How to use the PKA peripheral to execute modular exponentiation. This allows ciphering/deciphering a text in interrupt mode.

For this example, two .pem file have been created using openssl external tool. (https://www.openssl.org). Src/rsa_pub_2048.pem and Src/rsa_priv_2048.pem, contains respectively the public and private key used in this example.

To ease the usage of this .pem files, 2 set of files, Src/rsa_pub_2048.c and Inc/rsa_pub_2048.h and Src/rsa_priv_2048.c and Inc/rsa_priv_2048.h are present to reflect their content. The explanation of how to create those file from a .pem are embedded inside both .c files.

The selected algorithm is RSA.

Based on the public key, the plain text message is ciphered using the modular exponentiation and stored in cipheredBuffer.

Then this ciphered buffer is deciphered using the modular exponentiation again but based on the private key and stored in the decipheredBuffer. After this, the buffer is identical to the initial plain text.

In this example, the PKA interrupt is triggered at the end of the operation. The interrupt handler then call the pka callback where a global variable is used to notify the main function.

In case of success, the LED1 (GREEN) is ON. In case of any error, the LED1 (GREEN) is toggling slowly.

1. Care must be taken when using HAL_Delay(), this function provides accurate delay (in milliseconds) based on variable incremented in SysTick ISR. This implies that if HAL_Delay() is called from a peripheral ISR process, then the SysTick interrupt must have higher priority (numerically lower) than the peripheral interrupt. Otherwise the caller ISR process will be blocked. To change the SysTick interrupt priority you have to use HAL_NVIC_SetPriority() function.

2. The application needs to ensure that the SysTick time base is always set to 1 millisecond to have correct HAL operation.

Security, PKA, RSA, modular exponentiation, cipher ,decipher

• PKA/PKA_ModularExponentiation_IT/Inc/stm32h5xx_hal_conf.h HAL configuration file
• PKA/PKA_ModularExponentiation_IT/Inc/stm32h5xx_it.h Interrupt handlers header file
• PKA/PKA_ModularExponentiation_IT/Inc/main.h Header for main.c module
• PKA/PKA_ModularExponentiation_IT/Src/stm32h5xx_it.c Interrupt handlers
• PKA/PKA_ModularExponentiation_IT/Src/main.c Main program
• PKA/PKA_ModularExponentiation_IT/Src/stm32h5xx_hal_msp.c HAL MSP module
• PKA/PKA_ModularExponentiation_IT/Src/system_stm32h5xx.c STM32H5xx system source file
• PKA/PKA_ModularExponentiation_IT/Src/plaintext.bin Random 2048 bits
• PKA/PKA_ModularExponentiation_IT/Src/plaintext.c Reflect the content of plaintext.bin in array format
• PKA/PKA_ModularExponentiation_IT/Inc/plaintext.h Header for plaintext.c
• PKA/PKA_ModularExponentiation_IT/Src/ciphertext.bin Ciphered plaintext.bin using openssl
• PKA/PKA_ModularExponentiation_IT/Src/ciphertext.c Reflect the content of ciphertext.bin in array format
• PKA/PKA_ModularExponentiation_IT/Inc/ciphertext.h Header for ciphertext.c
• PKA/PKA_ModularExponentiation_IT/Src/rsa_priv_2048.pem RSA private key generated using openssl
• PKA/PKA_ModularExponentiation_IT/Src/rsa_priv_2048.c Reflect the content of rsa_priv_2048.pem in array format
• PKA/PKA_ModularExponentiation_IT/Inc/rsa_priv_2048.h Header for rsa_priv_2048.c
• PKA/PKA_ModularExponentiation_IT/Src/rsa_pub_2048.pem RSA public key extracted from rsa_priv_2048.pem using openssl
• PKA/PKA_ModularExponentiation_IT/Src/rsa_pub_2048.c Reflect the content of rsa_pub_2048.pem in array format
• PKA/PKA_ModularExponentiation_IT/Inc/rsa_pub_2048.h Header for rsa_pub_2048.c

• This example runs on STM32H573IIKxQ devices.

• This example has been tested with STMicroelectronics STM32H573I-DK board and can be easily tailored to any other supported device and development board.

In order to make the program work, you must do the following:

• Open your preferred toolchain
• Rebuild all files and load your image into target memory
• Run the example