How to use Static Passkey for a no-display-no-output peripheral

Question

I'm looking to implement the following security schema: 
 
 The peripheral does not have a display nor input capabilities 
 A smartphone will connect to the device 
 The peripheral will pair (but not bond) with the smartphone 
 Pairing will require the smartphone user to enter a 6-digit passkey 
 Upon successful pairing, the passkey can be changed via the mobile application 
 A physical button on the peripheral will enable resetting the passkey to a default 
 
 Some questions: 
 
 Is it possible to implement this schema in nRF52840, s140, SDK15? If so, what are the required steps. 
 Does this schema provide MITM protection (assuming I change the passkey often)?

I know there are a lot of security and passkey relate questions out there, but I couldn't quite find the answers I needed. Thanks!

Sigurd · Accepted Answer

Hi, 
 Is it possible to implement this schema in nRF52840, s140, SDK15? If so, what are the required steps. 
 Yes, but it will not provide 100% MITM protection(opens up for passive eavesdropping). See this post . A starting point would be to take a look at the Glucose Application example in the SDK, it shows you how to implement passkey. By default it will use a random passkey, printed over UART, and you will need to input the passkey on the phone. 
 If you want a static prepreogrammed passkey instead of generaring a randome one, you can do it like this: 
 #define STATIC_PASSKEY "123455"

static void set_static_passkey()

{

 static ble_opt_t m_static_pin_option;

 uint8_t passkey[] = STATIC_PASSKEY;

 m_static_pin_option.gap_opt.passkey.p_passkey = &passkey[0];

 uint32_t err_code = sd_ble_opt_set(BLE_GAP_OPT_PASSKEY, &m_static_pin_option);

 APP_ERROR_CHECK(err_code); 

} 
 
 Call the set_static_passkey() function after you have called ble_stack_init() in main() 
 Snippet: 
 int main(void)
{
.
.
ble_stack_init();
set_static_passkey();
.
.. 
 Then make sure that you have this: 
 #define SEC_PARAM_BOND 0 /**< Perform bonding. */
#define SEC_PARAM_MITM 1 /**< Man In The Middle protection required (applicable when display module is detected). */
#define SEC_PARAM_LESC 0 /**< LE Secure Connections not enabled. */
#define SEC_PARAM_KEYPRESS 0 /**< Keypress notifications not enabled. */
#define SEC_PARAM_IO_CAPABILITIES BLE_GAP_IO_CAPS_DISPLAY_ONLY /**< Display I/O capabilities. */
#define SEC_PARAM_OOB 0 /**< Out Of Band data not available. */
#define SEC_PARAM_MIN_KEY_SIZE 7 /**< Minimum encryption key size. */
#define SEC_PARAM_MAX_KEY_SIZE 16 /**< Maximum encryption key size. */ 
 and that in peer_manager_init(), this is commented out: 
 
 sec_param.kdist_own.enc = 1;
sec_param.kdist_own.id = 1;
sec_param.kdist_peer.enc = 1;
sec_param.kdist_peer.id = 1; 
 Also see this link . 
 
 Does this schema provide MITM protection (assuming I change the passkey often)? 
 The passkey method is generally considered to be secure from MITM attacks, but it's not 100% secure, and it opens up for passive eavesdropping. Please see this , this and this post. 
 Also read this document by Bluetooth SIG . Also this wiki-site provide some good information. 
 Note that what type of security you need will depend on what type of application you have. 
 (PS: bonding is generally recommended, so that the end-user don't have to enter the passkey every time he need to connect to the device.)

How to use Static Passkey for a no-display-no-output peripheral

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