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Fix biased BLE pass key generation

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Zorvalt 2021-12-19 17:10:50 +01:00 committed by JF
parent 5938b4b208
commit d967efa197
1 changed files with 22 additions and 1 deletions
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23
src/components/ble/NimbleController.cpp
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@ -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);