Steganography and Steganalysis in Digital Multimedia: Hype or Hallelujah?

Abstract

In this tutorial, we introduce the basic theory behind Steganography and Steganalysis, and present some recent algorithms and developments of these fields. We show how the existing techniques used nowadays are related to Image Processing and Computer Vision, point out several trendy applications of Steganography and Steganalysis, and list a few great research opportunities just waiting to be addressed.

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Steganography and Steganalysis in Digital Multimedia:
Hype or Hallelujah?
Anderson Rocha1,2
Siome Goldenstein1
Abstract: In this tutorial,we introducethe basic theorybehindSteganographyand
Steganalysis, and present some recent algorithms and developments of these fields.
We show how the existing techniques used nowadays are related to Image Process-
ing and Computer Vision, point out several trendy applications of Steganographyand
Steganalysis, and list a few great research opportunitiesjust waiting to be addressed.
1 Introduction
De artificio sine secreti latentis suspicione scribendi!3. (David Kahn)
More than just a science, Steganography is the art of secret communication. Its pur-
pose is to hide the presence of communication, a very different goal than Cryptography,
that aims to make communication unintelligible for those that do not possess the correct ac-
cess rights [1]. Applications of Steganography can include feature location (identification of
subcomponents within a data set), captioning, time-stamping, and tamper-proofing (demon-
stration that original contents have not been altered). Unfortunately, not all applications are
harmless, and there are strong indications that Steganography has been used to spread child
pornographypictures on the internet [2, 3].
In this way, it is important to study and develop algorithms to detect the existence of
hidden messages. Digital Steganalysis is the body of techniques that attempts to distinguish
between non-stego or cover objects, those that do not contain a hidden message, and stego-
objects, those that contain a hidden message.
Steganography and Steganalysis have received a lot of attention around the world in
the past few years. Some are interested in securing their communications through hiding the
very own fact that they are exchanging information. On the other hand, others are interested
in detecting the existence of these communications – possibly because they might be related
to illegal activities.
1Institute of Computing, University of Campinas (Unicamp).
2Corresponding author: anderson.rocha@ic.unicamp.br
3The effort of secret communication without raising suspicions.

