How to make the bluez bluetooth stack work with hex pin codes.

event.c

/*
 * An excerpt from https://github.com/pcacjr/bluez/blob/c70a4afe4328053f462d8e30a0a787245aa7b948/src/event.c#L132
 */

static size_t decode_hex(const char *pin, char *out)
{
	size_t i;

	for (i = 0; i < 16 && pin[i * 2] && pin[i * 2 + 1]; i++)
		sscanf(&pin[i * 2], "%02hhX", &out[i]);

	return i;
}

static size_t decode_pin(const char *pin, char *out)
{
	size_t len;

	if (!pin)
		return 0;

	if (pin[0] == '$') {
		len = decode_hex(&pin[1], out);
	} else {
		len = strnlen(pin, 16);
		memcpy(out, pin, len);
	}

	return len;
}

static void pincode_cb(struct agent *agent, DBusError *derr,
				const char *pincode, struct btd_device *device)
{
	struct btd_adapter *adapter = device_get_adapter(device);
	bdaddr_t dba;
	int err;
	size_t len;
	char rawpin[16];

	device_get_address(device, &dba);

	len = decode_pin(pincode, rawpin);
	if (derr || !len) {
		err = btd_adapter_pincode_reply(adapter, &dba, NULL, 0);
		if (err < 0)
			goto fail;
		return;
	}

	err = btd_adapter_pincode_reply(adapter, &dba, rawpin, len);
	if (err < 0)
		goto fail;

	return;

fail:
	error("Sending PIN code reply failed: %s (%d)", strerror(-err), -err);
}