So, for simplicity sake, lets say I keep the secret recipe and instructions for all of the famous soft drinks (aka pop) & certain types of chicken :dubious: on a CD… ( The typical kind you use with a computer for data storage…) Because this cd contains many files for all of the different drinks and chicken, 90% of the disk is used. I accidentally leave out the cd on my desk when an angry employee who was just fired comes to my office all upset… He see’s the cd and proceeds to break it into multiple pieces… So now I have multiple pieces of a cd, some really little, and some like a 1/4 size of the disk… Do I have any chance of getting my data back??? Can I get data of of the small pieces of the disk… (1/4 inch or smaller pieces of cd), the 1/4 piece of the whole disk??? Do I need to send it to the NSA for data retrieval (I know, they wouldn’t even talk to me, but for sake of argument!) or can I “easily” get the data of the 1/4inch pieces by sending it to a data retrieval company??? Help, my company is almost in ruins!!! and no, I didn’t back up my data :smack:…lol…
The data is spread out along a spiral track. So, it’s unlikely that breaking a CD into pieces will ever result in a piece big enough to allow you to recover the data.
Right. It would somehow have to be broken in concentric circles for there to be a realistic chance of any meaningful recovery.
This is the problem.
Assuming you collect almost all the pieces, piece them together carefully, and the NSA takes it to their top secret labs and reads what they can - odds are you’ll end up in a situation with a supposed file with huge chunks missing - analogous to the following:
Take all the pages of a printed book, cut them out and lay them down flat on the floor. Now randomly spray streaked of black paint across the pages. You have some whole pages blacked out, some pages where lines of every 10th word is missing, a lot of fragmentary pieces of data, etc. Now scramble a few pages because you can’t read the page numbers… In a lot of cases, the quantity or name of one ingredient in that recipe is missing to the whole file is useless.
IIRC the data is stored in blocks, on a spiral track. I think each block is labelled sequentially, so that helps. You can make educated guesses - at the point on the disc, there are X blocks per track. The write layer is a thin metallic coating and the act of breaking will have lost chunks of this, and certainly places along the break will be missing a few bits. The recording is pits (pressed CD) or dark/light areas (burned CD) and along the edges of the snaps you may los a lot of bits. If the angry employee stomped or scratched it, even more data loss. AFAIK the data is not recorded encrypted, so it’s easy to decipher one byte at a time. there are even checksums to help with hints.
The amount of effort required to decode the puzzle, and come up with a solution that resents only fragmentary data is enormous. This better be Bernie Madoff’s or Osama bin Laden’s secret recipes to make it worthwhile. If you are looking for a perfect copy you are outta luck. If you think that 5 out of 7 of the Coke secret recipe, or 9 out of 11 of the Colonel’s Herbs and Spices is better than nothing, then maybe the effort is worth it.
Well, CD data is 50% redundant, so it’s not inconceivable that the data could be recovered.
(FWIW - it’s Reed-Solomon encoded).
I wasn’t sure if this was just for music encoding (which needed to correct “missed bursts” or if the same multi-way block redundancy applies to data CD’s. note the encoding is sequential, but if you lose a large geometric chunk of the CD you will have incomplete data. If you only have a lot of “fault lines”, this correction may work.
The Red Book spec for Mode 1 (standard data) discs specify “288 bytes per sector for error detection and correction, leaving 2,048 bytes per sector available for data”. Much smaller fraction than 50%.
The error correction system is designed to overcome radial scratches. A few dozen bytes here and there in a track can be overcome. Linear scratches mean doom for those sectors.
The OP’s scenario requires unbelievable effort for recovery, but as a general rule figure a percent of area damaged considering each break to be a mm wide. If 90% of the disc in total is intact, then 90% of raw bytes could be recovered, very theoretically.
Unless the fragments are to be read individually by some sort of high resolution scanning laser, the disc would also have to be reassembled - which itself would require incredible precision, not to mention that the process of breaking it up will have distorted the plastic.
