I implemented the org.bluez.Agent1 interface using QDbus and I would like to set a fixed pin (that I will provide to the users) in order to authenticate all the pairing request and reject them if the pin is wrong. The agent capabilities should be "NoInputNoOutput" because the project will be deployed on the RaspberryPi 3 without keyboard or display. Is there a way to deal with this? Thank you
You should not initialize the capability as "NoInputOutput" for fixed key pairing. NoInputOutput means there is not display and there is no keyboard/UI possible for this device. Mentioning this capability for your Agent when registering with BlueZ means, instructing BlueZ (bluetoothd) not to call any Agent API for authentication.
This is typically useful to autopair without any manual intervention. You can see this sample code, which uses NoInputOutput capability to connect the device without any intervention.
What you need is "DisplayOnly" capability to instruct BlueZ to call "DisplayPinCode" or "DisplayPasskey" based on SSP support of your device.
You can implement DisplayPinCode/DisplayPasskey in your Agent to reply constant PIN always. So the Bluetooth device which tries to connect can use the same constant PIN for pairing.
Here Display Pin /Passkey is just the naming convention or hint to Agent developers to write wizard/UI or any form of display operations. But you can completely ignore the displaying operation and reply with static/constant 6 digit key for pairing.
Typically this Agent API is called by Bluez (in rasperry pi) when the device (iPhone/Android mobile/any bluetooth capable device) trying pair calls "RequestPasskey" or "RequestPinCode" from the device end.