Schneier on Security

Clive Robinson • December 8, 2011 8:54 AM

It looks like it was a fun competition but…

Shreading is known to be no good against even a technicaly unsophisticated enemy.

If you are old enough you may remember the “Iranian Students” who reconstructed thousands of CIA and US Diplomatic cables, files and other documents.

For those that are unfamiliar with shreaders they come in a variety of forms,

1, Strip.
2, Square cross cut.
3, Diamond cross cut.
4, Spiral cross cut.
5, Random feed Pin Punch.
6, Ball mill.
7, Liquidiser.

The first is pointless for anything other than making bedding for your pet rabbits. Because the width of the strip is generaly two or more times the character width, thus even the human eye can put the documents back together fairly quickly, especialy as the shreads of one page invariably end up next to each other in the shreded bag.

Likewise most commercial (square) cross cut shreaders, the advantage these have over the strip cutters is you can generaly get considerably more documents in a single waste sack. But again all that has happened is the same wide strips of the strip cutter get choped into length of an inch or less which still alows fairly easy orientation by human eye and thus reconstruction.

Some low grade square cross cut shreaders produce strips only 2mm wide and 5mm long but they jam easily and are just a pain to use. They also have a habit of putting visable tool marks on the “chads” such that not only can they be orientated with out refrence to what is printed on the page usually the tool marks can identify where across the page the chad came from…

Rarer and generaly better made are diamond and spiral cross cutters, the size of the chads can be very small and importantly because the way they work tends to leave uniform tool marks. It would not be impossible to put a document back together but at 1mm by 1mm there is one heck of a lot of them in a single A4 sheet, and they get easily miked up in the waste hopper and bag.

Randomised feed Pin Punch systems, I’ve only ever seen two of these and they don’t half suck up the electricity. Put simply they cut the paper into strips that then get feed randomly into a pin punch system with ten or more pin punches that reduces the strips into 1mm diameter or smaller disks that then get fed along different length channels before being re-mixed in the waste hopper and sack. Good luck with putting that back together.

However the prefered method of shreading for security apprears to be either the ball mill or liquidizer depending on the quantity to be dealt with.

Basicaly the ball mill is the same machine that is used to make “paper fiber” in a paper making factory, it breaks the paper down into a slury of celulose fibers that can then be pressed out to form sheets of low grade cardboard, toilet paper etc.

A liquidizer looks just like the juicer you have at home, your confidential information (KeyMat tapes and cards) because of their small size get poped in the top with a suitable liquid (light vegtable oil or other flamable liquid) the vortex effect ensures that the paper gets reduced to less than fiber and very very well mixed. The resulting “fuel” gets burnt or can be pressed into “barbecue brick” sized lumps

For those that have “optical media” to be got rid of a belt sander will if applied to the right side will grind of the foil and track lands and pits. If you are in a real hurry a microwave oven will do a fair job of making them irrecoverable in just a few seconds. However don’t do this in a poorly ventilated area the fumes coming off have what are politly called “Toxilogical Disadvantages” or in laymans terms “they are going to kill you today or in the near future with some horrible disease such as emphasima or worse”.