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marchat Protocol Specification

This document outlines the communication protocol used by marchat, a terminal-based chat application built with Go and Bubble Tea. It covers WebSocket interactions, message formats, and expected client behavior. The protocol is designed for simplicity, extensibility, and ease of implementation for alternative clients or integrations.

WebSocket Connection

Clients connect to the server via WebSocket:

/ws

The server may run over either ws:// (HTTP) or wss:// (HTTPS/TLS) depending on configuration. The connection scheme is determined by whether TLS certificates are provided to the server.

After a successful WebSocket upgrade, the client must immediately send a handshake message.

Handshake

The handshake message introduces the user to the server. It must be the first message sent after connection.

Format

{
  "username": "alice",
  "admin": true,
  "admin_key": "your-admin-key"
}

Fields

  • username (string): Required. Display name. Must be unique among currently connected users. The server reserves the name under lock during handshake before registering the session, so two simultaneous connections cannot claim the same username.
  • admin (bool): Optional. Request admin access. Defaults to false.
  • admin_key (string): Required only if admin is true. Must match the server-configured key.

If admin is requested:

  • The username must match one in the admin allowlist (case-insensitive).
  • The provided key must match the server’s configured key.
  • If the key does not match, the server sends a JSON message with type auth_failed and data containing reason: "invalid admin key", then a 1008 WebSocket close with reason Invalid admin key (same RFC 6455 close frame format as other handshake rejections).

Invalid handshakes (missing username, duplicate names, or invalid admin credentials) result in immediate connection termination. The server sends a standard WebSocket close frame (RFC 6455): a registered close code plus an optional UTF-8 reason string (not raw text without a code). Handshake JSON parse failures use 1002 (CloseProtocolError); policy rejections (empty username, invalid username, allowlist, non-admin claiming admin, duplicate username, ban, etc.) use 1008 (ClosePolicyViolation). Duplicate usernames typically include reason text such as Username already taken - please choose a different username. Alternative clients should read the close code and reason from their WebSocket API, not assume the payload is plain text.

Message Format

All messages exchanged after handshake use JSON.

Chat Messages

{
  "sender": "alice",
  "content": "hello world",
  "created_at": "2025-07-24T15:04:00Z",
  "type": "text",
  "channel": "general",
  "message_id": 42,
  "edited": false,
  "encrypted": false
}

Optional fields (recipient, reaction, etc.) are omitted from JSON when unset.

Fields

  • sender (string): Username of the sender.
  • content (string): Message text. Empty if type is file. For search, carries the query string.
  • created_at (string): RFC3339 timestamp.
  • type (string): Core types include "text", "file", and "admin_command". See Extended Message Types for additional values.
  • file (object, optional): Present only when type is "file".
  • message_id (int64, optional): Unique database identifier for a message; used for edits, deletes, reactions, and pins. Assigned by the server when a text message is persisted.
  • recipient (string, optional): Target username for direct messages (type: "dm"). Empty for normal channel/broadcast traffic.
  • edited (bool, optional): true if the message body was edited after send.
  • channel (string, optional): Channel name for scoped delivery; defaults to "general" when omitted on outgoing chat messages. See Channels.
  • reaction (object, optional): Reaction payload when type is "reaction". See Reaction object.
  • encrypted (bool, optional): true when content is end-to-end encrypted (opaque to the server). See End-to-end encryption.

File Object

{
  "filename": "screenshot.png",
  "size": 23456,
  "data": "<base64-encoded>"
}

Maximum file size is configurable (default 1MB). Files exceeding this size are rejected. Configure via environment variables on the server:

  • MARCHAT_MAX_FILE_BYTES: exact byte limit (takes precedence)
  • MARCHAT_MAX_FILE_MB: size in megabytes

If neither is set, the default is 1MB.

