Lightweight Extensible Message Format

LXMF is a simple and flexible messaging format and delivery protocol that allows a wide variety of implementations, while using as little bandwidth as possible. It is built on top of Reticulum and offers zero-conf message routing, end-to-end encryption and Forward Secrecy by default.

User-facing clients built on LXMF include:

• Sideband
• Nomad Network
• Nexus Messenger

Structure

LXMF messages are stored in a simple and efficient format, that's easy to parse and write.

The format follows this general structure:

• Destination
• Source
• Ed25519 Signature
• Payload
  • Timestamp
  • Content
  • Title
  • Fields

And these rules:

1. A LXMF message is identified by its message-id, which is a SHA-256 hash of the Destination, Source and Payload. The message-id is never included directly in the message, since it can always be inferred from the message itself.

   In some cases the actual message-id cannot be inferred, for example when a Propagation Node is storing an encrypted message for an offline user. In theses cases a transient-id is used to identify the message while in storage or transit.

2. Destination, Source, Signature and Payload parts are mandatory, as is the Timestamp part of the payload.

   • The Destination and Source fields are 10-byte Reticulum destination hashes
   • The Signature field is a 64-byte Ed25519 signature of the Destination, Source, Payload and message-id
   • The Payload part is a msgpacked list containing four items:
     1. The Timestamp is a double-precision floating point number representing the number of seconds since the UNIX epoch.
     2. The Content is the optional content or body of the message
     3. The Title is an optional title for the message
     4. The Fields is an optional dictionary

3. The Content, Title and Fields parts must be included in the message structure, but can be left empty.

4. The Fields part can be left empty, or contain a dictionary of any structure or depth.

Usage Examples

LXMF offers flexibility to implement many different messaging schemes, ranging from human communication to machine control and sensor monitoring. Here's a few examples:

• A messaging system for passing short, simple messages between human users, akin to SMS can be implemented using only the Content field, and leaving all other optional fields empty.

• For sending full-size mail, an email-like system can be implemented using the Title and Content fields to store "subject" and "body" parts of the message, and optionally the Fields part can be used to store attachments or other metadata.

• Machine-control messages or sensor readings can be implemented using command structures embedded in the Fields dictionary.

• Distributed discussion or news-groups, akin to USENET or similar systems, can be implemented using the relevant fields and LXMF Propagation Nodes. Broadcast bulletins can be implemented in a similar fashion.

Propagation Nodes

LXM Propagation Nodes offer a way to store and forward messages to users or endpoints that are not directly reachable at the time of message emission. Propagation Nodes can also provide infrastructure for distributed bulletin, news or discussion boards.

When Propagation Nodes exist on a Reticulum network, they will by default peer with each other and synchronise messages, automatically creating an encrypted, distributed message store. Users and other endpoints can retrieve messages destined for them from any available Propagation Nodes on the network.

The LXM Router

The LXM Router handles transporting messages over a Reticulum network, managing delivery receipts, outbound and inbound queues, and is the point of API interaction for client programs. The LXM Router also implements functionality for acting as an LXMF Propagation Node.

Programatically, using the LXM Router to send a message is as simple as:

import LXMF

lxm_router = LXMF.LXMRouter()

message = LXMF.LXMessage(destination, source, "This is a short, simple message.")

lxm_router.handle_outbound(message)

The LXM Router then handles the heavy lifting, such as message packing, encryption, delivery confirmation, path lookup, routing, retries and failure notifications.

Transport Encryption

LXMF uses encryption provided by Reticulum, and thus uses end-to-end encryption by default. The delivery method of a message will influence which transport encryption scheme is used.

• A message can be delivered opportunistically, embedded in a single Reticulum packet. In this cases the message will be opportunistically routed through the network, and will be encrypted with ephemeral keys derived with ECDH on Curve25519. This mode offers Perfect Forward Secrecy.

• If a message is delivered to the Reticulum GROUP destination type, the message will be transported using AES-128 encryption.

• If a message is delivered over a Reticulum link (which is the default method), the message will be encrypted with ephemeral keys derived with ECDH on Curve25519. This mode offers Perfect Forward Secrecy.

Wire Format & Overhead

Assuming the default Reticulum configuration, the binary wire-format is as follows:

• 16 bytes destination hash
• 16 bytes source hash
• 64 bytes Ed25519 signature
• Remaining bytes of msgpack payload data, in accordance with the structure defined above

The complete message overhead for LXMF is only 99 bytes, which in return gives you timestamped, digitally signed, infinitely extensible, end-to-end encrypted, zero-conf routed, minimal-infrastructure messaging that's easy to use and build applications with.

Caveat Emptor

LXMF is beta software, and should be considered experimental. While it has been built with cryptography best-practices very foremost in mind, it has not been externally security audited, and there could very well be privacy-breaking bugs. If you want to help out, or help sponsor an audit, please do get in touch.

Installation

If you want to try out LXMF, you can install it with pip:

pip3 install lxmf

Development Roadmap

LXMF is actively being developed, and the following improvements and features are currently planned for implementation:

• Write and release full API and protocol documentation
• Update examples in readme to actually work
• Content Destinations, and easy to use API for group messaging and discussion threads
• Sneakernet and physical transport functionality
• LXMF paper- and QR-encoded messages
• Documenting and possibly expanding LXMF limits and priorities
• Sync affinity based on link speeds and distances, for more intelligently choosing peer sync order
• Markets on LXMF