If the broken CD contained information of enormous value to the government of superpower nation, then huge effort and expense would be applied to recovering the data - and it might be partially successful.
For a CD containing details of drinks and chicken. Not a chance.
I couldn’t find how long of a string of valid data is needed to reconstruct the stored data with a 4000 bit missing section, but assuming the data needed to decode doesn’t completely circle the CD, you could cut a 2.5 mm segment out of a CD and still recover all the data.
Most of the data is gone. That said, if you’re specifically storing small files–like a text file containing a recipe–then there is a chance that it could be recovered. A CD track stores around 1000 bits/mm, so in a 6 mm chunk (1/4") you could fit about 750 bytes (enough for a few sentences), and in a 100 mm chunk (1/4 of a disc) you could store 12.5 KB (enough for a short document or very small picture). You would have to be lucky that you don’t cross an edge, but that probability goes up the smaller the file.
And yes, you would need “the NSA” (i.e., someone with specialized, sophisticated equipment) to get at the data. Actually, it could be done with a few hundred bucks worth of equipment in a garage, but to my knowledge no one has tried it. The software effort would be fairly involved.
In theory, you could glue a CD back together again as long as you aren’t missing any of the bits, the error correction would fix the glitches in the cracks. But you’d need to make sure the pieces were put back together with sub-micrometer accuracy, or the tracks wouldn’t properly align. So for all practical purposes, not possible.
So for any businesses not already in ruins from data storage policies like this, take note!
It would also theoretically be possible to design a filesystem or other encoding standard that would allow data to be recovered from broken CD fragments. But I’m not sure if one exists. Actually reading the data would be difficult without special equipment.
Any such filesystem would have abysmally slow access times.
Yesterday,
All those backups seemed a waste of pay.
Now my database has gone away,
Oh, I believe in yesterday.
Suddenly,
There’s not half the files there used to be,
There’s a milestone hanging over me,
The system crashed so suddenly.
I pushed
Something wrong
What it was, I could not say.
Now all
My data’s gone
And I long for yesterday-ay-ay-ay.
Yesterday,
The need for backups seemed so far away.
I knew my data was all here to stay,
Now I believe in yesterday.
Only on write. Not necessarily on read. CD/DVDs/etc. tend to be written in one large chunk anyway, so slow write access may not be an issue.
You would lose a good fraction of the total space for the error correction bits, but them’s the breaks.
Data CDs need a higher accuracy (or reconstruction rate) than music. If a block of music cannot be decoded, a previous block or two can be substituted without the human ear being able to detect it. Not so for a computer program.
For music, each block is corrected using Reed-Solomon Cross-Interleaved code first, then if that fails, another block can be substituted. Computer data doesn’t have that luxury.
No, you would need to spread the data all over the disc (so that any given block would not be killed by being cut by a crack), and that would entail a lot of head movement. So, both read an write times would be slowed.
Just want to re-emphasize that data and music CDs don’t have the same depth of encoding backup. My earlier post applied to data CDs, which the OP was talking about. (Unless the OP was doing steganography by hiding data in music.;))
Something we all seem to be glossing over: the mechanical strength of the reassembled CD.
Assuming you could find substantially all the pieces AND glue the pieces back together securely AND assuming you could make the resulting jumble flat enough, there’s still no way to spin it at any kind of serious RPM. Hell, apparently undamaged data CD’s routinely shatter from centrifugal force in modern 30x or whatever readers.
So we’d need to build a custom reader device which either held the platter stationary & scanned the surface with a moving laser, or one which spun the CD at silly-slow speeds like just 10s of RPM.
Impossible? No. Darn expensive? You bet.
And for the folks who were whooshed by the OP’s choice of example of the CD’s content. … I think he was talking about e.g. the only copy of Coca Cola’s secret formula or KFC’s secret recipe. In other words, corporate secrets of billion dollar value.