GIS & Steganography - Part 1: Hidden Secrets in the Digital Ether

By Bill Thoen

Steganography resurfaced into the public's awareness a few months ago when USA Today published a sensationalist story (1) claiming that terrorists were using the Internet to distribute secret messages hidden in pornographic images.While no actual proof has ever been uncovered that members of Al Qaeda really did use this method, what raises hairs among the intelligence community is that they could have, and that it could be very hard to intercept and decode those messages.But now the jinni of steganography is out of its bottle and all over the Internet.It could even be lurking in GIS data.

So, what is steganography? It is not a prehistoric armor-plated lizard with a tail like a 500-lb morningstar.The word comes from the Greek roots stego-, or steganos, and -graphy, and means "covered writing." It is the technique of hiding secret messages within other media, so that to all eyes but those of the sender and intended receiver, no secret message appears to be present.It is different from cryptography, in which the presence of an encrypted message is clearly obvious, though its meaning is not.In fact, cryptography and steganography are separate techniques, which can be used independently or together.The strength of the steganography idea lies in the assumption that a secret message that is not even noticed ever is subjected to cryptanalysis.In addition, if it is hidden well enough, the task of even finding it among all other media can be daunting, if not impossible.

Steganography is not a new idea, either.The technique dates back at least to the ancient Greeks and was especially popular with secret societies of the 14th and 15th centuries.The idea is so simple and effective that it has been used right into modern times.Examples can be found in written messages where a letter in a particular position in each word spells out a hidden message; or in the arrangement of notes in a piece of music; or in several other ways.In today's world, the ability of the personal computer to digitally process text, images, sound, and video opens a new dimension in this Spy vs.Spy game. However, there are uses for steganography other than terrorist activity.In the GIS field, it might be used to protect copyrights of digital files such as satellite photos or vector map data.The simplest method is to use the least significant few bits of selected bytes in a digital data set to store a hidden message.A typical aerial image, for example, always contains some amount of "noise" (shadows, reflections, vegetation variation, camera imperfections, distortions caused by topography, etc.) so that trading a few least significant bits of original image and replacing them with a coded message is undetectable to the eye, nor would it adversely affect the scientific value of the altered image.

A data vendor might use this technique to embed a standard copyright message in the dataset (this is known as watermarking).They may also include a registration number specific to the party that licenses the data from them (this is known as fingerprinting), so that if the data turns up in an unauthorized application, the data vendor can use the digital watermark to prove ownership and the fingerprint to prosecute. Another useful purpose for steganography in GIS may be to embed metadata within a file.Too often, the information about map data (e.g.geographic extents, accuracy, precision, date, source, etc.) is separated from the actual image or data file.This sort of application is already being used by hospitals to embed a patient's identity within medical images such as X-rays or CAT scans taken of the patient.

Although not all situations require absolute robustness, a desirable goal of steganography for use as a GIS tool would be the ability to withstand typical GIS operation such as reprojection, resampling, and line and polygon generalization to name a few.Indeed, here lies a formidable problem for the least significant bits approach to steganography for vector data, but this argument does not apply to all situations.Nor is this the only approach one may use to embed a message in vector data set.

Steganography can be applied to GIS in many ways to help solve several different problems.There are no perfect solutions that work in all situations, but two different methods for applying steganography are described in the following article, and you can judge for yourself.The concept is sound, the need is there, and the idea is even now circulating through the Internet, so you have probably not heard the last of it.

(1) Terror groups hide behind Web encryption:
See USA Today Article

See Part 2

See also these resources on the web...
Go here.


Published Wednesday, April 10th, 2002

Written by Bill Thoen



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