diff options
| -rw-r--r-- | src/components/ble/NimbleController.cpp | 23 |
1 files changed, 22 insertions, 1 deletions
diff --git a/src/components/ble/NimbleController.cpp b/src/components/ble/NimbleController.cpp index d8510bd3..00a394fe 100644 --- a/src/components/ble/NimbleController.cpp +++ b/src/components/ble/NimbleController.cpp @@ -278,7 +278,28 @@ int NimbleController::OnGAPEvent(ble_gap_event* event) { if (event->passkey.params.action == BLE_SM_IOACT_DISP) { struct ble_sm_io pkey = {0}; pkey.action = event->passkey.params.action; - pkey.passkey = ble_ll_rand() % 1000000; + + /* + * Passkey is a 6 digits code (1'000'000 possibilities). + * It is important every possible value has an equal probability + * of getting generated. Simply applying a modulo creates a bias + * since 2^32 is not a multiple of 1'000'000. + * To prevent that, we can reject values greater than 999'999. + * + * Rejecting values would happen a lot since 2^32-1 is way greater + * than 1'000'000. An optimisation is to use a multiple of 1'000'000. + * The greatest multiple of 1'000'000 lesser than 2^32-1 is + * 4'294'000'000. + * + * Great explanation at: + * https://research.kudelskisecurity.com/2020/07/28/the-definitive-guide-to-modulo-bias-and-how-to-avoid-it/ + */ + uint32_t passkey_rand; + do { + passkey_rand = ble_ll_rand(); + } while (passkey_rand > 4293999999); + pkey.passkey = passkey_rand % 1000000; + bleController.SetPairingKey(pkey.passkey); systemTask.PushMessage(Pinetime::System::Messages::OnPairing); ble_sm_inject_io(event->passkey.conn_handle, &pkey); |