76

I was wondering why CD/DVD burners can't just overwrite a CD or DVD after it's been finalized and set to read-only mode. I understand that read-only mode prevents further writes, but can't a burner just ignore that and burn over the original data anyway?

I know you wouldn't really be able to store any new data on the disc because it would most likely corrupt everything, but for data-destruction purposes, what's preventing this?

Is there just no software made for this purpose? Does the firmware on CD/DVD burners boards prevent this? Or has nobody had enough interest because discs are easy to destroy anyway?

Braiam
  • 4,709
  • 3
  • 26
  • 57
Luke
  • 801
  • 1
  • 6
  • 5
  • 23
    I guess the "nobody has enough interest", together with the stance that "a CD is a read-only medium", is the reason it's not implemented. (What if a bug or malware destroyed that backup CD you just put into your drive?) – JimmyB Jul 05 '16 at 09:15
  • 11
    Note: StackExchange sites are **not** *forums*. They are Q&A sites. The difference is *significant*: forums are about creating possibly open ended *discussions*, while Q&A sites are about *specific and answerable* questions. – Bakuriu Jul 05 '16 at 10:54
  • 1
    The OP wrote a cd/dvd, but also tagged it with [burning]. I assume CD-R / DVD-R was meant. – Hennes Jul 05 '16 at 11:11
  • 7
    Why waste electrical power to destroy data from a CD when you can have fun and physically destroy it? You won't be able to use the CD anyway, so, why bother with it's physical integrity? – Ismael Miguel Jul 05 '16 at 18:27
  • Well, observations on the ease of other destructive methods does skip one use case. If an evil person wanted to destroy a disk that there was no physical access to, a software method could be useful – infixed Jul 05 '16 at 19:24
  • 1
    To really get a better answer you'd need to illustrate what you hope to achieve vs. the other available methods (like a cross-cut paper shredder.) – James Snell Jul 06 '16 at 10:19
  • I am 99% sure that it would NOT be a reliable method. It would require burning in exactly same spots as previously written data, and even slightest offset would result in the data being forensically recoverable. But that's just a guess. – Agent_L Jul 06 '16 at 14:51
  • 2
    @Agent_L Actually, forcing it to write all 1, would do it. But, there are some problems. The disc could be of extremelly low quality and the laser could actually burn through the CD if it writes data where data was already written before. – Ismael Miguel Jul 06 '16 at 17:24
  • Another method to "erase" a CD quickly: Start with *encryted* data in the first place, with the key stored elsewhere. Then forget the key. – Hagen von Eitzen Jul 06 '16 at 21:07
  • Alternatively you could put the CD-R in a sun-bed. – Aron Jul 07 '16 at 05:12
  • @Agent_L [That is what the ATIP is for](https://en.wikipedia.org/wiki/Absolute_Time_in_Pregroove) – Aron Jul 07 '16 at 05:13
  • Because burning a CD with fire is a more effective way to destroy a CD? You only bother to use software approach to clean a hard disk because you want to reuse it. – cytsunny Jul 07 '16 at 05:23
  • To avoid damage due to a mistake or malware. Also, 2 seconds in a microwave oven can rather reliably destroy a bunch of discs. Don't snap or burn - it's dangerous. – kaay Jul 07 '16 at 13:56

5 Answers5

71

I understand that read-only mode prevents further writes, but can't a burner just ignore that and burn over the original data anyway?

Yes, if you modify the firmware on the burner then you could do that.

That means either having access to the firmware's source in the first place (read: support from the manufacturer) or some tricky manipulation.

I know you wouldn't really be able to store any new data on the disc because it would most likely corrupt everything,

Correct. Once CD-R or DVD-R (not -RW) is written to, part of the disc is physically changed. There is no way to revert this.

For more details please see this post: What prevents CD-R from being rewritten?

but for data-destruction purposes, what's preventing this?

Nothing but the safety checks in the firmware.

Community
  • 1
Hennes
  • 64,768
  • 7
  • 111
  • 168
  • 24
    For data destruction purposes, the simpler way to burn data out of a finalized CD-R could involve a candle. That's basically what you would be doing with a hacked firmware, just with lasers. – Sunny Molini Jul 06 '16 at 13:46
  • 32
    Even cooler would be to toss the CD into a pool with *sharks with laser mounted on their forehead*. :) – Hennes Jul 06 '16 at 13:51
  • 2
    @SunnyMolini burning cd's (cd's are made of polycarbonate, aluminum and some lacquer) is bad for the environment. Punch holes in them or just break them to make them unreadable and [put them in the plastics recycling bin](http://earth911.com/recycling-guide/how-to-recycle-cds-tapes/). Don't burn them. – agtoever Jul 09 '16 at 07:33
38

This actually exists and is called smart erase. It is Lite-On proprietary and I have never seen anyone use this.

