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14/WAKU2-MESSAGE

Abstract​

Waku v2 is a family of modular peer-to-peer protocols for secure communication. These protocols are designed to be secure, privacy-preserving, and censorship-resistant and can run in resource-restricted environments. At a high level, Waku v2 implements a Pub/Sub messaging pattern over libp2p and adds capabilities.

The present document specifies the Waku v2 message format, a way to encapsulate the messages sent with specific information security goals, and Whisper/Waku v1 backward compatibility.

Motivation​

When sending messages over Waku, there are multiple requirements:

  • One may have a separate encryption layer as part of the application.
  • One may want to provide efficient routing for resource-restricted devices.
  • One may want to provide compatibility with Waku v1 envelopes.
  • One may want encrypted payloads by default.
  • One may want to provide unlinkability to get metadata protection.

This specification attempts to provide for these various requirements.

Semantics​

Waku Message​

A Waku message is constituted by the combination of data payload and attributes that, for example, a publisher sends to a topic and is eventually delivered to subscribers.

Waku message attributes are key-value pairs of metadata associated with a message. And the message data payload is the part of the transmitted Waku message that is the actual message information. The data payload is also treated as a Waku message attribute for convenience.

Message Attributes​

  • The payload attribute MUST contain the message data payload to be sent.

  • The content_topic attribute MUST specify a string identifier that can be used for content-based filtering, as described in 23/WAKU2-TOPICS.

  • The meta attribute, if present, contains an arbitrary application-specific variable-length byte array with a maximum length limit of 64 bytes. This attribute can be utilized to convey supplementary details to various Waku protocols, thereby enabling customized processing based on its contents.

  • The version attribute, if present, contains a version number to discriminate different types of payload encryption. If omitted, the value SHOULD be interpreted as version 0.

  • The timestamp attribute, if present, signifies the time at which the message was generated by its sender. This attribute MAY contain the Unix epoch time in nanoseconds. If the attribute is omitted, it SHOULD be interpreted as timestamp 0.

  • The ephemeral attribute, if present, signifies the transient nature of the message. For example, an ephemeral message SHOULD not be persisted by the Waku network. If this attribute is set to true, the message SHOULD be interpreted as ephemeral. If, instead, the attribute is omitted or set to false, the message SHOULD be interpreted as non-ephemeral.

Wire Format​

The Waku message wire format is specified using protocol buffers v3.

syntax = "proto3";

message WakuMessage {
  bytes payload = 1;
  string content_topic = 2;
  optional uint32 version = 3;
  optional sint64 timestamp = 10;
  optional bytes meta = 11;
  optional bool ephemeral = 31;
}

An example proto file following this specification can be found here (vacp2p/waku).

Payload encryption​

The Waku message payload MAY be encrypted. The message version attribute indicates the schema used to encrypt the payload data.

  • Version 0: The payload SHOULD be interpreted as unencrypted; additionally, it CAN indicate that the message payload has been encrypted at the application layer.

  • Version 1: The payload SHOULD be encrypted using Waku v1 payload encryption specified in 26/WAKU-PAYLOAD. This provides asymmetric and symmetric encryption. The key agreement is performed out of band. And provides an encrypted signature and padding for some form of unlinkability.

  • Version 2: The payload SHOULD be encoded according to 35/WAKU2-NOISE. Waku Noise protocol provides symmetric encryption and asymmetric key exchange.

Any version value not included in this list is reserved for future specification. And, in this case, the payload SHOULD be interpreted as unencrypted by the Waku layer.

Whisper/Waku v1 envelope compatibility​

Whisper/Waku v1 envelopes are compatible with Waku v2 messages format.

  • Whisper/Waku v1 topic field SHOULD be mapped to Waku v2 message's content_topic attribute.
  • Whisper/Waku v1 data field SHOULD be mapped to Waku v2 message's payload attribute.

Waku v2 implements a pub/sub messaging pattern over libp2p. This makes redundant some Whisper/Waku v1 envelope fields (e.g., expiry, ttl, topic, etc.), so they can be ignored.

Deterministic message hashing​

In Protocol Buffers v3, the deterministic serialization is not canonical across the different implementations and languages. It is also unstable across different builds with schema changes due to unknown fields.

