Linux devices with host and either USB OTG or device ports can be used as USB to USB converter boards, with the keyboard connected to the USB host port and the PC to the USB OTG or device port.
Under this kind of setup, the Linux USB HID gadget driver can be used to emulate
a HID device and keyd can be configured to translate evdev input events to
HID reports.
sudo apt-get install libudev-dev # Debian specific, install the corresponding package on your distribution
git clone https://github.com/rvaiya/keyd
cd keyd
make vkbd-usb-gadget && sudo make install-usb-gadget
sudo systemctl enable usb-gadget && sudo systemctl start usb-gadget
sudo systemctl enable keyd && sudo systemctl start keyd
The device should show up as 1d6b:0104 Linux Foundation Multifunction Composite Gadget
on the host machine. This can be observed on a linus host by checking the output of
lsof or the existence of /dev/input/by-id/Tux_USB_Gadget_Keyboard.