With SmartErase users can erase the data on DVD±R (DL) and CD-R discs that contain confidential data and which should not be retrievable in any way. The feature works by overwriting the original data on the disc with random meaningless characters so that the original data can no longer be read by any drive.

Source

Hastur
  • 18,764
  • 9
  • 52
  • 95
bot47
  • 1,802
  • 3
  • 21
  • 35
  • 8
    It was present in [Nero 7](http://www.tweaktown.com/pressrelease/523/erase_your_discs_with_lite_on_smarterase/index.html). – Hastur Jul 06 '16 at 08:01
  • 1
    This is where I remembered it from. – bot47 Jul 06 '16 at 08:36
  • 14
    I'm surprised a company bothered to develop this. I mean, most people wanting to make a disc unreadable would just smash it up. It's not only quicker, but much more fun. – daiscog Jul 06 '16 at 11:18
  • 2
    Maybe it has been certified in a way your smashing hand hasn't been. Or it's been the marketing department. At least it's a unique selling point. – bot47 Jul 06 '16 at 11:58
  • 8
    Microwaves are fairly effective (though the ambitious could still recover some of the information) – Wayne Werner Jul 06 '16 at 16:57
  • 5
    Snapping CDs can lead to thin splinters of plastic flying everywhere. Don't do this. You really don't want plastic splinters in your eye. – Baldrickk Jul 07 '16 at 07:27
  • 1
    Random bits are not good enough, because there will be a bias towards 1s or toward 0s (only one of the 2 value can be overwritten), basically that means that someone determinated could reverse partially the data (he know that at least all remaining 0s or 1s are correct), Instead I would do multiple random pass and finally a total pass (the data could still be retrievable in some way of course.) – CoffeDeveloper Jul 07 '16 at 08:24
  • 1
    @Baldrickk Wrap it in a towel or multiple layers of newspaper and you'll be fine. The splinters are a feature, they make data retrieval harder. –  Jul 07 '16 at 10:44
  • @Baldrickk If you're worried just wear safety goggles. – Casey Jul 08 '16 at 13:37
  • @WayneWerner Having microwaved CDs several times myself for fun (but sadly not profit), I would be surprised if anyone could recover data from them. Not only does the foil fracture and burn in the mini lightning storm, but even the "undamaged" areas often warp visibly. Guess it depends on the make how quickly and thoroughly the damage is done. Either way, I prefer putting my discs through a heavy paper shredder - don't want the fumes from burning plastic & foil in the same enclosed space where I put my food. And to save time for those who've never done it: https://youtu.be/0JkCIfLE_-M?t=32 – brichins Jul 12 '16 at 16:00
21

If your intent is to destructively erase data, then yes this is possible. But there are easier and faster ways. shredding, burning, melting, solvents. There simply has not been the demand to allow it, and some demand to prevent it. Optical drives are heading the way of the floppy, so like the optical serial mouse, this may never be commercial reality. Lightscribe was cool.

Remember, the code written is not really binary, there are limits to the run lengths of the mark and space ratios. You cannot "touch up" this code.

In the past I used dd myself to duplicate (to the sector) certain unfinalised DVD-R's burned by particular video cameras, when nothing else would work.

