This guide covers hardware and software methods for encrypting and decrypting firmware in [microcontrollers](https://www.ampheo.com/c/microcontrollers) ([STM32](https://www.ampheo.com/search/STM32), [PIC](https://www.ampheo.com/search/PIC), [AVR](https://www.ampheo.com/search/AVR), ESP32, etc.), including security best practices and attack mitigation.

**1. Why Encrypt MCU Firmware?**

**2. Hardware-Based Encryption**
**A. Built-in MCU Security Features**

STM32 Read Protection (RDP)
* Level 0: No protection (default)
* Level 1: Flash read disabled (JTAG/SWD locked)
* Level 2: Irreversible bricking if tampered

How to enable:

c
// In STM32CubeIDE (or via ST-Link CLI)
HAL_FLASH_OB_Unlock();
FLASH_OBProgramInitTypeDef OB_Config;
OB_Config.OptionType = OPTIONBYTE_RDP;
OB_Config.RDPLevel = OB_RDP_LEVEL_1;
HAL_FLASHEx_OBProgram(&OB_Config);
HAL_FLASH_OB_Lock();

**B. External Secure Elements**

Example: AES-128 Encryption with ATECC608A

c
uint8_t plaintext[16] = "Hello MCU!";
uint8_t key[16] = {0x2B, 0x7E, 0x15, ... }; // AES-128 key
uint8_t ciphertext[16];

ATECC608A_Encrypt(key, plaintext, ciphertext); // Uses I2C

**3. Software-Based Encryption**
**A. Symmetric Encryption (AES)**
Best for real-time decryption in resource-constrained MCUs.

STM32 Hardware AES (CRYP peripheral)

c
// STM32Cube HAL Example
AES_HandleTypeDef haes;
haes.Init.DataType = AES_DATATYPE_8B;
haes.Init.KeySize = AES_KEYSIZE_128;
HAL_CRYP_Init(&haes);

uint8_t key[16] = "..."; // 128-bit key
uint8_t plaintext[16] = "Encrypt this!";
uint8_t ciphertext[16];

HAL_CRYP_AESECB_Encrypt(&haes, plaintext, 16, ciphertext, HAL_MAX_DELAY);

**B. Asymmetric Encryption (RSA/ECC)**
Used for secure boot and key exchange.

* RSA-2048: Slow (~1 sec on Cortex-M4)
* ECC P-256: Faster than RSA

Example: ECDSA Signature Verification

c
// Using MbedTLS on STM32
mbedtls_ecdsa_context ctx;
mbedtls_ecdsa_init(&ctx);
mbedtls_ecp_group_load(&ctx.grp, MBEDTLS_ECP_DP_SECP256R1);

mbedtls_ecdsa_verify(&ctx.grp, hash, sig, &ctx.Q);

**4. Firmware Decryption Methods (Attack Vectors)**
**A. Side-Channel Attacks**

**B. Fault Injection**
* Clock Glitching → Reset MCU during secure boot
* Laser Fault Injection → Bypass password checks

Mitigation:

* Enable Tamper Protection (STM32 RDP Level 2)
* Use Secure Bootloaders

**C. JTAG/SWD Exploitation**
* Disable Debug Ports (STM32 RDP=1)
* Use Secure Firmware Updates (Signed DFU)

**5. Secure Boot Implementation**
**A. STM32 Secure Boot (TrustZone)**
1. Enable TrustZone in STM32CubeMX

2. Sign Firmware with ECDSA

bash
openssl ecparam -genkey -name prime256v1 -noout -out priv.key
openssl dgst -sha256 -sign priv.key firmware.bin > firmware.sig

3. Verify Signature in Bootloader

**B. ESP32 Secure Boot**

bash
# Enable Secure Boot in ESP-IDF
idf.py secure-boot enable
idf.py flash

**6. Best Practices**
1. Never Store Keys in Flash → Use OTP (One-Time Programmable) memory
2. Use Randomized Nonces → Prevent replay attacks
3. Secure Firmware Updates → Encrypted + Signed OTA
4. Disable Unused Peripherals → Reduce attack surface

**7. Tools for MCU Security Testing**

**Conclusion**

Next Steps:

* Try [STM32](https://www.ampheoelec.de/search/STM32) TrustZone for secure/non-secure partitions
* Implement AES-256 + Secure Boot on ESP32
* Test your MCU with ChipWhisperer