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Poor man in the middle

· 6 min read
Alejandro Revilla

This is a personal story not related to jPOS, but it's somewhat related to payment networks and security, so I hope you enjoy it.

Back in the 80s here in Uruguay, when I was in my early 20s, credit cards started to become popular and merchants started to use CATs (credit authorization terminals) that used some mysterious protocol to talk to some servers in order to get authorizations.

I didn't have a card, but my partner in crime since age 7 -- my friend @dflc -- got one, I think it was a VISA.

We analyzed the card and of course, we were very interested to figure out what was written in that magnetic stripe, but we didn't have a reader. We probably tried with some tape recorder heads in order to get some audio, I don't remember, but I'm sure we had to try that.

One day, we called a store in the new mall in the city, Montevideo Shopping Center, for personal reasons (probably wanted to buy a present or something like that). Not 100% sure I was the one that placed the call, but I think I was. I'm very anxious, so I never asked our secretary to place the calls for me, if a number was busy I would dial 100 times in a minute until the call completes (and this was rotary dialing). If I recall correctly, the store was Pascualini, still popular these days. After busy, busy, busy, I got to hear some 'click click click click', followed by silence...

When you are into modems, and BBSs, there's not doubt what you do in a situation like that, you whistle! A simple short whistle starting at around 900Hz and going up to 1200~1300Hz is easy to whistle and you get V.21 and Bell 103 modems to start their connection establishing dance.

So I whistled (or my friend), and heard the modem, we knew it wasn't a FAX (no birps), we knew exactly what was that, that new tiny CAT thing, an Omron CAT 90 that we've started to see at some stores.


We knew exactly what was happening, that thing wasn't detecting that the line was free before blindly start dialing. Our eyes opened, we simultaneously smiled, it took us probably a few milliseconds to know what was next: Man in the middle!

We also saw a business opportunity (we were hungry): we knew we could build a little hardware to sense the DC voltage of a free versus busy line and sell it to the local acquirers (free line tone were not standard those rotary-dialing days).

As a first step, we planned for a proof of concept. We wanted to monitor a transaction, record it on tape, and let the real acquirer process the transaction. We were into BBSs and ran our own BBS those days. My friend had his home land line, plus 8 BBS lines in his bedroom (along side with a MicroVAX with two SCSI 500MB mirrored noisy disks spinning day and night), so we could use one line to dial the merchant, and another one to dial the acquirer. We had to do some war dialing and small social engineering to get the acquirer's listed phone numbers, lucky for us, numbers were in the phone book.

We played with phones since we were 8 or 10 years old, I remember I used to short-circuit the phone to break my mother's long calls when I needed it. We did phone patches for the ham radio stations, my friend @dflc used to develop his own telephone answering machine using discrete IC components (4011s and 4001s here and there) and a pair of cassette recorders, so the required hardware was ready in a couple days.

We needed a way to know when to initiate a call to the store exactly when the transaction was going to be initiated. There were no cell phones those days, but of course, we had VHF handhelds, actually a pair of Icom IC-02ATs. I used handhelds since high school, I thought they were the coolest thing to have and I still don't understand why ladies were not impressed by a guy with that kind of technology hanging in his belt, unbelievable...


The distance between my friends' and the mall was small, just 600 meters


The plan was easy: My friend (who owned the card) would go to the mall, buy something, send some signals (without talking, just a few push to talk pushes - for those in the know, that would be an A1 encoding) at the right time when the lady at the store was about to process the transaction. I'd be in our NOC (his bedroom) calling the merchant, the acquirer, hitting 'REC' on the recorder, and patching both lines with our little hardware (also monitoring with headphones).

We did a test VHF connection and although those handheld transceivers could be used to cover 80+ kilometers with good conditions, the mall was a Faraday cage, I didn't hear him. So we needed a plan B. My friend's brother got a mobile VHF in his car, plenty of power (50W). So we called him (via radio) and luckily he was close to the area. We explained the mission, although no questions were asked, he would take us seriously. He parked the car close to the mall (so he could listen the short transmission from inside the mall) and QSP to me. (QSP, Q2 aka QSP version 2, rings a bell?). FTW, QSP is the Q-signal code for "relay message".

So we did the transaction, everything worked on the first try, @dflc bought himself a leather wallet or a belt, can't remember, knowing him, I'm sure he still have it as a trophy and a way to remember that fun hacking day. The transaction was properly approved by the Visa acquirer (who BTW, now runs jPOS), we were just men in the middle.

We've got the transaction and I remember we analyzed it in several ways, replayed it against different modems, etc.

On the business side, we had meetings with the local card acquirers where we explained their vulnerability and offered a solution. Of course, they didn't like the fact that we, young suspicious "hackers/crackers" were telling them what to do so they did nothing.

We kept our grin for a good while, it was a nice, albeit pretty simple, hack.