AES Encryption with SD

[leehong]

Hi there,

I'm using the softdevice sd_ecb_block_encrypt to encrypt my 16byte block data as follows:

When my data buffer is filled, it called aes_encrypt(),

void aes_encrypt(void) { int error = 0; uint8_t clear_text[16] = {Data[0], Data[1], Data[2], Data[3], Data[4], Data[5],Data[6], Data[7], Data[8], Data[9], Data[10], Data[11], Data[12], 0x00, 0x00, 0x00};

nrf_ecb_hal_data_t encryption_parm;
memset(&encryption_parm, 0, sizeof(encryption_parm));
memcpy(encryption_parm.key, key, 16);
memcpy(encryption_parm.cleartext, clear_text, 16);
memcpy(encryption_parm.ciphertext, cipher_text, 16);

error = sd_ecb_block_encrypt(&encryption_parm);
printf("%d", error);
printf("%s", cipher_text);

/* write to eddystone UID frame */
write_uid_frame_buffer();
eddystone_set_adv_data(EDDYSTONE_UID);

}

A snipplet of my write_uid_frame_buffer is as follows:

encoded_advdata[(*len_advdata)++] = cipher_text[0];
encoded_advdata[(*len_advdata)++] = cipher_text[1];
encoded_advdata[(*len_advdata)++] = cipher_text[2];
encoded_advdata[(*len_advdata)++] = cipher_text[3];
encoded_advdata[(*len_advdata)++] = cipher_text[4];
encoded_advdata[(*len_advdata)++] = cipher_text[5];
encoded_advdata[(*len_advdata)++] = cipher_text[6];

I'm basically encrypting a segment of my eddystone broadcasting frame to be deciphered by specific users only. I'm not getting any new values from cipher_text after sd_ecb_block_encrypt(&encryption_parm) is called, any idea?

[run_ar]

What do you mean when you say that you are not getting any new values from cipher text? Do you mean you are not getting a value at all or that the value don't change? What data are you putting in, static or do you have a counter (ref counter mode)?

[leehong]

The value doesn't change. data in (clear_text) is data coming in from another chip at 10sec intervals. It seems the softdevice is too busy to process encryption?

[run_ar]

I haven't seen this before. Can you confirm that the data coming in from the other chip is actually changing? And which Softdevice version are you using?

[leehong]

Yes, new data comes in every 10s. I'm using the S110 sd.

[run_ar]

The AES block is used to create a chippertext, this can be used to encrypt/decrypt data as shown below:

nrf_ecb_hal_data_t aes_struct;

uint8_t aes_data[16];

uint8_t aes_data_decrypted[16];



//Initializing arrays

memset (&aes_struct, 0, sizeof(aes_struct));

memset (aes_data, 0, sizeof(aes_data));

memset (aes_data_decrypted, 0, sizeof(aes_data_decrypted));



//Initializing key

for (int i = 0; i < 16; i++)

{

    aes_struct.key [i] = i;

}



//Initializing nouncence

memset (aes_struct.cleartext, 0xaa, sizeof(aes_struct.cleartext)); //todo: use more random data and add counter



//Creating chipertext

sd_ecb_block_encrypt(&aes_struct);  



//Encrypt -> XOR chipertext with p_data:

for (int i = 0; i < length; i++)

{  

    aes_data[i] = p_data [i] ^ aes_struct.ciphertext[i];

}



//decrypt -> XOR chipertext with Encrypted data:

for (int i = 0; i < length; i++)

{  

    aes_data_decrypted[i] = aes_data [i] ^ aes_struct.ciphertext[i];

}