Targeted Fragmentation: Covert Channels Using Hard Drives

A team of researchers has presented a steganographic technique which can be used to conceal data on a hard drive. The technique is essentially based on targeted fragmentation of clusters when saving a file in the FAT file system. When decoded, the distance between clusters reveals the binary sequence of the hidden data. Two (numerically) sequential clusters, for example, mean that the following bit is equal to the previous one.

If the distance to the next cluster is greater, this means that the next bit is not equal to the previous bit. In this way, a series of clusters making up a saved file yields a defined bit stream. If the reader knows the state of the starting bit, he is able to obtain the correct bit stream.

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