document we detail how we use Whisper to provide for the various chat use cases, as well how offline inboxing works.

Requirements

An Ethereum node that is connected to peers and implements the Whisper v6 specifications.

Design goals

Provide routing, metadata protection, topic-based multicasting and basic encryption properties to support asynchronous chat.

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 in Whisper V6 extensions (or Status Whisper Node)
  • 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

Basic Assumption

This protocol assumes the following:

  1. There MUST be an Ethereum node that is capable of discovering peers and implements Whisper V6 specification.
  2. Participants of a given Whisper network in order to communicate with each other MUST accept messages with lowered PoW value. More in (Whisper node configuration)(#whisper-node-configuration).
  3. Time MUST be synced between all nodes participating in the given network (this is intrinsic requirement of the Whisper specification as well). A clock drift between two peers larger than 20 seconds MAY result in discarding incoming messages.

Protocol Overview

Notice: this protocol is documented post factum. The goal of it is to clearly present the current design and prepare the ground for its second version.

The implementation of this protocol is mainly done in https://github.com/status-im/status-react and https://github.com/status-im/status-go repositories.

The goal of this protocol is to allow people running Ethereum nodes with Whisper service enabled to exchange messages that are end-to-end encrypted in a way that guarantees darkness to some extent.

It's important to notice that messages are not limited to be text messages only. They can also have special meaning depending on the client implementation. For example, in the current implementation, there are message which informs about Eth requests.

This protocol consist of three components:

  • payload
  • Whisper adapter
  • offline messaging.

The payload section describes how the messages are encoded and decoded and what each fields of a message means. This is required to properly interpret messages by the client.

Whisper adapter specifies interaction with the Whisper service with regards to keys management, configuration and attaching metadata required to properly forward and process messages.

Offline messaging describes how the protocol handles delivering messages when one or more participants are offline and the messages expire in the Whisper network. For more, see [Status Whisper Mailserver Spec)[status-whisper-mailserver-spec.md].

The protocol does not specify additional things like peers discovery, running Whisper nodes, underlying p2p protocols etc.

Whisper adapter

Whisper in version 6 has been chosen as an messages exchange protocol because it was designed as an off-chain communication layer for the Ethereum nodes. It supports e2e encryption and uses epidemic spread to route data to all members of the network. It also provides darkness to some extent.

However, using Whisper has a few tradeoffs:

  • was not designed to handle huge number of messages
  • was not designed to be real-time; some delays over a few seconods are expected
  • does not scale well with the number of messages in the network

This protocol can operate using a Whisper service which requires this protocol implementation to run in the same process as well as Whisper's RPC API which might be provided by a separate Whisper node process via IPC or WebSocket.

There is some tight coupling between the payload and Whisper:

  • Whisper message topic depends on the actual message type (see Topic)
  • Whisper message uses a different key (asymmetric or symmetric) depending on the actual message type (see Keys management)

Whisper node configuration

If you want to run a Whisper node and receive messages from Status clients, it must be properly cnofigured.

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 performance 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

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.

Keys management for PFS is described in Perfect forward secrecy section.

Encryption algorithms

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

Cryptographic algoritms used by PFS are described in Perfect forward secrecy section.

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]
}

Message encryption

The protocol distinguishes messages encrypted using asymmetric and symmetric encryption.

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.

Encryption of messages supporting PFS is described in Perfect Forward Secrecy section.

Perfect Forward Secrecy (PFS)

Additionally to encrypting messages on the Whisper level, the protocol supports PFS specification.

A message payload is first encrypted following the PFS specification and then it is encrypted once again following the Whisper specification and this protocol.

As not all messages are encrypted with PFS, a following strategy MAY be used:

  1. First, message is decrypted on the Whisper level
  2. Try to decrypt the message payload using PFS algorithm 2.1. If successful, pass the decrypted value to (3) 2.2. If failed, pass the unchanged payload to (3)
  3. Decode the payload as described in Paylooad section

TODO: link to a separate document (currently in the PR).

