You’ll have to try harder than that (Image: Ryan McVay/Getty)

GOOD news for spies. There is now a way to hide data on a hard drive without using encryption. Instead of using a cipher to scramble text, the method involves manipulating the location of data fragments.

The inventors say their method makes it possible to encode a 20-megabyte message on a 160-gigabyte portable hard drive. It hides data so well that its existence would be “unreasonably complex” to detect, they say.

Encryption should sometimes be avoided, says Hassan Khan at the University of Southern California in Los Angeles, because the gobbledegook it creates is a dead giveaway: it shows someone might have something to hide. That could spell disaster for someone trying to smuggle information out of a repressive country.


So “steganography“, hiding data in plain sight, is coming to the fore. Normally, data intended to be secret is added to the pixels in digital images, or used to change the transmission timing of internet packets. But these techniques are well known and easily detected, says Khan. So, with colleagues at the National University of Science and Technology in Islamabad, Pakistan, he has developed an alternative.

Their technique exploits the way hard drives store file data in numerous small chunks, called clusters. The operating system stores these clusters all over the disc, wherever there is free space between fragments of other files.

Khan and his colleagues have written software that ensures clusters of a file, rather than being positioned at the whim of the disc drive controller chip, as is usually the case, are positioned according to a code. All the person at the other end needs to know is which file’s cluster positions have been encoded.

The code depends on whether sequential clusters in a file are situated adjacent to each other on the hard disc or not. If they are adjacent, this corresponds to a binary 1 in the secret message. If sequential clusters are stored in different places on the disc, this encodes a binary 0 (Computers and Security, DOI: 10.1016/j.cose.2010.10.005). The recipient then uses the same software to tell them the file’s cluster positions, and hence the message. The researchers intend to make their software open source.

“An investigator can’t tell the cluster fragmentation pattern is intentional- it looks like what you’d get after addition and deletion of files over time,” says Khan. Tests show the technique works, as long as none of the files on the hard disc are modified before handover.

“The real strength of this technique is that even a completely full drive can still have secret data added to it – simply by rearranging the clusters,” adds Khan.

Others are impressed with the technique but see limitations.

“This type of steganography could be used by spies, police or informants – but the risk is that it requires direct contact to physically exchange the USB device containing the secret data,” says Wojciech Mazurcyk, a steganographer at Warsaw University of Technology in Poland. “So it lacks the flexibility of internet steganography. Once you embed the secret data on the disk it is not easy to modify it.”

But won’t making the covert hard disk software open source – as the group plans – encourage its use by criminals and terror groups?

“It’s how security vulnerability disclosure works,” says Khan. “We have identified that this is possible. Now security agencies can devise techniques to detect it.” He adds that his team have had no issues with either US or Pakistani security agencies over their development of this secret medium – despite current political tensions between the two nations.

“The use of steganographic techniques like this is likely to increase,” says Fred Piper, director of information security at Royal Holloway, University of London. “Eavesdroppers can learn much from the fact that somebody is encrypting a message.”