- [Differences between shh/6 waku/0](#differences-between-shh6-waku0)
- [Acknowledgements](#acknowledgements)
- [Copyright](#copyright)
## Abstract
This specification describes the format of Waku messages within the ÐΞVp2p Wire Protocol. This spec substitutes [EIP- 627](https://eips.ethereum.org/EIPS/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.
<!-- TODO: Add waku mode in v1, it isn't in the spec yet:
> Waku is a fork of the original Whisper protocol that enables better scalability and offline messaging, at the cost of some metadata protection guarantees. It does this through (a) light node support (b) historic messages (through a mailserver) (d) basic rate limiting.
-->
## 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.
All Waku messages are sent as devp2p RLPx transport protocol, version 5<sup>[1](https://github.com/ethereum/devp2p/blob/master/rlpx.md)</sup> 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](https://github.com/ethereum/wiki/wiki/RLP) and the [Ethereum Yellow Paper, appendix B](https://ethereum.github.io/yellowpaper/paper.pdf) for more details on RLP.
; 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."
This packet is used by Waku 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.
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.
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 Waku Mail Server for delivery of old (expired) messages, which is otherwise not allowed.
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 `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 Waku 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.
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.
Mailservers are waku nodes that can archive messages and delivering them to its peers on-demand. A node which wants to provide mailserver functionality MUST store envelopes from incoming message packets (Waku packet-code 0x01). The envelopes can be stored in any format, however they MUST be serialized and deserialized to the Waku envelope format.
A mailserver SHOULD store envelopes for all topics to be generally useful for any peer, however for specific use cases it MAY store envelopes for a subset of topics.
In order to request historic messages, a node MUST send a packet P2P Request (`0x7e`) to a peer providing mailserver functionality. This packet requires one argument which MUST be a Waku envelope.
`Limit`: 4-byte wide unsigned integer limiting the number of returned envelopes
`Cursor`: 32-byte wide array of a cursor returned from the previous request (optional)
The `Cursor` field SHOULD be filled in if a number of envelopes between `Lower` and `Upper` is greater than `Limit` so that the requester can send another request using the obtained `Cursor` value. What exactly is in the `Cursor` is up to the implementation. The requester SHOULD NOT use a `Cursor` obtained from one mailserver in a request to another mailserver because the format or the result MAY be different.
The envelope MUST be signed with a symmetric key agreed between the requester and Mailserver.
Historic messages MUST be sent to a peer as a packet with a P2P Message code (`0x7f`) followed by an array of Waku envelopes. It is incompatible with the original Whisper spec (EIP-627) because it allows only a single envelope, however, an array of envelopes is much more performant. In order to stay compatible with EIP-627, a peer receiving historic message MUST handle both cases.
In order to receive historic messages from a mailserver, a node MUST trust the selected mailserver, that is allow to receive packets with the P2P Message code. By default, such packets are discarded.
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.
<!-- TODO: Elaborate on waku/1 would be directly compatible with waku/0 if version don't match -->
### 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:**
1. A posts message; B posts message.
2. C picks up message from A and B and relays them both to Waku and Whisper.
3. 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.
<!-- TODO: Outline difference between_bridging_ and data format -->
<!-- TODO: Think about how to maintain forwards capability for waku/v0 -> v1 -> v2, etc. -->
<!-- Example user story: changing version number to 1; moving to libp2p; changing routing to PSS style; remove PoW; replacing PoW with zkSNARKs; adding packet codes for rate limit / accounting for resources feedback; additional disconnect features -->
There are several security considerations to take into account when running Waku. Chief among them are: scalability, DDoS-resistance and privacy. These also vary depending on what capabilities are used, such as mailserver, light node, and so on.
<!-- TODO: elaborate on security considerations -->
<!-- TODO: Light node security considerations
> Running a node as a light node mode impacts privacy due to the fact that it becomes identifiable what nodes care about if they aren't relaying traffic.
Replace with:
> I think the main privacy concern with light nodes is that the directly connected peers will know that a message originates from them (as it are the only ones it sends). And yes, based on that they can make some assumptions on which messages (topics) they are interested in also.
The golang implementation of Whisper (v.6) already uses packet codes `0x00` - `0x03`. Parity's implementation of v.6 will also use codes `0x00` - `0x03`. Codes `0x7E` and `0x7F` are reserved, but still unused and left for custom implementation of Whisper Mail Server.
