Could you store data by magnetizing a DVD or CD-ROM?

I’ve had a concept for a form of ‘physical steganography’ whereby one hides data (encrypted or not) by magnetizing an aluminum optical disk such as a DVD or CD-ROM, in the manner of a hard drive platter. Ideally, this should be reasonably robust against the kinds of use an optical disk is likely to see; that is, no hugely strong magnetic fields but plenty of spinning and possible storage near home stereo systems and in hot cars and so on. I’m willing to compromise on bit density but secrecy is part of the point, after all.

Also, would it be possible to do this with a CD-R or DVD-R?

Aluminum is paramagnetic. I don’t know much about paramagnetism, but it is harder to measure and detect than ferromagnetism. Why not just use encryption?

Because this is just a thought experiment, really. I don’t ever plan on actually doing it. I’m just curious about whether it could be made to work.

Not sure about the magnetism, for the same reasons as Tripolar, but it might be worth mentioning that DVDs and CDs have error-correction redundancy built in. I think this means it would be possible to encode information in the form of deliberate errors - that would be undetected (or rather, corrected out of detection) by the normal read processes, but easily retrievable if you know they’re in there.

Right, and if you store a bunch of your vacation photos as JPEGs there’s quite a bit of storage available to you in the form of things like compression artefacts. I’m aware of multiple different ways to hide data, all of them a lot more practical than what I proposed. I just want to know whether what I proposed would work.

Ok, let’s go with no for the time being, for standard DVDs and CDs. Does your thought experiment include disks made with some iron included in the plastic? That might work. The optical technology is just looking at the disk surface. Another approach would be the optical aluminum on one side, and an iron based coating on the other side.


Only if normal optical disks come that way.

As I understand it, no: It’s looking (shining a laser on) the aluminum layer beneath the plastic surface, where all the pits are etched.

If you have to coat the disks with iron, that’s going to stand out and be fairly suspicious. Which would be directly contrary to the whole purpose.

The iron coating could be disguised in a picture on the disk. Not likely that anyone would be looking for magnetic encoding on a DVD anyway.

There was such a thing once. They were called floppy disks. But an asteroid hit the earth and destroyed them all millions of years ago.

Maybe a different way of thinking about it – could you disguise a floppy disk to look like a CD? They came in a bunch of sizes – more than the two that were popular consumer products for so long.

How about a “hollowed out” CD that can be opened, and a floppy disk smuggled inside?

I’m trying to think of ways where you wouldn’t have to design and build special drives to read/write.

Oops. forgot – the suggestion in the OP will certainly need custom designed drives. If you’re going to do that, you may as well also have custom designed disks that merely look like CDs.

Or you want the disks to be both? So if inspected/tested, a something will play if it gets put into an optical drive.

In this case, I’d just go further and have dual-layer CDs, or triple-layer DVDs or something.

If not, I’d be more inclined to get LightScribe disks and encode information on the label side somehow.

You could use magnetic ink on the printable side. Your data density is going to be pretty low, though, as I suspect the magnetic domain size for magnetic ink will be larger than that of a hard disk, and your picture will not be all black. Also, the domains may not be stable.


Oh, I was unaware of that. Because I was born five years ago. In a shipping crate. Outside an Apple Store. In the middle of that one part of Kansas where they will stick you in a wooden effigy of Steve Jobs and burn you to death if you dare mention anything the iMac doesn’t support, in hopes of bringing back the Newton.

I think you’re going through more mental gymnastics than would be required to try and figure out the magnetic properties of aluminum and determine if my idea is Howard Hughes-nuts or Gary Busey-nuts. (Remember, the Spruce Goose flew.)

I’m getting the idea that this is simply impossible anyway, but I will respond to this: They do need to be able to be used as normal optical disks, which means that if the government agent sticks one in my CD player he’ll be comforted by the dulcet tones of that sweet siren Miley “Don’t You Dare Judge Me” Cyrus.

si_blakely’s idea about magnetic ink is intriguing and probably the best way to rescue my plan from the junk heap. Nanoda mentioned something else interesting, but since I’ve never heard of this LightScribe witchery I’ll have to research it tomorrow after I’ve [del]ensured I’m no longer a fit sacrifice to the minor deity in charge of PDAs[/del] had a good, healthy eight hours of sleep far away from Kansas.

<slow clap> :smiley:

Wouldn’t particularly need to disguise it, if the top surface of the CD/DVD is covered with a uniform coating of it. It would just look like a blank disc. any picture ink laid down on top of that would make it more difficult to get a read/write head close to the magnetic medium, though this might only reduce the information density (rather than making the whole thing unfeasible).

The question remains open about magnetizing the aluminum. Can a paramagnetic substance work like magnetic substances to record data?

Paramagnetic substances, by definition, do not retain any residual magnetism after the externally applied magnetic field is removed.

So, no.

Right. Thank you. That answers the original question, then.