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Comment by taliesinb | Hacker News Reader

I implemented a physical port-knocking daemon, once. I lived in a block of flats — you needed an expensive fob to open the outer gate, and it didn’t even work reliably.

I lost my fob. So I took my intercom apart and wired a band pass filter circuit between the buzzer input and actuator output. If you pressed my buzzer with the right steady pattern, it would automatically let you in.

It worked great and I didn’t buy a replacement fob.

A few months later I started getting woken up by someone buzzing my buzzer. They would always scurry in before I could open my door to see who it was.

Turns out my upstairs neighbor saw my shenanigans a few times and started freeloading off my invention. Honestly amazed he/she reverse engineered what was going on!

Needless to say, I bought a new fob.

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I port knocked called a friend once while traveling overseas when a second friend decided to act like an idiot and got us both some attention by the local authorities.

First friend-a local-is the type who doesn’t answer his phone from numbers he doesn’t know/isn’t saved as a contact-which I knew, so I dialed him, then dialed his wife (who also doesn’t answer unknown numbers), then dialed him again. He picked up the second time and after some explaining he chuckled and goes “yeah I figured it must be important when the same number called $wife immediately after calling me”.

He was there to pick us (including drunk mutual friend) up within the hour. And yes, he’s a network engineer heh.

Back in the day of payphones and expensive long-distance calls, hitchhikers had a protocol for calling their parents/friends for free: they ring them up but don't say anything. The callee reads a list of expected answers (like the towns where the caller would stay), and the caller hangs up on the right answer, then dials again.

Not sure how payment worked and why you could listen for free—I'm only familiar with the method from written accounts.

Sounds like 1800 reverse where you could send a short 2-3 second message that’s supposed to be your name.

The other person would get an option to receive a call from <whoever> for $5, but usually <whoever> is the name or code of a location to be picked up.

I must have done this 100 times as a kid. Once an angry man clued onto it, switched my line across and told me off. This is back in the days where mobile calls were expensive and very few kids had phones.

I think that's just good old Mr. Bob Itsaboy: https://www.youtube.com/watch?v=9JxhTnWrKYs

I have one of those newer intercoms, where you first select who to call, and then it calls the correct flat. So your system wouldn't work.

But you just gave me an idea for something I'll probably never implement: an audio key. The intercom auto-responds and if the audio matches the key (hello 90s modem sounds), then it lets you in. You could add more automation on top and provide temporary keys for guests, etc. Just play the audio you received on whatsapp, it will open the door. Please someone implement this and get rich doing it so that I can just buy it. It needs to take into account when you're at home and you still want to answer the mailman.

I'd have gone the overly complicated route - throw a relay in there and tie it to something that detects whether or not your phone is connected to your WiFi network. Automatically disable it when you're already home!

Until you forget your phone at home and proceed to lock yourself out of your apartment with no way to call for help!

Ha, that sounds exactly like something I'd do.

This reminded me of a project Paweł Zadrożniak made on the same idea. https://www.youtube.com/watch?v=e9GWteVUYD0 http://silent.org.pl/home/2015/10/04/entry-phone-hack/

I enjoyed your story, but can you please elaborate on what your intercom is / was for?

I understand the key fob/card concept for access purposes, but have never heard of an “intercom” in the context of home/apartment/etc security.

An intercom that lets someone at the gate talk to the apartment in question to try to convince them to buzz them in?

Yes. They press an apartment number which dials your phone. "Hey your pizza is here", then you press a button on your phone which opens the outer gate or the front door to the building. You then meet them at your front door.

It's a system like a local telephone system, where you dial up to a certain apartment and then they press a button that opens the door for you.

It's like a phone for the yard door. Someone buzzes you, you talk to them and let them in or not.

The intercom would be for a guest to use to request access. See Seinfeld episodes for an example.

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