ABSTRACT
In this paper we present a method of embedding information
within digital imagesj called Spread Spectrum
Image Steganography (SS1S) along with its payload capacity.
Steganography is the science of communicating
in a hidden manner. SS1S conceals a message of substantial
length within digital imagery while maintaining
the original image size and dynamic range. The
hidden message can be recovered using the appropriate
keys without any knowledge of the original image. The
capacity of the steganographic channel is described and
the performance of the technique is illustrated. Applications
for such a data-hiding scheme include in-band
captioning, hidden communication, image tamperproofing,
authentication ~ invisible map overlays, embedded
control, and revision tracking.
INTRODUCTION
Steganographic techniques allow communication between
two authorized parties without an observer being
aware that the communication is actually taking
place. These techniques have many Army applications
in the defensive information warfare arena, such as hidden
communication, in-band captioning, and tamperproofing.
A useful steganographic system must provide
a method to embed data in an imperceptible manner,
allow the data to be readily extracted, promote a high
information rate or payload capacity, and incorporate a
certain amount of robustness to removal [1], [2].
Digit al steganography, or information-hiding schemes,
can be characterized utilizing the theories of communication
[3]. The parameters of information hiding such
as the amount of data bits that can be hidden, the
perceptibility of the message, and its robustness to re-
*Prepared through collaborative participation in the Advanced
Telecommunication/Information Distribution Research Program
(ATIRP) Consortium sponsored by the U.S. Army Research Laboratory
under Cooperative Agreement DAAL01-96-2-OO02
moval can be related to the characteristics of communication
systems: capacity, signal-to-noise ratio (SNR],
and jamming margin. The notion of payload capacity
in data hiding indicates the total number of bits hidden
and successfully recovered by the stegosystem. The
signal-to-noise ratio serves as a measure of detectability.
In this context, the message we are trying to conceal
— the embedded signal — represents the informationbearing
signal, and the cover image is viewed as noise.
Contrary to typical communication scenarios where a
high SNR is desired, a very low SNR for a stegosystem
corresponds to lower perceptibility and therefore
greater success when concealing the embedded signal.
The measure of jamming resistance can be used to describe
a level of robustness to removal or destruction of
the embedded signal, intentional or accidental,
It is not possible to simultaneously maximize robustness,
imperceptiveness, and capacity. Therefore, the
acceptable balance of these items must be dictated by
the application. For example, an information-hiding
scheme may forgo robustness in favor of capacity and
low perceptibility, whereas a watermarking scheme,
which may not require large capacity or low perceptibilityy,
would cert airily support increased robustness.
Finally, steganography used as a method of secret communication
would adopt the utmost imperceptiveness
while sacrificing robustness and possibly capacity.

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