How to use MCE for encryption/decryption on STM32 MCU?

 The MCE (Math Coprocessor Engine) on STM32 MCUs is often part of STM32 devices featuring advanced hardware acceleration for cryptographic operations. It is used to enhance encryption and decryption performance for algorithms like AES, DES, and others. Here's how to use MCE for encryption and decryption:

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Steps to Use MCE for Encryption/Decryption

1. Verify MCE Availability

Ensure that your STM32 microcontroller supports the MCE. This is typically found in STM32 families with cryptographic hardware accelerators, such as:

• STM32L4/L5
• STM32H7
• STM32WB/MP1 (selected models)

Check the datasheet or reference manual for confirmation.

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2. Initialize the Cryptographic Library

STM32 provides the HAL/LL drivers or the STM32 Cryptographic Library (STM32 Cryptolib) for interacting with the MCE.

Include Required Headers

Ensure you include the required headers in your code:

c

#include "stm32_hal.h"
#include "stm32l4xx_hal_cryp.h" // For STM32L4 (example)

Initialize the HAL Library

Call the HAL_Init() function and configure the system clock:

c

HAL_Init();
SystemClock_Config(); // Ensure the clock is set for the crypto engine

Enable the Crypto Peripheral Clock

Enable the clock for the cryptographic hardware:

c

__HAL_RCC_CRYP_CLK_ENABLE();

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3. Configure the MCE for Encryption/Decryption

Use the HAL_CRYP_Init() function to configure the MCE with your desired cryptographic parameters.

Example: AES Encryption/Decryption

1. Configure the CRYP_HandleTypeDef structure:

c

CRYP_HandleTypeDef hcryp;
uint8_t key[16] = { /* Your 128-bit AES key */ };
uint8_t iv[16] = { /* Initialization vector for CBC mode */ };

hcryp.Instance = CRYP; // Use the CRYP hardware instance
hcryp.Init.DataType = CRYP_DATATYPE_8B; // Process data as 8-bit chunks
hcryp.Init.KeySize = CRYP_KEYSIZE_128B; // AES-128 key size
hcryp.Init.Algorithm = CRYP_AES_CBC;    // AES in CBC mode
hcryp.Init.pKey = key;                  // Key pointer
hcryp.Init.pInitVect = iv;              // IV pointer
HAL_CRYP_Init(&hcryp);

2. Encrypt or decrypt data:
   • Use HAL_CRYP_Encrypt() for encryption.
   • Use HAL_CRYP_Decrypt() for decryption.

Example Code

c

uint8_t plaintext[16] = { /* 16 bytes of data to encrypt */ };
uint8_t ciphertext[16]; // Buffer for encrypted data
uint8_t decryptedtext[16]; // Buffer for decrypted data
uint16_t dataLength = 16; // Data length in bytes

// Encrypt data
HAL_CRYP_Encrypt(&hcryp, plaintext, dataLength, ciphertext, HAL_MAX_DELAY);

// Decrypt data
HAL_CRYP_Decrypt(&hcryp, ciphertext, dataLength, decryptedtext, HAL_MAX_DELAY);

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4. Verify Results

After encryption and decryption, compare the decrypted text with the original plaintext to ensure correctness.

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5. Clean Up

When the cryptographic operation is complete, deinitialize the MCE to save power and free resources:

c

HAL_CRYP_DeInit(&hcryp);

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Additional Tips

1. Check HAL Documentation The STM32 HAL documentation contains detailed examples and parameter explanations for the CRYP_HandleTypeDef structure and functions.

2. Interrupt/DMA Mode For larger datasets, use interrupt or DMA mode to offload CPU processing:

   • Use HAL_CRYP_Encrypt_IT() or HAL_CRYP_Encrypt_DMA().
   • Similarly, use HAL_CRYP_Decrypt_IT() or HAL_CRYP_Decrypt_DMA().

3. Algorithm Support The MCE supports various algorithms like AES, DES, and TDES. Configure the Algorithm field of the CRYP_HandleTypeDef structure accordingly.

4. Security Considerations

   • Use secure key storage mechanisms.
   • Ensure proper handling of initialization vectors (IVs) for modes like CBC.

5. Debugging Check the return values of HAL functions to handle errors appropriately, such as HAL_ERROR or HAL_BUSY.

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This approach provides efficient and secure encryption/decryption leveraging the MCE on STM32 MCUs. For complex setups, consult the STM32Cube examples specific to your microcontroller.