My idea is to take a raspberry pi zero (or any embedded linux board) and shove GPG inside it to encrypt/decrypt/sign data with RSA/ECC (for bitcoin) via USB. Like a USB thing that holds your (encrypted) keys, you unlock them and you're ready to go. When the computer is trying to do anything (sign/encrypt or read/write to your keychain) a message will appear on the small physical screen of the device that will require you to press a physical button on the device to accept it.
The key generation will also take place inside the device. The device will be completely air-gaped and will require user authentication to enter the system (via serial, no wireless communication at all)
Is it a good idea? Why? Is it secure? I know that some1 can dump your memory while the device is running via ultra-leet X-Ray machine but I don't mean THAT type of security. I want to know that I'm safe even if I lose my device. Is this done already? Where?