How do I encrypt a string?

[sdi_kei]

Hello.
We are developing using nrf52832 (S132 v7.0.1, SDK v17.0.0).

I'm looking at how to encrypt a string with AES CBC.
Below is the code we are considering.

static uint8_t m_key[16] = {'N', 'O', 'R', 'D', 'I', 'C', 'A', 'E', 'S', 'C', 'B', 'C', 'T', 'E', 'S', 'T'};
static char m_plain_text[NRF_CRYPTO_EXAMPLE_AES_MAX_TEXT_SIZE] =
{
    "Example string to demonstrate basic usage of AES CBC mode."
};
static char m_encrypted_text[NRF_CRYPTO_EXAMPLE_AES_MAX_TEXT_SIZE];

static void crypto_test(void){
    static nrf_crypto_aes_context_t cbc_encr_ctx;
    static nrf_crypto_aes_context_t cbc_decr_ctx;
           ret_code_t               ret_val;
           uint8_t                  iv[NRF_CRYPTO_MBEDTLS_AES_IV_SIZE];  // 16
           size_t                   len_in;
           size_t                   len_out;

    memset(m_encrypted_text, 0, sizeof(m_encrypted_text));
    memset(m_decrypted_text, 0, sizeof(m_decrypted_text));
    memset(iv, 0, sizeof(iv));

    len_in = strlen(m_plain_text);
    len_out = sizeof(m_encrypted_text);

    ret_val = nrf_crypto_init();
    APP_ERROR_CHECK(ret_val);

    ret_val = nrf_crypto_aes_init(&cbc_encr_ctx, &g_nrf_crypto_aes_cbc_128_info, NRF_CRYPTO_ENCRYPT);
    AES_ERROR_CHECK(ret_val);

    ret_val = nrf_crypto_aes_key_set(&cbc_encr_ctx, m_key);
    AES_ERROR_CHECK(ret_val);

    ret_val = nrf_crypto_aes_iv_set(&cbc_encr_ctx, iv);
    AES_ERROR_CHECK(ret_val);

    ret_val = nrf_crypto_aes_finalize(&cbc_encr_ctx,
                                      (uint8_t *)m_plain_text,
                                      len_in,
                                      (uint8_t *)m_encrypted_text,
                                      &len_out);
    AES_ERROR_CHECK(ret_val);

    return;
}

Currently, it works, but the strings are not encrypted and are all "0x00".
How can I solve it?

Best Regards.

[Carl Richard]

Hello, sdi_kei!

There seems to be an issue with the input length of your plain text string. With nrf_crypto_aes the input must be a multiple of 16 bytes, so you must pad the input data.

Best regards,
Carl Richard

[sdi_kei]

Hello,Carl Richard!

Thank you for teaching.
When I changed the character length of m_plain_text to 64, the 0x8511 error disappeared.

However, HardFault_Handler started to occur.
The condition that occurs is when you call nrf_crypto_aes_finalize.

The debugger shows "Exception occurred at PC = 0x39602, LR = 0x395d3".
How can I solve it?

Best Regards.

[Carl Richard]

Hello again!

This means that the program crashed somehow. Are you able to check where in the code Program Counter (PC) stopped?

Best regards,
Carl Richard

[sdi_kei]

Hello,Carl Richard!

When I checked the location of the PC, it was in SaSi_AesFinish on line 495 of cc310_backend_aes.c.

Best regards.

[Carl Richard]

Hello!

You must use a different backend, as the CryptoCell peripheral (CC310) isn't available on the nRF52832. Please check out the documentation for configuring nrf_crypto backends.

Best regards,
Carl Richard

[sdi_kei]

Hello!

I changed the backend from CC310 to mbed TLS and now I can encrypt and decrypt!
Thank you!

One question is that m_plain_text must be a multiple of 16 bytes. Is there a way to encrypt 15 byte characters?

Best regards.

[Carl Richard]

Hello again!

Good to hear. You must pad any input data shorter than 16 bytes. I.e, add empty bytes at the end of the array.

Best regards,
Carl Richard