17 KiB
Waku Whisper Specification
Version 0.1.0 (Initial release)
Authors: Oskar Thorén oskar@status.im, Dean Eigenmann dean@status.im
Table of Contents
- Abstract
- Motivation
- Specification
- Additional capabilities
- Backwards Compatibility
- Forwards Compatibility
- Security considerations
- Implementation Notes
- Footnotes
- Changelog
- Acknowledgements
- Copyright
Abstract
This specification describes the format of Waku messages within the ÐΞVp2p Wire Protocol. This spec substitutes EIP- 627. Waku is a fork of the original Whisper protocol that enables better usability for resource restricted devices, such as mostly-offline bandwidth-constrained smartphones. It does this primarily through (a) light node support and (b) historic messages (with a mailserver). Additionally, other experimental features for better scalability and DDoS resistance are under development and will be part of future versions.
Motivation
Waku was created to incrementally improve in areas that Whisper is lacking in, with special attention to resource restricted devices. We specify the standard for Waku messages in order to ensure forward compatibility of different Waku clients, backwards compatibility with Whisper clients, as well as to allow multiple implementations of Waku and its capabilities. We also modify the language to be more unambiguous, concise and consistent.
Specification
Use of RLPx transport protocol
All Waku messages are sent as devp2p RLPx transport protocol, version 5 1 packets. These packets MUST be RLP-encoded arrays of data containing two objects: packet code followed by another object (whose type depends on the packet code). See informal RLP spec and the Ethereum Yellow Paper, appendix B for more details on RLP.
Waku nodes that do not support a particular packet code MUST ignore the packet without generating any error.
Waku is a RLPx subprotocol called waku with version 0. The version number corresponds to the major version in the header spec.
ABNF specification
Using Augmented Backus-Naur form (ABNF) we have the following format:
; Packet codes 0 - 127 are reserved for Waku protocol
packet-code = 0 / 1 / 2 / 3 / [ x4-127 ]
packet-format = "[" packet-code packet-format "]"
; packet-format needs to be paired with its corresponding packet-format
packet = "[" required-packet / [ optional-packet ] "]"
required-packet = 0 status / 1 messages / 2 pow-requirement / 3 bloom-filter
optional-packet = 126 p2p-request / 127 p2p-message
packet-format = "[" packet-code packet-format "]"
status = "[" version pow-requirement [ bloom-filter ] [ light-node ] "]"
; version is "an integer (as specified in RLP)"
version = integer
messages = *whisper-envelope
; pow is "a single floating point value of PoW. This value is the IEEE 754 binary representation of 64-bit floating point number. Values of qNAN, sNAN, INF and -INF are not allowed. Negative values are also not allowed."
pow-requirement = pow
; bloom filter is "a byte array"
bloom-filter = bytes
light-node = bool
p2p-request = whisper-envelope
p2p-message = whisper-envelope
whisper-envelope = "[" expiry ttl topic data nonce "]"
; 4 bytes (UNIX time in seconds)
expiry = bytes
; 4 bytes (time-to-live in seconds)
ttl = bytes
; 4 bytes of arbitrary data
topic = bytes
; byte array of arbitrary size (contains encrypted message)
data = bytes
; 8 bytes of arbitrary data (used for PoW calculation)
nonce = bytes
All primitive types are RLP encoded. Note that, per RLP specification, integers are encoded starting from 0x00.
Packet Codes
The message codes reserved for Waku protocol: 0 - 127.
Messages with unknown codes MUST be ignored, for forward compatibility of future versions.
The Waku sub-protocol MUST support the following packet codes:
| Name | Int Value |
|---|---|
| Status | 0 |
| Messages | 1 |
| PoW Requirement | 2 |
| Bloom Filter | 3 |
The following message codes are optional, but they are reserved for specific purpose.
| Name | Int Value |
|---|---|
| P2P Request | 126 |
| P2P Message | 127 |
Packet usage
Status
The bloom filter paramenter is optional; if it is missing or nil, the node is considered to be full node (i.e. accepts all messages).
Status message should be sent after the RLPx handshake and prior to any other messages.
When a node does not receive the status message from a peer, before a configurable timeout, it MUST disconnect from that peer.
Messages
This packet is used for sending the standard Whisper envelopes.
PoW Requirement
This packet is used by Whisper nodes for dynamic adjustment of their individual PoW requirements. Recipient of this message should no longer deliver the sender messages with PoW lower than specified in this message.
PoW is defined as average number of iterations, required to find the current BestBit (the number of leading zero bits in the hash), divided by message size and TTL:
PoW = (2**BestBit) / (size * TTL)
PoW calculation:
fn short_rlp(envelope) = rlp of envelope, excluding env_nonce field.
fn pow_hash(envelope, env_nonce) = sha3(short_rlp(envelope) ++ env_nonce)
fn pow(pow_hash, size, ttl) = 2**leading_zeros(pow_hash) / (size * ttl)
where size is the size of the RLP-encoded envelope, excluding env_nonce field (size of short_rlp(envelope)).
Bloom Filter
This packet is used by Whisper nodes for sharing their interest in messages with specific topics.
The Bloom filter is used to identify a number of topics to a peer without compromising (too much) privacy over precisely what topics are of interest. Precise control over the information content (and thus efficiency of the filter) may be maintained through the addition of bits.
Blooms are formed by the bitwise OR operation on a number of bloomed topics. The bloom function takes the topic and projects them onto a 512-bit slice. At most, three bits are marked for each bloomed topic.
The projection function is defined as a mapping from a 4-byte slice S to a 512-bit slice D; for ease of explanation, S will dereference to bytes, whereas D will dereference to bits.
