Magnetic Stripes & Stuff

Apparently you can visually read magnetic stripes, if you can somehow see the magnetization patterns encoded on the stripe. I recently found out that this is indeed possible. The density is not that high and following methods will allow us to visualize the encoding.

You could use a “magnetic developer” solution and learn how each bit is encoded:


Or you could use iron (oxide) filings:


In this video linked from the Hackaday article, they also tried it on various magnetic media like cassette tape (yes who remembers those?) and floppy disks:

Also related, here’s a teardown of the Coin Card, an electronic card that can emulate a set of pre-programmed magstripe cards on-the-fly. There are more teardown photos where this came from:


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PlaidCTF 2015 “RE GEX” Write-up

This challenge came with a simple Python TCP server and a monstrous regex in a separate file. It consists of multiple sub-patterns (2563 to be exact), separated by OR operators1. To get the flag, the input supplied must NOT match any of these sub-patterns. If you break up the large regex expression, you see something like this:


The 3 conditions listed up front give you a hint to the requirements: must be made up of the letters plaidctf and be of length 171. Any patterns that DO match will NOT reveal the flag.

I tried the dumb way, which is to brute force the sequence. Python’s itertools.permutations killed the process, so I thought to represent the string using a large number instead, composed of 3 bits (to represent one of plaidctf) for each “character”. Python supports arbitrary sized integers, so manipulating and counting integers up to 171 * 3 bits wasn’t an issue. Well, of course that process took forever…

Z3 to the rescue!

The sane (and probably only) way to solve it was to use a theorem prover like Z3, so I decided to invest some time to play around with it, even though it was too late to submit the flag.

The regex patterns were probably emitted by some program itself, so it followed some regular syntax, which meant that I didn’t have to resort to using a regex parser. There were only 3 types of expressions used: a single character (.), n-characters (.{n}), and character classes ([xyz]). The patterns were parsed into Z3 constraints, in which we only had to restrict the character classes imposed by the each pattern at particular positions. A regex pattern like .{88}[padt].{60}[licf].{6}[plai].{14} is turned into a constraint expression like this:

    Or(And(pattern >> 246 & 7 != 0,
           pattern >> 246 & 7 != 2,
           pattern >> 246 & 7 != 4,
           pattern >> 246 & 7 != 6),
       And(pattern >> 63 & 7 != 1,
           pattern >> 63 & 7 != 3,
           pattern >> 63 & 7 != 5,
           pattern >> 63 & 7 != 7),
       And(pattern >> 42 & 7 != 0,
           pattern >> 42 & 7 != 1,
           pattern >> 42 & 7 != 2,
           pattern >> 42 & 7 != 3))

Solving this contraint yields the correct solution for this particular regex pattern. By combining all constraints with an And operator, the model will provide the correct solution for this challenge. You can see that I retained the representation of the pattern as a BitVector of length 171 * 3.

You can find my code here, which expects the regex patterns to be pre-processed and provided as input. The model took 11min 50sec to solve on my machine. With that, the server returns flag{np_hard_reg3x_ftw!!!1_ftdtatililactldtadf}.

Seeing as how Z3 is getting more popular in CTFs, I hope to use this new found ability for future challenges.

If you’d like to see what else Z3 can do, check out some of the inspiring entries on Diary of a reverse-engineer.

  1. Well it’s not really called that, but you get the point 

Exploring HDMI CEC

Consumer Electronics Control (CEC) allows control of AV devices that are connected via HDMI. This is the feature of HDMI that enables your TV to automatically turn on and switch to the correct input when you switch on your set-top box, for example. It also allows you to control your set-top box using the TV remote (in some cases).

Electrically, the CEC bus is a single-wire bus that is shared between all HDMI devices, thus any CEC message can be received by all connected devices. Each device then claims one or more logical addresses on which it will receive direct CEC commands.

LG remote

One interesting feature in the HDMI CEC specifications is Remote Control Pass Through, which allows button presses on the remote control to be passed through to HDMI-connected devices. I thought this feature could be used to unify the various remotes in my living room.

However, not all CEC devices are created equal. As usual, some manufacturers will deviate from the specifications, and/or introduce some quirks in their implementation (as you will see later). They also love to brand CEC with their own funky name, such as SimpLink or Anynet+.

Raspberry Pi as a CEC Bridge

As a quick and dirty way to check out the capabilities of my TV, I used a Raspberry Pi which has a HDMI connection that can be software-controlled. This also meant that I didn’t have to build my own CEC transceiver circuit.

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After some deliberation, I have decided to get a domain name as part of my online identity. Henceforth, this blog shall be known as…

The existing WordPress links will redirect to the new domain, and everything else should be functioning as normal (please let me know if it’s not). Please update your bookmarks and your RSS feed URLs to point to the new domain.

Although I may not have a lot of time, I will try my best to continue working on what I love and writing about it. There are already a couple of projects sitting here on my table, so stay tuned! 🙂