Young Iruka (14 or 15 idk) being actually assigned to do some fuunijutsu work for T&I and working with Ibiki. Over time Iruka is still a little scared of him, but not as much as before, Ibiki doesn't talk much, but it's no problem for Iruka.
One day after finishing decoding scrols Ibiki can see Iruka wants to say something so he asks him. Turns out Iruka saw shinobies doing special hand greeting (idk what it's called) and they told him it's a way to identify if the other is actually that person or someone doing a henge and Iruka wants them to have something like that to, because "fuunijutsu is very dangerous and has to stay a secret". Ibiki looks at him for what feels like forever and eventully agrees.
Years later when Kakashi's Anbu team is finishing an impotent report, Iruka walks in with some documents and dubs Ibiki up with a weird 10 seconds long handshake/greeting.
I also see it in modern au when Ibiki, Tenzou, Kakashi, Genma etc. are out for an unknown reason. Kakashi wants to talk to Iruka who is sitting at the bar and Ibiki is like "I'm going to talk to him for you", Kakashi tries to stop him, because it's Ibiki talking to someone. To their surprise Ibiki sits down next to the brunett, dubs him up and starts chatting.
