Are smartphones equipped to read the screen display for ChipTAN or do you need special hardware?
I assume that you could similarly transmit more information than a single QR code by generating a series of QR codes which could then be recorded by video instead of camera, although I don’t know if there are any existing libraries to decode this.
In any case, apart from the possibility of one-time pads, a higher
capacity data channel such as the suggested screen display for ChipTAN would allow Delta Chat to overcome the obstacle to including the encryption key in the QR code and allow new contacts to send encrypted messages immediately (although this wouldn’t be possible for invite links).