Secure Telemetry Shell

MQTT client with End-to-End Payload Encryption for Embedded Linux

Features

• Lightweight MQTT client for Embedded Linux
• End-to-End Payload Encryption with symmetric cipher
• Encapsulation in a shell for easy of use and possible modules development

Introduction

STS is an application-layer protocol on top of MQTT that allows payload E2EE. You can find more information about MQTT payload encryption on this link:

https://www.hivemq.com/blog/mqtt-security-fundamentals-payload-encryption/

When to use STS?

• TLS is not available

• Dealing with untrusted broker

• Use of constrained devices

• Need for secure E2EE of application data

• Need for an additional layer of security working in addition with TLS

  

Dependencies

STS uses a number of open source projects to work properly:

• paho-mqtt-embedded-c - Eclipse Paho MQTT C/C++ client for Embedded platforms
• mbedtls - Mbed TLS is a C library that implements cryptographic primitives, X.509 certificate manipulation and the SSL/TLS and DTLS protocols.

sudo apt install cmake g++

Installation

make deps
make

• make deps will download and build paho-mqtt and mbedtls
• make will build sts

Tests

make build-tests
make run-tests

• make build-tests will build tests
• make run-tests will run tests

HOWTO

STS can be used in 2 modes: with or without encryption: No encryption means a simple MQTT client with minimal features like subscribing to a topic and publishing messages.

With encryption means that an end-2-end encrypted communication is established

MQTT features that aren't implemented are:

• Quality Of Service
• Keep Alive
• Persistent session
• Multiple subscriptions per client

Configuration files

STS needs to use configuration files, those contain parameters for MQTT and STS, the ones provided work just fine, it uses a public broker:

broker.hivemq.com

It is also possible to test it in local using mosquitto, launch a mosquitto broker and simply add "localhost" to the url field.

KEY	VALUE (128 char max)
url	broker url
port	usually 1883 for TCP
username	if broker requires login
password	if broker requires login
subtop	topic to subscribe
pubtop	topic to publish
clientid	mqtt id
sts_mode	nosec, master, slave
aes	null, ecb, cbc

STS no encryption

cd bin/
./sts

Start a session and send messages

start ../configs/nosec
send my message

Stop a session

stop

STS with encryption

2 clients are needed, one in slave mode and the other in master mode. Protocol is designed as so you need to start a session with slave first then with master.

start ../configs/secslave

start ../configs/secmaster

Once encryption is established you can send encrypted message with:

sendenc my message

Architecture

STS Message

STS uses MQTT so a STS message is encapsulated in a MQTT message. It has one header which contains the message type and a data. Total size of a STS message is the same as MQTT, in this implementation it is set to 1024 bytes: [--HEADER--][--DATA--]

Message Types

INITREQ	Initialization request
INITACK	Initialization acknowledgement
KEYREQ	Key request
KEYACK	Key acknowledgement
ENC	Encrypted message
KILL	Message that tells remote client to terminate its session

Connection Protocol

Key Exchange and Shared Secret

master sends KEYREQ with its public key, upon receipt slave computes the shared secret and acknowledges with KEYACK. Now it is slave's turn to send its public key with KEYREQ, upon receipt master computes the shared secret and acknowledges with KEYACK. At this point encrypted communication is available and every message will be sent with an ENC header.

Disconnection

If any of the two clients ends its session a KILL message is sent to the remote client to notify it to terminate its session too.

Algorithms

Key exchange agreement protocol used is ECDH, elliptic curve is SECP256K1, finally encryption used is AES-256, 2 block cipher modes operation are available, ECB and CBC.

TODO

• maybe sequence number on msg?
• maybe msg integrity with SHA256?
• and maybe digital signature with ECDSA?