[gruturo]

Upload a file consisting of all zeroes (or any other known content for that matter) and now the contents of the screen is the secret used to XOR the input. Surely you meant something a bit more substantial?

[oxplot]

You can check for a magic string after decryption and only update the screen if valid.

[nybble41]

At this point you might as well just require a plaintext password to update the screen, and forget the XOR.