Unidirectional Data Transmission to a Smartphone
I'm going to use an old Android phone to store sensitive data (e.g., Bitcoin wallet private key), with no SIM card and WiFi and Bluetooth turned off. I could disable wireless connections physically if I get paranoid enough. This way, even if the system is compromised, it won't leak any data to the outside World.
The question is, if I want to install or update software on the phone, how can I send the apk files? The obvious way is through SD card, but what if some malware in the phone writes the key to the card? I'd have to dispose of the SD card every time, which can be costly and inconvenient.
I guess there may be microSD cards with a write protection lock switch, but I've never seen those before.
Ideally, I'd like to send something like audio signals or series of QR codes which the receiver will decode as a binary file.