PFS in Status.im docs

Device syncing

TODO: link to a separate document.

One-to-one messages

One-to-one messages are also known as private messages. These are the messages sent beween two participants of the conversation.

Sending

Sending a message is fairly easy and relies on the Whisper RPC API, however, some preparation is needed:

  1. Obtain a public key of the recipient of the message
  2. Add your private key to Whisper using shh_addPrivateKey and save the result as sigKeyID
  3. Call [shh_post(https://github.com/ethereum/go-ethereum/wiki/Whisper-v6-RPC-API#shh_post) with the following settings:
    1. pubKey MUST be a hex-encoded public key of the message recipient
    2. sig MUST be set to sigKeyID
    3. ttl MUST be at least 10 (it is in seconds)
    4. topic MUST be set accordingly to Topic section and hex-encoded
    5. payload MUST be a hex-encoded string
    6. powTime MAY be arbitrary but should be enough to perform proof-of-work
    7. powTarget MUST be equal or lower than 0.002.

Note: these instructions are for the Whisper V6 RPC API. If you use Whisper service directly or Go shhclient, the parameters might have different types.

Learn more following Whisper V6 RPC API.

Sending using PFS

When one decides to use PFS, the flow is the same but the payload MUST be additionally encrypted following the PFS specification before being hex-endoded and passed to shh_post.

Receiving

Receiving private messages depends on Whisper filters idea. Upon receiving, messages are first matched by a topic and then trying to be decrypted using user's private key.

  1. Add your private key to Whisper using shh_addPrivateKey and save the result as sigKeyID
  2. Call shh_subscribe with criteria:
    1. minPow MUST be at least 0.002
    2. topics MUST be list of hex-encoded topics you expect messages to receive from (follow Topic section)
    3. allowP2P MUST be set to true if offline messages are supported, otherwise can be false.

Alternative method is to use shh_newMessageFilter which takes the same criteria object and then periodically calling shh_getFilterMessages method.

Learn more following Whisper V6 RPC API.

Public messages

Public messages are encrypted with a symmetric key which is publicly known so anyone can participate in the conversation.

The fact that anyone can participate makes the public chats voulnerable to spam attacks. Also, there are no moderators of these chats.

Sending

  1. Calculate a symmetric key using shh_generateSymKeyFromPassword passing a public chat name as a string and save the result to symKeyID
  2. Call shh_post with the following settings:
    1. symKeyID MUST be set to symKeyID
    2. sig MUST be set to sigKeyID
    3. ttl MUST be at least 10 (it is in seconds)
    4. topic MUST be set accordingly to Topic section and hex-encoded,
    5. payload MUST be a hex-encoded string,
    6. powTime MAY be arbitrary but should be enough to perform proof-of-work
    7. powTarget MUST be equal or lower than 0.002.

Learn more following Whisper V6 RPC API.

Receiving

Receiving public messages depends on Whisper filters idea. Upon receiving, messages are first matched by a topic and then trying to be decrypted using a symmetric key.

  1. Calculate a symmetric key using shh_generateSymKeyFromPassword passing public chat name as a string and save the result to symKeyID
  2. Call shh_subscribe with criteria:
    1. minPow MUST be at least 0.002
    2. topics MUST be list of hex-encoded topics you expect messages to receive from (follow Topic section)
    3. allowP2P MUST be set to true if offline messages are supported, otherwise can be false.

Alternative method is to use shh_newMessageFilter which takes the same criteria object and then periodically calling shh_getFilterMessages method.

Learn more following Whisper V6 RPC API.

Offline messages

A client SHOULD implement mailserver client mode. See Status Whisper Mailserver Spec. A Status node MAY implement the server mode as well.

Whisper V6 extensions (or Status Whisper Node)

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.

Security concerns

TBD.