Is it possible to program Android to act as physical USB keyboard?

What I really want to know is whether it is a hardware problem, or a software problem. Could I plug my android phone into a computer via USB and have it act as a hardware keyboard. I do not want to install anything on the computer, I want android to behave like the standard hardware.

Edit: Clarification I want to write a program/library for android that enables the device to fully emulate an ordinary keyboard, so that the operating system reports it as a standard keyboard device, and it would work in the BIOS or anywhere else that a keyboard works.

Solution 1:

I've modified kernel on Nexus 7 to act like standard HID keyboard and mouse, without losing MTP/ADB/other USB functionality.

You can use usb-gadget-test commandline utility to send keystrokes and mouse movements to your PC. I want to create a remote admin app later, which will send key events and receive video from camera.

Kernel patch, binaries and instructions: https://github.com/pelya/android-keyboard-gadget

Edit: I've published a proper app to Google Play, if your Nexus 7 is rooted you can flash kernel right from the app, and send keypresses with it.

Solution 2:

Most USB keyboards need drivers to run. Any keyboard functionality (non-standard buttons) beyond the capabilities of the standard HID drivers will need to install some software on the computer.

That being said, It may be possible to use Android's USB capabilities, as well as writing a custom driver if default HID is not sufficient, to achieve your goal. It is likely a very non-trivial undertaking.

Edit: I think KristopherMicinski is right that the level of control you get with the stock Android USB API is inadequate for this purpose. His two solutions of modifying the firmware to communicate using HID standards, as well as a hardware middleman that translates from the Android Accessory protocol to HID both seem valid to me. If installing drivers on the computer is out of the question, these may be the only two options.

However, if you're open to installing a driver for this behavior, It should be possible to write a custom driver that can handle Android USB protocol, and correctly translate to the correct calls/interrupts for keyboard functionality. If memory serves, every peripheral keyboard I've used in the last 10 years has needed to install a driver for full functionality, so this may not be considered non-standard behavior. (The though just occurs that this approach will only allow the device to function as a keyboard inside windows, not during the boot process)