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