mckenzm
  • 899
  • 4
  • 14
  • "there are limits to the run lengths of the mark and space ratios" - Do you have a source for that? – JimmyB Jul 05 '16 at 11:43
  • The limit is the wave length of the laser used for burning/reading. – Maximilian Riegler Jul 05 '16 at 13:13
  • @jimmyB. Not authoritative reference, but it is RLL encoding, ergo its limited by definition :) https://en.wikipedia.org/wiki/Run-length_limited – infixed Jul 05 '16 at 13:45
  • 2
    @rinukkusu I think we're talking about the *maximum* lengths here... – JimmyB Jul 05 '16 at 13:46
  • @JimmyB: A laser will heat up when it's on and cool down when it's off. The amount of cooling for a laser that can be on continuously would be much greater than for a laser that will only be powered on for short bursts at a limited duty cycle. – supercat Jul 05 '16 at 18:01
  • 2
    @supercat The idea of the run length is not about the laser getting hot, it is to do with it being a self-clocking code. You can't just write "zeros" forever, the reader would lose sync. The same is true for Ethernet: "bit-stuffing" is used to prevent too many same-value bits in a row. Manchester coding prevents that, but has other issues (higher frequency, mostly). –  Jul 05 '16 at 18:21
  • 3
    @infixed doesn't meant that you can't violate those limits, it just means that if you do, you no longer have valid data. Which is a perfectly acceptable result if you're trying to destroy the data on the disc :) – hobbs Jul 05 '16 at 18:52
  • @supercat Even if RLL was not made in mind with laser thermal power dissipation, that doesn't mean power disspaption is unimportant. The fact that usual usage uses RLL might have affected a design choice and omitted additional overload protect on the circuit. Firmware instead of hardware protection. Unexpected uses like this might exceed it. – infixed Jul 05 '16 at 19:20
  • @infixed: I think you're agreeing with me; did you mean to ping hobbs? – supercat Jul 05 '16 at 19:23
  • @supercat Sorry, I meant to ping nocomprende who seemed to be saying RLL was just about sync. I was trying to point out it had other important effects – infixed Jul 05 '16 at 19:33
  • 1
    You said there are faster and easier ways than having the CD drive overwrite it, and while _ shredding, burning, melting, and solvents_ may be faster, they all sound much harder (and/or require special equipment) than just putting the disk in the drive and clicking "Erase", even if it takes 15 minutes. Even shredding via scissors is surprisingly hard if you want to cut in small enough pieces to prevent easy data recovery. – Johnny Jul 05 '16 at 20:11
  • @JimmyB, the encoding is necessary because of the limited number of states that can be represented by the "eye" or "eye pattern", which is the analogue signal generated in reading the pits, so when encoding in binary there are limits to the number of contiguous bits of either state. – mckenzm Jul 05 '16 at 21:44
  • 1
    @mckenzm I know. But note that the question is not about writing data to a disc but to *destroy* data already written. There's no need to write 'valid' data, nor readable data and not even synchronization marks to *destroy* the previous data. Hence the discussion is out of place here. – JimmyB Jul 06 '16 at 09:59
  • @JimmyB, agreed. It was a stretch to literally interpret the question in this way. I drifted out of scope with the solvents too. I was thinking of bulk destruction and not just a single disk. If I was in a hurry and had a few to get through.... – mckenzm Jul 06 '16 at 12:15
8

It would probably not be overly difficult to have firmware overwrite a portion of the disk so that it could no longer be read via ordinary means (if one wanted to destroy an entire disk, physical destruction is much more practical than overwriting). It would be difficult, however, to ensure that the data could not be recovered via microscopic examination or other similar techniques. While there might be some limited value in a level of erasure which would require some work to undo, it doesn't really fit well with either of the two main purposes of erasure: (1) make space available for other purposes, or (2) make information genuinely unrecoverable.

I could imagine some potential usefulness for a specialized CD-ROM drive which was designed to be capable of destroying information on a disk in very targeted fashion in cases where it was necessary simultaneously to ensure that some information on a disk was truly destroyed but still have the remainder of the disk remain usable as physical evidence (if a disk is copied and destroyed, it may be hard to prove that the contents of the copy match what was contained on the original). Such destruction, however, would require a burning laser significantly more powerful than would normally be required to store information onto a disk, as well as a means of examining the disk in much more detail than a typical drive would be able to manage.

supercat
  • 1,779
  • 9
  • 8
1

Writable discs contain something called a "pre-groove wobble", as well as physical addressing information called the "ATIP" (CD-R), "ADIP" (DVD+R/DVD+RW/BD-R/BD-RE), or "land pre-pits" (DVD-R/DVD-RW) — collectively, pre-groove information. What this does is let the burner keep track of where it is on the disc, and make sure that it's burning in the correct location and at a precise speed, so that the final result will be readable.

Although the details vary between the different formats, what they have in common is that the pre-groove information is a very small signal, designed so that read-only drives won't even notice them, and only a burner looking for them on a blank disc will be able to see them. Because the pre-groove signal is so weak, burning to the disk completely buries it in the much stronger signal of the recorded data.

So a drive trying to write to a disc that's already been written would be "running blind" without its main source of information for how to behave. But they're not made to do that — all the burner's actions are pretty much synced to the pre-groove info, and if it can't find that, it stops with an error.

hobbs
  • 1,276
  • 8
  • 11