To overcome this interoperability limitation, a Waku v2 message's hash MUST be computed following this schema:

message_hash = sha256(concat(pubsub_topic, message.payload, message.content_topic, message.meta, message.timestamp))

If an optional attribute, such as meta, is absent, the concatenation of attributes SHOULD exclude it. This recommendation is made to ensure that the concatenation process proceeds smoothly when certain attributes are missing and to maintain backward compatibility.

This hashing schema is deemed appropriate for use cases where a cross-implementation deterministic hash is needed, such as message deduplication and integrity validation. The collision probability offered by this hashing schema can be considered negligible. This is due to the deterministic concatenation order of the message attributes, coupled with using a SHA-2 (256-bit) hashing algorithm.

Test vectors​

Waku message hash computation (meta size of 12 bytes):

pubsub_topic = "/waku/2/default-waku/proto" (0x2f77616b752f322f64656661756c742d77616b752f70726f746f)
message.payload = 0x010203045445535405060708
message.content_topic = "/waku/2/default-content/proto" (0x2f77616b752f322f64656661756c742d636f6e74656e742f70726f746f)
message.meta = 0x73757065722d736563726574
message.timestamp = 0x175789bfa23f8400

message_hash = 0x64cce733fed134e83da02b02c6f689814872b1a0ac97ea56b76095c3c72bfe05

Waku message hash computation (meta size of 64 bytes):

pubsub_topic = "/waku/2/default-waku/proto" (0x2f77616b752f322f64656661756c742d77616b752f70726f746f)
message.payload = 0x010203045445535405060708
message.content_topic = "/waku/2/default-content/proto" (0x2f77616b752f322f64656661756c742d636f6e74656e742f70726f746f)
message.meta = 0x000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f
message.timestamp = 0x175789bfa23f8400

message_hash = 0x7158b6498753313368b9af8f6e0a0a05104f68f972981da42a43bc53fb0c1b27

Waku message hash computation (meta attribute not present):

pubsub_topic = "/waku/2/default-waku/proto" (0x2f77616b752f322f64656661756c742d77616b752f70726f746f)
message.payload = 0x010203045445535405060708
message.content_topic = "/waku/2/default-content/proto" (0x2f77616b752f322f64656661756c742d636f6e74656e742f70726f746f)
message.meta = <not-present>
message.timestamp = 0x175789bfa23f8400

message_hash = 0xa2554498b31f5bcdfcbf7fa58ad1c2d45f0254f3f8110a85588ec3cf10720fd8

Waku message hash computation (payload length 0):

pubsub_topic = "/waku/2/default-waku/proto" (0x2f77616b752f322f64656661756c742d77616b752f70726f746f)
message.payload = []
message.content_topic = "/waku/2/default-content/proto" (0x2f77616b752f322f64656661756c742d636f6e74656e742f70726f746f)
message.meta = 0x73757065722d736563726574
message.timestamp = 0x175789bfa23f8400

message_hash = 0x483ea950cb63f9b9d6926b262bb36194d3f40a0463ce8446228350bd44e96de4

Security Considerations​

Confidentiality, integrity, and authenticity​

The level of confidentiality, integrity, and authenticity of the Waku message payload is discretionary. Accordingly, the application layer shall utilize the encryption and signature schemes supported by Waku v2 to meet the application-specific privacy needs.

Reliability of the timestamp attribute​

The Waku message timestamp attribute is set by the sender. Therefore, because message timestamps aren’t independently verified, this attribute is prone to exploitation and misuse. It should not solely be relied upon for operations such as message ordering. For example, a malicious actor can arbitrarily set the timestamp of a Waku message to a high value so that it always shows up as the most recent message in a chat application. Applications using Waku messages’ timestamp attribute are recommended to use additional methods for more robust message ordering. An example of how to deal with message ordering against adversarial message timestamps can be found in the Status protocol, see 62/STATUS-PAYLOADS.

Reliability of the ephemeral attribute​

The Waku message ephemeral attribute is set by the sender. Since there is currently no incentive mechanism for network participants to behave correctly, this attribute is inherently insecure. A malicious actor can tamper with the value of a Waku message’s ephemeral attribute, and the receiver would not be able to verify the integrity of the message.

Copyright and related rights waived via CC0.

References​