Netizens with extreme privacy needs got a new tool for their cyber utility belts recently with the release of an application that lets users hide secret messages in virtually any executable computer program, without changing the program's size or affecting its operation.

The tool is called "Hydan," an old English word for the act of hiding something, and it's part of a research project by Columbia University computer science masters student Rakan El-Khalil, who showed off the program to a small group of open-source programmers and hackers gathered at the second annual CodeCon conference in San Francisco on Sunday.

Hydan is a novel development in the field of steganography -- the science of burying secret messages in seemingly innocuous content. Popular stego programs operate on image and music files, where a secret missive can be hidden without altering the content enough to be perceived by human senses. But because they contain instructions for a computer's processor, executable files are less forgiving of tampering. Improperly changing a single bit of executable code can render an application completely unusable.

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. "The problem with program binaries is there is just not a lot of redundancy in them," said El-Khalil.

He found some of that useful redundancy in the instructions that tell the computer to add or subtract.

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. "If we use a scenario in which addition is zero, and subtraction is one, we can just go through and flip them as needed," El-Khalil explained.

El-Khalil concedes that the method is imperfect -- an application that's been impressed with a secret message has considerably more "negative subtractions" than an unadulterated program, making it easy to pick out through a statistical analysis. Hydan could also break programs that are self-modifying or employ other unconventional techniques. And it's less efficient than stego programs for image and sound files: good steganography for a JPEG file can hide one byte of storage in 17 bytes of image, while Hydan's ratio is one byte of storage to 150 bytes of code.

Future versions of Hydan will boost that capacity by finding different places to code data, such as in the order of a program's functions, and the order in which arguments are passed to those functions. For now, the application is still powerful enough to secretly stash the United States Constitution and the Declaration of Independence in a single copy of Microsoft Word.

Beyond the covert uses, the technology could be used to attach a digital signature to an application, or to embed an executable with a virtual watermark.