Unshredding
In some cases it is technically possible to reassemble the pieces of shredded documents; the feasibility of such a project is a cost-to-benefit calculation. If the chad is not further randomized, the strips that belonged to the same document tend to come out of the shredder close to each other and remain roughly in that configuration. Furthermore, when the documents are fed to the shredder in a way that the lines of text are not perpendicular to the shredder blades, portions of text may remain legible on the strips.
If the shredder doesn't cut paper small enough, confidential documents could be removed from the trash, reassembled and read. This can lead to corporate espionage as dumpster diving is the easiest way for professional thieves to steal sensitive information from businesses. For this reason, several new shredders provide protection from information theft by cutting paper into pieces significantly smaller than the length of a staple.
These micro cut shredders make it difficult for criminals to assemble a document by cutting a piece of paper into about 3,770 bits vs. the average confetti cut shredder, which cuts a piece of paper into 300 pieces or the average strip cut shredder, which cuts a piece of paper into 34 strips.
After the Iranian Revolution and the takeover of the U.S. embassy in Tehran in 1979, Iranians enlisted local carpet weavers who reconstructed the pieces by hand. The recovered documents would be later released by the Iranian regime in a series of books called "Documents from the US espionage Den". The US government subsequently improved its shredding techniques by adding pulverizing, pulping, and chemical decomposition protocols.
Modern computer technology considerably speeds up the process of reassembling shredded documents. The strips are scanned on both sides, and then a computer determines how the strips should be put together. Robert Johnson of the National Association for Information Destruction; has stated that there is a huge demand for document reconstruction. Several companies offer commercial document reconstruction services. For maximum security, documents should be shredded so that the words of the document go through the shredder horizontally (i.e. perpendicular to the blades). Many of the documents in the Enron Accounting scandals were fed through the shredder the wrong way, making them easier to reassemble. There is currently an effort underway to recover the shredded archives of the Stasi, the East German secret police. There are "millions of shreds of paper that panicked Stasi officials threw into garbage bags during the regime's final days in the fall of 1989". As it took three dozen people six years to reconstruct 300 of the 16,000 bags, the Fraunhofer-IPK institute has developed the "Stasi-Schnipselmaschine" (Stasi snippet machine) for computerized reconstruction and is testing it in a pilot project.
DARPA’s Shredder Challenge calls upon computer scientists, puzzle enthusiasts and anyone else who likes solving complex problems to compete for up to $50,000 by piecing together a series of shredded documents. The Shredder Challenge consists of five separate puzzles in which the number of documents, the document subject matter and the method of shredding will be varied to present challenges of increasing difficulty. To complete each problem, participants must provide the answer to a puzzle embedded in the content of the reconstructed document. The overall prizewinner and prize awarded will depend on the number and difficulty of the problems solved. DARPA declared a winner on December 2, 2011 (the winning entry was submitted 33 days after the challenge began) - the winner was "All Your Shreds Are Belong To U.S." using a combination system that used automated sorting to pick the best fragment combinations to be reviewed by humans.
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