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Steganographyand Steganalysis in Digital Multimedia: Hype or Hallelujah?
In this tutorial, we introduce the basic theory behind Steganographyand Steganalysis,
and present some recent algorithms and developments of these fields. We show how the
existing techniques used nowadays are related to Image Processing and Computer Vision,
point out several trendy applications of Steganography and Steganalysis, and list a few great
research opportunities just waiting to be addressed.
The remainder of this tutorial is organized as follows. In Section 2, we introduce
the main concepts of Steganography and Steganalysis. Then, we present historical remarks
and social impacts in Sections 3 and 4, respectively. In Section 5, we discuss information
hiding for scientific and commercial applications. In Sections 6 and 7, we point out the main
techniques of Steganography and Steganalysis. In Section 8, we present common-available
information hiding tools and software. Finally, in Sections 9 and 10, we point out open
research topics and conclusions.
2Terminology
Accordingto the generalmodel of InformationHiding: embeddeddata is the message
we want to send secretly. Often, we hide the embedded data in an innocuous medium, called
cover message. There are many kinds of cover messages such as cover text, when we use text
to hide a message; or cover image, when we use an image to hide a message. The embedding
process produces a stego object which contains the hidden message. We can use a stego key
to control the embedding process, so we can also restrict detection and/or recovery of the
embedded data to other parties with the appropriate permissions to access this data.
Figure 1 shows the process of hiding a message in an image. First we choose the data
we want to hide. Further, we use a selected key to hide the message in a previously selected
cover image which produces the stego image.
When designing information hiding techniques, we have to consider three competing
aspects: capacity, security, and robustness [4]. Capacity refers to the amount of information
we can embed in a cover object. Security relates to an eavesdropper’s inability to detect
the hidden information. Robustness refers to the amount of modification the stego-object
can withstand before an adversary can destroy the information [4]. Steganography strives
for high security and capacity. Hence, a successful attack to the Steganography consists
of the detection of the hidden content. On the other hand, in some applications, such as
watermarking, there is the additional requirement of robustness. In these cases, a successful
attack consists in the detection and removal of the copyright marking.
Figure2 presents the InformationHiding hierarchy[5]. Covert channelsconsist of the
use of a secret and secure channel for communication purposes (e.g., military covert chan-
nels). Steganography is the art, and science, of hiding the information to avoid its detection.
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Steganographyand Steganalysis in Digital Multimedia: Hype or Hallelujah?
Message to be hidden
The cover medium
to be used
The produced stego image
Figure 1. A data hiding example.
It derives from the Greek steganos ∼ “hide, embed” and graph ∼ “writing”.
We classify Steganography as technical and linguistic. When we use physical means
to conceal the information, such as invisible inks or micro-dots, we are using technical
Steganography. On the other hand, if we use only “linguistic” properties of the cover ob-
ject, such as changes in image pixels or letter positions, in a cover text we are using linguistic
Steganography.
Copyright marking refers to the group of techniques devised to identify the ownership
of intellectual property over information. It can be fragile, when any modification on the me-
dia leads to the loss of the marking; or robust, when the markingis robust to some destructive
attacks.
Robust copyrightmarkingcan be of two types: fingerprintingand watermarking. Fin-
gerprinting hides an unique identifier of the customer who originally acquired the informa-
tion, recording in the media its ownership. If the copyright owner finds the document in
the possession of an unwanted party, she can use the fingerprint information to identify, and
prosecute, the customer who violated the license agreement.
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Steganographyand Steganalysis in Digital Multimedia: Hype or Hallelujah?
Information Hiding
Covert channels
Steganography
Linguistic
Technical
Anonymity Copyright marking
Robust watermarking Fragile watermarking
Fingerprinting Watermarking
Perceptible Imperceptible
Figure 2. Information Hiding hierarchy.
Unlike fingerprints, watermarks identify the copyrightowner of the document,not the
identity of the owner. Furthermore, we can classify watermarking according to its visibility
to the naked eye as perceptible or imperceptible.
In short, fingerprints are used to identify violators of the license agreement, while
watermarkshelp with prosecutingthose who havean illegal copyofa digital document[5, 6].
Anonymity is the body of techniques devised to surf the Web secretly. This is done
using sites like Anonymizer4or remailers (blind e-mailing services).
3 Historical remarks
Throughouthistory, people always have aspired to more privacy and security for their
communications [7, 8]. One of the first documents describing Steganography comes from
Histories by Herodotus, the Father of History. In this work, Herodotus gives us several cases
of such activities. A man named Harpagus killed a hare and hid a message in its belly. Then,
he sent the hare with a messenger who pretended to be a hunter [7].
In order to convince his allies that it was time to begin a revolt against Medes and
the Persians, Histaieus shaved the head of his most trusted slave, tattooed the message on his
head and waited until his hair grew back. After that, he sent him along with the instruction to
shave his head only in the presence of his allies.
Another technique was the use of tablets covered by wax, first used by Demeratus, a
Greek who wanted to report from the Persian court back to his friends in Greece that Xerxes,
the Great, was about to invade them. The normal use of wax tablets consisted in writing
the text in the wax over the wood. Demeratus, however, decided to melt the wax, write the
message directly to the wood, and then put a new layer of wax on the wood in such a way
4www.anonymizer.com
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Steganographyand Steganalysis in Digital Multimedia: Hype or Hallelujah?
that the message was not visible anymore. With this ingenious action, the tablets were sent
as apparently blank tablets to Greece. This worked for a while, until a woman named Gorgo
guessed that maybe the wax was hiding something. She removed the wax and became the
first woman cryptanalyst in History.
During the Renaissance, the Harpagus’ hare technique was “improved” by Giovanni
Porta, one of the greatest cryptologists of his time, who proposed feeding a message to a dog
and then killing the dog [8].
Drawings were also used to conceal information. It is a simple matter to hide infor-
mation by varying the length of a line, shadings, or other elements of the picture. Nowadays,
we have proof that great artists, such as Leonardo Da Vinci, Michelangelo, and Rafael, have
used their drawings to conceal information [8]. However, we still do not have any means to
identify the real contents, or even intention, of these messages.
Sympatheticinkswerea widespreadtechnique. Whohasnotheardaboutlemon-based
ink during childhood? With this type of ink, it is possible to write an innocent letter having a
very different message written between its lines.
Sciencehasdevelopednewchemicalsubstancesthat, combinedwith othersubstances,
cause a reaction that makes the result visible. One of them is gallotanic acid, made from gall
nuts, that becomes visible when coming in contact with copper sulfate [9].
With the continuous improvement of lenses, photo cameras, and films, people were
able to reduce the size of a photo down to the size of a printed period [7, 8]. One such
example is micro-dot technology, developed by the Germans during the Second World War,
referred to as the “enemy’s masterpiece of espionage” by the FBI’s director J. Edgar Hoover.
Micro-dots are photographs the size of a printed period that have the clarity of standard-
sized typewritten pages. Generally, micro-dots were not hidden, nor encrypted messages.
They were just so small as to not draw attention to themselves. The micro-dots allowed the
transmission of large amounts of data (e.g., texts, drawings, and photographs)duringthe war.
There are also other forms of hidden communications, like null ciphers. Using such
techniques, the real message is “camouflaged” in an innocuous message. The messages are
very hard to construct and usually look like strange text. This strangeness factor can be
reduced if the constructor has enough space and time. A famous case of a null cipher is the
bookHypteronomachiaPoliphiliof 1499. A Catholic priest namedColona decidedto declare
his love to a young lady named Polya by putting the message “Father Colona Passionately
loves Polia” in the first letter of each chapter of his book.
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