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]

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];

}

[run_ar]

Compared to the "block cipher mode of operation" article on wikipedia: p_data is the actuall data you want to encrypt (plaintext). aes_struct.ciphertext is the same as the "block chiper encryption" block output. So when aes_struct.ciphertext is xor'ed with p_data you get what is named ciphertext in the counter mode overview in the wikipedia article (encrypted data ready to send). Obviously it's a bit confusion since there are two chipertexts... But the AES block supports encryption, not decryption which is why it's done like this.

[Naralasetty Praneeth]

hi @run_ar

i need encrypt the advertising data like uuid,major&minor of my beacon so that it cannot be visible to other app or nrf connect it should be visible only to self designed app

for this which code i should use

[codebreaker176]

Hi, 

Did you get what to do for this 

Naralasetty Praneeth said:

i need encrypt the advertising data like uuid,major&minor of my beacon so that it cannot be visible to other app or nrf connect it should be visible only to self designed app

If so can you please share

[run_ar]

Hi,

You can use the same procedure as described earlier to encrypt the data before placing it in the advertisement data as manufacturer specific data. However you also need a good (secure) way to exchange the secure key, if that is in place and you use counter mode it should be quite secure.

[codebreaker176]

Hi,

Can you give an example how to use the security key in the code?

Like how and where is the security key supposed to be used.

And how to automatically generate a security key?

[run_ar]

I think that's already described in the code example in this thread. Just need to replace p_data with whatever date you want to encrypt. once that is done you add the encrypted data to manufacturer specific data in the advertisement packet.

[codebreaker176]

Alright thanks!