Rakan El-Khalil, a Columbia University computer science Masters candidate, has recently released an application that lets users hide a secret message in virtually any executable computer program without changing the program's size or affecting its operation. The tool is called Hydan, which means "the act of hiding something."
El-Khalil's research focused on redundancies in the Intel x86 instruction set — places where at least two different instructions are effectively the same. Each choice between two redundant options can represent a single bit of data. A computer instruction to add the number 50 to another value, for example, can be replaced with an instruction to subtract the number -50 instead. Mathematically, the instructions are the same. In choosing between the two, a stego program can get one bit of covert storage out of each addition or subtraction operation in the executable, without changing the way the application runs or adding a single byte to its size.
This technology could also be used to attach a digital signature to an application or to embed an executable with a virtual watermark.