The Hidden Art of Steganography
 What is hiding in your picture?
Summer 2005
CS 4953 The Hidden Art of Steganography
The Hidden Art of Steganography
 What is hiding in your picture?
Summer 2005
CS 4953 The Hidden Art of Steganography
The Hidden Art of Steganography
 What is hiding in your picture?
Summer 2005
CS 4953 The Hidden Art of Steganography
Information Hiding
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Information Hiding is a branch of computer science that deals with
concealing the existence of a message
It is related to cryptography whose intent is to render messages
unreadable except by the intended recipients
It employs technologies from numerous science disciplines:
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Digital Signal Processing (Images, Audio, Video)
Cryptography
Information Theory\Coding Theory
Data Compression
Human Visual/Auditory perception
There are four primary sub-disciplines of Information Hiding
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Steganography
Watermarking
Covert Channels
Anonymity
Summer 2005
CS 4953 The Hidden Art of Steganography
Information Hiding
F. A. P. Petitcolas, R. J. