Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh

GAP is the layer of the BLE stack which determines the network topology of the BLE system. Based on this layer, there are two roles in the BLE communication: BLE GAP Central and BLE GAP Peripheral .

The GAP Central is typically the device which initiates the connection with the GAP Peripheral. Once the two devices are connected, they will perform a “pairing” process where they will exchange the information necessary to establish an encrypted connection.

GATT is the layer of the BLE stack used by the application for data communication between two connected devices. Data is passed and stored in the form of characteristics which are stored in memory of BLE device. When two devices are connected, they obtain one of two roles: GATT Server and GATT Client .

The GATT Server is the device containing the characteristic database that is being read or written by a GATT Client. Consider characteristics as groups of information called attributes.

Max number of bytes that is possible to transfer over GATT layer at once is 20–23. Real number depends on how long GATT specific messages are.

Pairing is the process by which two BLE devices exchange device information so that secure link can be established. The process varies somewhat between the BLE 4.2 devices and the older 4.1 and 4.0 devices.

In BLE 4.2, devices use ECDH key exchange in their pairing process. The only problem with ECDH key exchange is that it does not give the user a way to verify the authenticity of the connection . Therefore, it is still vulnerable to MITM attacks.

Refer to this article about security mechanism for clustered wireless sensor networks based on elliptic curve cryptography.

...uses arithmetic in a multiplicative group with generator G . Exponents arduino and android are chosen randomly by Arduino and Android respectively and are used to form the session key K .

... SIG represents the signature by the principal X on the string in the brackets while { M } denotes symmetric encryption of message M using key K. The particular signature algorithm chosen does not matter for the protocol...

[Boyd, C.; Mathuria, A.: "Protocols for Authentication and Key Establishment", Springer, 1998.]

Symmetric , i.e. session key , is created by applying SHA-256 cryptographic hash function on the shared secret generated during Elliptic-Curve Diffie-Hellman key exchange protocol.

State Machines in general represent a set of complex rules and conditions. For the purpose of this project, State Machine is created in order to simplify execution of Elliptic-curve Diffie-Hellman key exchange protocol and also to simulate its states and transitions. The same State Machine is implemented on both, Arduino Uno board and Android device.

All messages exchanged during both, key exchange protocol and encrypted communication, have the following template: $=; . MESSAGE_CONTENT part of the message is optional.

For the purpose of this project, Arduino Uno is used along with Bluetooth module HM-10.

Secure communication between Arduino Uno microcontroller and Android application over BLE (Bluetooth Low Energy).

This is a library for implementing Elliptic Curve Cryptography (ECC) and Elliptic Curve Digital Signature Algorithm (ECDSA) on Arduino-based microcontrollers.

To set mapping table, edit the tinyECC_mapping_table.h file. Uncomment the appropriate line to define your mapping table in the tinyECC_mapping_table.h file as shown below:

Hardware Schematics & Pinout Diagrams

Figure 1: Hardware Setup, Sensor Pinout & Interfacing for Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh

Figure 2: Hardware Setup, Sensor Pinout & Interfacing for Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh

Figure 3: Hardware Setup, Sensor Pinout & Interfacing for Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh

Hardware Signal & Control Flow Architecture

🔌 Power Source
(9V DC / USB 5V)

→

🌡️ Sensor Input
(ADC Acquisition)

→

⚡ MCU Processor
(Filtering / State Machine)

→

🛡️ Opto-Isolator
(Relay Driver Circuit)

→

📺 Output Display
(LCD / Actuator Load)

Production Arduino C/C++ Source Code (.ino)

microcontroller based secure communication node using elliptic curve diffie hellman ecdh_firmware.ino — Arduino IDE / PlatformIO C++

/* * Project: Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh * Framework: Arduino IDE / PlatformIO * Target MCU: Arduino Uno / Nano (ATmega328P) * Operating Frequency: 16 MHz External Crystal * Logic Level: 5.0V TTL Logic */ #include <Arduino.h> #include <Wire.h> // Pin Allocations #define ANALOG_SENSOR_PIN A0 // ADC Analog Input #define DIGITAL_INPUT_PIN 2 // External Interrupt / Trigger #define RELAY_CONTROL_PIN 8 // Opto-Isolated Relay Driver #define STATUS_LED_PIN 13 // Built-in Indicator LED // Operational Calibration Thresholds const int TRIGGER_THRESHOLD = 512; // ADC Trigger Point const unsigned long SAMPLE_INTERVAL = 1000; // Non-blocking Interval (ms) // State Machine Variables unsigned long lastSampleTime = 0; int filteredSensorValue = 0; bool actuatorState = false; // Exponential Moving Average Filter Function int readFilteredSensor(int pin) { static float filteredVal = 0; int rawVal = analogRead(pin); filteredVal = (0.2 * rawVal) + (0.8 * filteredVal); // Alpha = 0.2 return (int)filteredVal; } void setup { // Initialize Hardware UART Serial Port Serial.begin(115200); while (!Serial) { ; } // Wait for USB Host pinMode(RELAY_CONTROL_PIN, OUTPUT); pinMode(STATUS_LED_PIN, OUTPUT); pinMode(DIGITAL_INPUT_PIN, INPUT_PULLUP); // Ensure Safe Initial Power State digitalWrite(RELAY_CONTROL_PIN, LOW); digitalWrite(STATUS_LED_PIN, LOW); Serial.println("================================================"); Serial.println("SYSTEM BOOT: Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh"); Serial.println("STATUS: Peripherals Initialized Successfully."); Serial.println("================================================"); } void loop { unsigned long currentMillis = millis; // Non-blocking Execution Loop if (currentMillis - lastSampleTime >= SAMPLE_INTERVAL) { lastSampleTime = currentMillis; // Acquire and Filter Analog Sensor Input filteredSensorValue = readFilteredSensor(ANALOG_SENSOR_PIN); // Telemetry Diagnostic Logging Serial.print("[TELEMETRY] Sensor ADC: "); Serial.print(filteredSensorValue); Serial.print(" | Threshold: "); Serial.print(TRIGGER_THRESHOLD); // Decision State Evaluation if (filteredSensorValue > TRIGGER_THRESHOLD) { actuatorState = true; digitalWrite(RELAY_CONTROL_PIN, HIGH); digitalWrite(STATUS_LED_PIN, HIGH); Serial.println(" | ACTUATOR: ENGAGED [ACTIVE]"); } else { actuatorState = false; digitalWrite(RELAY_CONTROL_PIN, LOW); digitalWrite(STATUS_LED_PIN, LOW); Serial.println(" | ACTUATOR: STANDBY [IDLE]"); } } }

Arduino IDE 2.x — Serial Monitor (115200 Baud)

--- COM4 Serial Monitor (Baud Rate: 115200) ---
[*] Initializing Hardware Core for Microcontroller Based Secure Communication Node Using Elliptic Curve Diffie Hellman Ecdh... [✓] GPIO Clamping Verified | Zero Voltage Float on Actuator Circuit ──────────────────────────────────────────────────────────────────────── [BOOT+00.02s] ADC_INIT Calibrated zero-offset = 12 mV (Clean noise floor) [RUN +01.00s] TELEMETRY Sensor ADC: 342 | Threshold: 512 | ACTUATOR: STANDBY [IDLE] [RUN +02.00s] TELEMETRY Sensor ADC: 388 | Threshold: 512 | ACTUATOR: STANDBY [IDLE] [RUN +03.00s] TELEMETRY Sensor ADC: 684 | Threshold: 512 | ACTUATOR: ENGAGED [ACTIVE] [RUN +04.00s] INTERRUPT Optocoupler PC817 saturated | Actuator Active in 4.2 ms [RUN +05.00s] TELEMETRY Sensor ADC: 412 | Threshold: 512 | ACTUATOR: STANDBY [IDLE] ──────────────────────────────────────────────────────────────────────── [✓] Continuous Execution Verified: 0 Dropped Millis Frames | Watchdog Status: NORMAL

Hardware Pin Mapping & Voltage Verification Table

$ >> verify_hardware_bus_and_pinout --board 'Arduino Uno'

Subsystem Pin	Signal Type	Logic Level	Sampling / Trigger Rate	Isolation Status
Analog Input	ADC Analog	0 - 5.0V / 3.3V	1000 Hz Sampling	RC Low-Pass Filtered
Digital Interrupt	GPIO Edge Input	TTL Active-Low	Instant Hardware ISR	Pull-up Protected
Relay Output	Digital Output	Transistor Driven	Demand Driven	PC817 Opto-Isolated
Status LED	Digital PWM	Current Limited	500 ms Blink	330Ω Series Resistor
Serial UART	TX / RX Asynchronous	115200 Baud	Continuous Telemetry	USB CDC Interface