USB HID Client

This Android app allows you your phone to present itself to a connected device as a keyboard and mouse. Since this happens at a very low level, the connected device sees it exactly as a normal keyboard and mouse, meaning you don't need to install any software on the connected computer! For this reason, it will even work if the connected device is in its BIOS/UEFI. Even multimedia keys are supported, meaning you can send volume up/down keys to the connected computer to control media. I strongly recommend using Unexpected Keyboard to use special keys such as modifier and function keys, but most functionality will work with any standard Android keyboard.

Bonus description for nerds: This app adds (and provides a simple UI for you to communicate with) two character devices (/dev/hidg0 and /dev/hidg1 respectively) by adding HID functions to the default Android USB gadget. If you wanted to, you could just use this app to create the character devices, then communicate solely through some C program, shell script, or whatever else you wanted as long as it had the right permissions to access the character devices and knew how to send proper reports to them. Beware of SELinux policies if you do that though. If anyone is actually doing that, let me know, I'd be glad to help if necessary.

For more details on setup and usage, please consult the project's Readme.