Reaction object

When type is "reaction", reaction must be set:

{
  "emoji": "👍",
  "target_id": 42,
  "is_removal": false
}
  • emoji (string): Emoji character or shorthand alias. The client resolves aliases before sending (e.g. +1 becomes 👍, heart becomes ❤️). Raw emoji characters are also accepted. Supported aliases: +1, -1, heart, laugh, fire, party, eyes, check, x, think, clap, rocket, wave, 100, sad, wow, angry, skull, pray, star.
  • target_id (int64): message_id of the message being reacted to.
  • is_removal (bool, optional): When true, removes the sender’s reaction instead of adding it.

Extended Message Types

These values of type extend the core chat protocol:

type Purpose
edit Replace the text of an existing message. Requires message_id and new body in content. Set encrypted to true when content is E2E ciphertext (same base64 nonce ‖ ciphertext layout as text); the server persists that flag to is_encrypted so history and reconnects stay consistent. Only the original sender may edit; admins cannot edit someone else's message (unlike delete, where an admin may remove any message). Enforced by matching WebSocket sender to the stored row.
delete Soft-delete a message. Requires message_id. Authors may delete their own messages; admins may delete any message.
typing Typing indicator; content is not required. The server sets sender and broadcasts to clients (see Channels for delivery scope when channel is set).
reaction Add or remove a reaction. Requires the reaction object (emoji, target_id, optional is_removal).
dm Direct message. Requires recipient (target username). Delivered only to sender and recipient.
search Full-text search. content is the search query; the server replies with a private text message from System listing up to 20 matches (not broadcast).
pin Toggle pinned state for message_id. Admin only; non-admins receive an error text from System. On success, a System text notice is broadcast.
read_receipt Read-receipt notification. Relayed to connected clients; payload conventions (e.g. which message was read) may use content and/or message_id as agreed by clients.
join_channel Join a channel. Requires channel (name). If the client was in another channel, they leave it first. The server sends a confirmation text from System.
leave_channel Leave the current channel and return to #general. No content required. If already in general, no-op. The server sends a confirmation text from System.

Server Events

Messages initiated by the server to update client state.

User List

{
  "type": "userlist",
  "data": {
    "users": ["alice", "bob"]
  }
}

End-to-end encryption

Optional chat encryption uses a shared symmetric key for all participants (global model), not per-user public-key exchange.

  • Key material: 32 random bytes, distributed out-of-band. Clients load the same key via MARCHAT_GLOBAL_E2E_KEY (standard base64) or a locally generated key that operators then share manually. Persisted keys live in the client’s passphrase-protected keystore.dat (format and migration details: README.md / ARCHITECTURE.md). The reference client does not print the full auto-generated key to stdout (only a Key ID); use env, keystore copy, or another confidential channel. When the env var is set, it wins for that client process; the keystore file is unchanged unless the client saves a new key without the env var set.
  • Algorithm: ChaCha20-Poly1305 (RFC 8439). Each encrypted payload uses a random 12-byte nonce (typical for this AEAD).
  • Text messages on the wire: Same JSON message shape as plaintext chat; set encrypted to true. The content field is standard base64 encoding of nonce ‖ ciphertext (nonce first, 12 bytes, then the Poly1305-sealed ciphertext). The plaintext decrypted by the AEAD is a JSON object representing the inner chat message (e.g. sender, content, type, timestamp) as produced by the reference client.
  • Files: When type is file and E2E is enabled, the reference client encrypts file bytes with the same global key; see client implementation for the exact binary layout (nonce-prefixed ciphertext).
  • Edits (type: edit): When E2E is enabled, the reference client encrypts the new plaintext the same way as a normal text message and sets encrypted accordingly. The server stores the new opaque content and updates is_encrypted from the incoming encrypted field (it does not force plaintext).

The server stores and relays opaque content (and encrypted file blobs) without performing decryption.

