Sunday, March 25, 2007

Damaged CD media

Last year I signed up for the DC3 forensics challenge while at the Blackhat conference. I never really got around to participating in the challenge due to time constraints though. I still have the packet sitting in my office and one day decided to play with the CD challenge. In this challenge I was presented with a CD that has some data on it, and it's been cut in to two unevenly sized pieces. The task is to retrieve the data. The data rings on the cd were small, roughly 50MB by the looks of it. Of course at this point, the 2006 challenge is over with and AccessData won the challenge but I never saw the reports posted anywhere. So, on to some CD fun!

I found the CD to be one of the more interesting challenges so I did some experimenting. In the name of experimentation some of the things I tried were:

Take a CD, write 50MB of data to it, cut it with an exacto knife, then trying to piece it back together using electrical tape. This actually worked...once. However it never worked in subsequent trials so this method was abandoned.

Take a CD, write 50MB of data to it, cut it with an exacto knife, then try to glue it to another CD. This failed miserably. The glue seeped in to the wrong places and the disc wouldn't budge in the reader.

Finally, I took a CD, wrote 50MB to it, and cut it with an exacto knife, then cut it again, and again and again and again, until I got something that resembled a credit card cd. This worked, over and over and over again. It worked in all 5 trials and was labeled the winner as you can see below what one disk looks like on the front and back.



As you might notice this particular test copy was broken - it's been in my bag for 6 months.

So, what's the trick here? Remember that CD's must maintain balance to spin properly inside of your reader. By cutting the disk to a smaller size, some extra weight was removed and the CD is no longer lopsided, so it balances easier and it now fits within the inner ring of the reader drive much like the credit card cd's do.

One other thing to remember, while experimenting with this some fun with a paper cutter was had. There's nothing like applying obscene amounts of cutting force to optical media!


The 2007 DC3 challenge has been posted on the DC3 site. Registration isn't open yet, but the topics look really interesting this year. Pay attention to the first three and you might notice that I've been talking a little about each one ;)
* BitLocker Breaker
* TPM Trickery
* Damaged Media Recovery
* Real vs. Fake Image Analysis
* SAM Attack
* Sizzling Thumb Drive
* Steganography Tracks
* Password Cracking
* Hack That
* and more...

3 comments:

Iain said...

Interesting. When I read the challenge, I assumed that the CD had been broken in half (across the central hole) so I wondered "how on earth will the tracks be lined up?". It took just a bit of lateral thinking on your part to come up with a workable solution. I suppose the only worry might be about actually trimming the "live" (one and only) copy of the CD to be examined.

I'm surprised that the results of the challenge haven't been published. Surely such information ("how I/we tackled the problem") could be of great benefit to the community.

hogfly said...

Iain,
I'm actually really annoyed that the results haven't been published. It goes to show that working with the government is still largely a one way street even though Jim Christy says the 'gov needs better tools..it'd be nice if they'd open a few lists or maybe invite people to work with them to understand their needs a little better. Instead they keep public contests like this private. At the very least they should have sent the methods used/developed to other contestants.

This year's contest is supposed to have many many new damaged media challenges.

Iain said...

Yes, I see there are damaged media problems. As you're interested in the challenge this year, couldn't you pose the question to the organisers regarding feedback of other folks' methodology to contestants? Yes, the methods will leak out (hopefully!) but, as I said, surely that's to the benefit of everyone working in the field. That applies to the professional as well as keen amateurs, like me.