Documentation about RF packets

witc · September 30, 2020, 1:37pm

Dear,

Is there any documentation about how the Meshtastic really works? I mean especially the RF communication beetween nodes (crypting, forwarding,…)

Thank you very much. Jan.

geeksville · September 30, 2020, 6:55pm

Sure. Some of those docs are a bit sparse but:

Mesh algorithm:

github.com

meshtastic/Meshtastic-device/blob/master/docs/software/mesh-alg.md

# Mesh broadcast algorithm

## Current algorithm

The routing protocol for Meshtastic is really quite simple (and suboptimal). It is heavily influenced by the mesh routing algorithm used in [Radiohead](https://www.airspayce.com/mikem/arduino/RadioHead/) (which was used in very early versions of this project). It has four conceptual layers.

### A note about protocol buffers

Because we want our devices to work across various vendors and implementations, we use [Protocol Buffers](https://github.com/meshtastic/Meshtastic-protobufs) pervasively. For information on how the protocol buffers are used wrt API clients see [sw-design](sw-design.md), for purposes of this document you mostly only
need to consider the MeshPacket and Subpacket message types.

### Layer 1: Non reliable zero hop messaging

This layer is conventional non-reliable lora packet transmission. The transmitted packet has the following representation on the ether:

- A 32 bit LORA preamble (to allow receiving radios to synchronize clocks and start framing). We use a longer than minimum (8 bit) preamble to maximize the amount of time the LORA receivers can stay asleep, which dramatically lowers power consumption.

After the preamble the 16 byte packet header is transmitted. This header is described directly by the PacketHeader class in the C++ source code. But indirectly it matches the first portion of the "MeshPacket" protobuf definition. But notably: this portion of the packet is sent directly as the following 16 bytes (rather than using the protobuf encoding). We do this to both save airtime and to allow receiving radio hardware the option of filtering packets before even waking the main CPU.

- to (4 bytes): the unique NodeId of the destination (or 0xffffffff for NodeNum_BROADCAST)

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And crypto:

github.com

meshtastic/Meshtastic-device/blob/master/docs/software/crypto.md

# Encryption in Meshtastic

Cryptography is tricky, so we've tried to 'simply' apply standard crypto solutions to our implementation. However,
the project developers are not cryptography experts. Therefore we ask two things:

- If you are a cryptography expert, please review these notes and our questions below. Can you help us by reviewing our
  notes below and offering advice? We will happily give as much or as little credit as you wish ;-).
- Consider our existing solution 'alpha' and probably fairly secure against a not particularly aggressive adversary
(but we can't yet make a more confident statement).

## Summary of strengths/weaknesses of our current implementation

Based on comments from reviewers (see below), here's some tips for usage of these radios.  So you can know the level of protection offered:

* It is pretty likely that the AES256 security is implemented 'correctly' and an observer will not be able to decode your messages.
* Warning: If an attacker is able to get one of the radios in their position, they could either a) extract the channel key from that device or b) use that radio to listen to new communications.
* Warning: If an attacker is able to get the "Channel QR code/URL" that you share with others - that attacker could then be able to read any messages sent on the channel (either tomorrow or in the past - if they kept a raw copy of those broadcast packets)

Possible future areas of work (if there is enough interest - post in our [forum](https://meshtastic.discourse.group) if you want this):

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Documentation about RF packets

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