Server Behavior

  • On successful handshake:
    • Sends up to 50 recent messages from history (newest first; clients typically display in chronological order).
    • Sends current user list.
  • On user connect/disconnect:
    • Broadcasts updated user list.
  • On message send:
    • Persists eligible messages to the configured SQL backend selected by MARCHAT_DB_PATH (SQLite path, PostgreSQL DSN, or MySQL DSN).
    • Delivers to all connected clients or only to members of a channel when channel is non-empty and sender is not System (see Channels). Direct messages use a separate path (sender and recipient only).
  • Reactions, read receipts, and last channel per user may be persisted server-side and replayed to reconnecting clients.
  • Message history is capped at 1000 messages.
  • On successful type: edit, the server updates content, sets edited, and sets stored encryption metadata from the incoming encrypted field (so edited ciphertext rows remain ciphertext with is_encrypted aligned to the wire).

Channels

  • Every connection is placed in the general channel after a successful handshake.
  • Channel names are carried on messages via the optional channel field. When it is set on a non-system message, the server routes that message only to clients currently joined to that channel.
  • Clients switch channels by sending type: join_channel with channel set to the target name. Sending join_channel while already in another channel leaves the previous channel first.
  • Clients return to general with type: leave_channel (no fields required beyond the message envelope). If the client is already in general, the server performs no channel change.

Rate Limiting

Per WebSocket connection, the server enforces rate limiting on all incoming JSON messages (including typing indicators and control types):

  • Burst: at most 20 messages per 5 second sliding window.
  • Cooldown: if the limit is exceeded, further incoming messages from that connection are ignored until 10 seconds have elapsed since the violation, then counting resumes.

Exceeded messages are dropped silently from the client’s perspective (the server logs the event). Alternative clients should pace high-frequency traffic accordingly.

Message Retention

  • Messages are stored in the backend configured by MARCHAT_DB_PATH (SQLite/PostgreSQL/MySQL).
  • The most recent 1000 messages are retained.
  • Older messages are deleted automatically.

Authentication

Admin status is optional and granted only if:

  • admin is set to true in the handshake.
  • admin_key matches the server’s configured key.
  • username is on the allowed admin list, if configured.

If admin is not requested, admin_key is not required.

Configuration

Client settings are usually stored in config.json under the client configuration directory (per-user application data, or MARCHAT_CONFIG_DIR). That directory is separate from the server directory (.env plus local DB file when SQLite is used). Use marchat-client -doctor / marchat-server -doctor to print resolved paths.

Example config.json shape:

{
  "username": "YOUR_USERNAME",
  "server_url": "ws://localhost:9090/ws",
  "theme": "patriot",
  "twenty_four_hour": true
}

Note: The server_url should use ws:// for HTTP connections or wss:// for HTTPS/TLS connections, depending on the server's TLS configuration.

Sensitive values (like admin_key) are passed only during handshake and are not stored in config.

Notes on Extensibility

The protocol is intentionally JSON-based. Plugins extend the server through a managed plugin host (separate processes, JSON IPC); chat messages with type "text" can be forwarded to plugins for automation, while :-prefixed lines and admin_command messages participate in the command pipeline (including built-in admin commands where authorized). The plugin SDK's Message struct mirrors the core wire fields (sender, content, created_at, type, channel, encrypted, message_id, recipient, edited) so plugins receive full message context. All extended fields use omitempty, making the wire format backwards-compatible with older compiled plugins. Clients and tools should use the type field and optional structured fields (message_id, channel, reaction, etc.) to interpret each payload.

Example Workflow

  1. Client connects via WebSocket to /ws
  2. Sends handshake:
{
  "username": "carol",
  "admin": false
}
  1. Server responds with history and user list
  2. Client sends message (optional channel; if omitted, the server uses the client’s current channel, default general):
{
  "sender": "carol",
  "content": "hey there",
  "created_at": "2025-07-24T15:09:00Z",
  "type": "text",
  "channel": "general"
}
  1. Server delivers the message to every connected client joined to that channel (clients in other channels do not receive it)

This document is intended to help developers build compatible clients, bots, or tools for marchat, or understand how the protocol works.

For questions or suggestions, please open a GitHub Discussion or Issue.