specs/x8.md

sip	title	status	type	author	created	updated
3	Initial Message Payload Specification	Draft	Standard	Adam Babik <adam@status.im>, Oskar Thorén <oskar@status.im>	2019-04-22

• Wrapper
• Encoding
• Upgradability

This specification describes how the payload of each message in the Status Protocol looks like. It does not care how the payload is encrypted or exchanged later.

The payload must be flexible enough to support messaging but also cases described in Status Whitepaper as well as various clients created using vastly different technologies.

Wrapper

Payloads are wrapped in a protobuf record record:

    message StatusProtocolMessage {
      bytes signature = 1;
      bytes payload = 2;
    }

Where signature is the bytes of the signed SHA3-256 of the payload, signed with the key of the author of the message. The signature is needed to validate authorship of the message, so that the message can be relayed to third parties. If a signature is not present but an author is provided by a layer below, the message is to be relayed to third parties and its considered plausibly deniable.

Encoding

The payload is encoded using Transit format. It is a text-based format so even encoded version is easily readable by humans. Transit was chosen over JSON in order to reduce the bandwidth.

Example of a valid encoded payload:

["~#c4",["abc123","text/plain","~:public-group-user-message",154593077368201,1545930773682,["^ ","~:chat-id","testing-adamb","~:text","abc123"]]]

As you can see, the message is an array and each index value has its meaning:

• 0: c4 is a decoder handler identification for the current payload format. Identifications allow to register handlers for many different types of payload
• 1: array which items correspond to the described payload fields above

For more details regarding serialization and deserialization please consult transit format specification.

Message types

• Message

Message

The type Message represents a text message exchanged between clients.

• Payload
• Content types
• Message types
• Clock vs Timestamp and message ordering
• Replies

Payload

Payload is a struct (a compound data type) with the following fields (order is important):

1. text string
2. content type enum (more in Content types)
3. message type enum (more in Message types)
4. clock int64
5. timestamp int64
6. content struct { chat-id string, text string }

Content types

Content types are required for a proper interpretation of incoming messages. Not each message is a plain text but may carry a different information.

The following content types MUST be supported:

• text/plain identifies a message which content is a plain text.

There are also other content types that MAY be implemented by the client:

• sticker TODO
• status TODO
• command TODO
• command-request TODO
• emoji TODO

Message types

Message types are required to decide how a particular message is encrypted (more in Whisper > Message encryption) and what metadata needs to be attached (more in Whisper > Topic) when passing a message to the transport layer.

The following messages types MUST be supported:

• public-group-user-message is a message to the public group
• user-message is a private message
• group-user-message is a message to the private group.

Clock vs Timestamp and message ordering

timestamp MUST be Unix time calculated when the message is created. Because the peers in the Whisper network should have synchronized time, timestamp values should be fairly accurate among all Whisper network participants.

clock SHOULD be calculated using the algorithm of Lamport timestamps. When there are messages available in a chat, clock's value is calculated based on the last received message in a particular chat: last-message-clock-value + 1. If there are no messages, clock is initialized with timestamp's value.

clock value is used for the message ordering. Due to the used algorithm and distributed nature of the system, we achieve casual ordering which might produce counterintuitive results in some edge cases. For example, when one joins a public chat and sends a message before receiving the exist messages, their message clock value might be lower and the message will end up in the past when the historical messages are fetched.

Replies

TODO

Upgradability

The current protocol format is hardly upgradable without breaking backward compatibility. Because Transit is used in this particular way described above, the only reliable option is to append a new field to the Transit record definition. It will be simply ignored by the old clients.