2A-Trust — Lightweight Authentication & Replay Protection for ESP-NOW

Overview

2A-Trust is a beginner-friendly extension to the ESP-NOW communication protocol on ESP32 devices. It adds simple lightweight authentication and replay protection using a shared secret and monotonic sequence numbers.

This project prioritizes practical trust guarantees without heavy cryptography, making it suitable for undergraduate work in communication and embedded systems.

Threat Model

Assumptions:

• The wireless channel is open and untrusted.
• An attacker can inject or replay packets.
• Firmware secrets are not extracted by the attacker.

Goal:

The receiver should only accept messages from a trusted sender that contain valid fields.

Mechanisms Implemented

1. Shared Secret – A fixed token included in every packet.
2. Sequence Number – A monotonic field to defend against replayed packets.

These mechanisms are easy to explain and implement using Arduino for ESP32.

Packet Format

Each packet consists of:

• secret: 16-bit shared token
• seq: 16-bit sequence number
• payload: application data

At the receiver:

• Shared secret is checked first.
• Sequence number is checked next for freshness.

Reproducibility Setup

Hardware

• 2× ESP32-WROOM-32 development boards
• USB cables for programming

Software Requirements

• Arduino IDE 1.8.19 or later
• ESP32 Arduino Core v2.0.5
• No external libraries required (ESP-NOW is built-in)

Configuration Steps

1. Open receiver/receiver.ino
2. Note the receiver's MAC address printed on board or find via:

   WiFi.macAddress()

3. Open sender/sender.ino
4. Update line 10:

   uint8_t receiverMAC[] = {0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF};  // Your receiver MAC

5. Set shared secret (must match on both devices):

   #define SHARED_SECRET 0xBEEF

6. Flash receiver first, then sender
7. Open Serial Monitor at 115200 baud

Limitations

• No cryptographically strong authentication.
• Payloads are not encrypted.
• Shared secret is static and visible in firmware.
• Only one sender–receiver relationship is shown.

Evaluation

The receiver logs:

• Number of accepted packets
• Number of rejected packets
• Reason for rejection

These logs can be exported from the Serial Monitor into CSV for basic analysis.

Security Analysis

Shared Secret Strength

• 16-bit secret = 65,536 possible values
• Vulnerable to brute force in <1 minute with automated scanning
• Not suitable for production, intended for educational demonstration only

Sequence Number Limitations

• 16-bit counter = 65,536 messages maximum
• Counter overflow requires manual reset
• No persistence across device reboots
• Sequential messages can be replayed in-order after power cycle

Appropriate Use Cases

• Classroom demonstration of protocol validation concepts
• Low-stakes sensor networks in controlled environments
• Educational exploration of replay attack prevention

Repository Structure

2A-Trust/

├── sender/

│ └── sender.ino

|____ License

├── receiver/

│ └── receiver.ino

├── results/

│ └── *.csv └── README.md

Educational Value

By building and evaluating this project, you learn:

• Wireless protocol message validation
• Replay attack defenses
• Embedded engineering trade-offs

Djaman670@gmail.com