LET D[*] = 0
FOREACH i IN { 0, 1, 2 } DO
LET n = S[i]
IF S[3] & (2 ** i) THEN n += 256
D[n] = 1
END FOR
OPTIONAL
P2P Request
This packet is used for sending Dapp-level peer-to-peer requests, e.g. Whisper Mail Client requesting old messages from the Whisper Mail Server.
P2P Message
This packet is used for sending the peer-to-peer messages, which are not supposed to be forwarded any further. E.g. it might be used by the Whisper Mail Server for delivery of old (expired) messages, which is otherwise not allowed.
Whisper Envelope data field (Optional)
This section outlines the optional description of Data Field to set up an example. Later it may be moved to a separate spec.
It is only relevant if you want to decrypt the incoming message, but if you only want to send a message, any other format would be perfectly valid and must be forwarded to the peers.
Data field contains encrypted message of the Envelope. In case of symmetric encryption, it also contains appended Salt (a.k.a. AES Nonce, 12 bytes). Plaintext (unencrypted) payload consists of the following concatenated fields: flags, auxiliary field, payload, padding and signature (in this sequence).
flags: 1 byte; first two bits contain the size of auxiliary field, third bit indicates whether the signature is present.
auxiliary field: up to 4 bytes; contains the size of payload.
payload: byte array of arbitrary size (may be zero).
padding: byte array of arbitrary size (may be zero).
signature: 65 bytes, if present.
salt: 12 bytes, if present (in case of symmetric encryption).
Those unable to decrypt the message data are also unable to access the signature. The signature, if provided, is the ECDSA signature of the Keccak-256 hash of the unencrypted data using the secret key of the originator identity. The signature is serialised as the concatenation of the R, S and V parameters of the SECP-256k1 ECDSA signature, in that order. R and S are both big-endian encoded, fixed-width 256-bit unsigned. V is an 8-bit big-endian encoded, non-normalised and should be either 27 or 28.
The padding field was introduced in order to align the message size, since message size alone might reveal important metainformation. Padding can be arbitrary size. However, it is recommended that the size of Data Field (excuding the Salt) before encryption (i.e. plain text) should be factor of 256 bytes.
Payload Encryption
Asymmetric encryption uses the standard Elliptic Curve Integrated Encryption Scheme with SECP-256k1 public key.
Symmetric encryption uses AES GCM algorithm with random 96-bit nonce.
Packet code Rationale
Packet codes 0x00 and 0x01 are already used in all Whisper versions.
Packet code 0x02 will be necessary for the future development of Whisper. It will provide possiblitity to adjust the PoW requirement in real time. It is better to allow the network to govern itself, rather than hardcode any specific value for minimal PoW requirement.
Packet code 0x03 will be necessary for scalability of the network. In case of too much traffic, the nodes will be able to request and receive only the messages they are interested in.
Packet codes 0x7E and 0x7F may be used to implement Whisper Mail Server and Client. Without P2P messages it would be impossible to deliver the old messages, since they will be recognized as expired, and the peer will be disconnected for violating the Whisper protocol. They might be useful for other purposes when it is not possible to spend time on PoW, e.g. if a stock exchange will want to provide live feed about the latest trades.
Additional capabilities
Waku supports multiple capabilities. These include light node, rate limting, mailserver (client and server) and bridging of traffic. Here we list these capabilities, how they are identified, what properties they have and what invariants they must maintain.
Light node
The rationale for light nodes is to allow for interaction with waku on resource restricted devices as bandwidth can often be an issue.
Light nodes MUST NOT forward any incoming messages, they MUST only send their own messages. When light nodes happen to connect to each other, they SHOULD disconnect. As this would result in messages being dropped between the two.
Light nodes are identified by the light_node value in the status message.
Backwards Compatibility
Waku is a different subprotocol from Whisper so it isn't directly compatible. However, the data format is the same, so compatibility can be achieved by the use of a bridging mode as described below. Any client which does not implement certain packet codes should gracefully ignore the packets with those codes. This will ensure the forward compatibility.
Waku-Whisper bridging
waku/0 and shh/6 are different DevP2P subprotocols. In order to achieve backwards compatibility, bridging is required. It works as follows.
Roles:
- Waku client A, only Waku capability
- Whisper client B, only Whisper capability
- WakuWhisper bridge C, both Waku and Whisper capability
Flow:
- A posts message; B posts message.
- C picks up message from A and B and relays them both to Waku and Whisper.
- A receives message on Waku; B on Whisper.
Note: This flow means if another bridge C1 is active, we might get duplicate relaying for a message between C1 and C2. I.e. Whisper(<>Waku<>Whisper)<>Waku, A-C1-C2-B. Theoretically this bridging chain can get as long as TTL permits.
Forwards Compatibility
It is desirable to have a strategy for maintaining forward compatibility between waku/0 and future version of waku. Here we outline some concerns and strategy for this.
Security considerations
There are several security considerations to take into account when running Waku. Chief among them are: scalability, DDoS-resistance and privacy. These also varies depending on what capabilities are used, such as mailserver, light node, and so on.
Implementation Notes
Notes useful for implementing Waku mode.
Footnotes
Changelog
| Version | Comment |
|---|---|
| 0.1.0 (current) | Initial Release |
Differences between shh/6 waku/0
Summary of main differences between this spec and Whisper v6, as described in EIP-627:
- RLPx subprotocol is changed from
shh/6towaku/0 - Light node capability
- Whisper Mail Server and Whisper Mail Client implemented
Acknowledgements
- Kim De Mey
- Andrea Maria Piana
- Adam Babik
Copyright
Copyright and related rights waived via CC0.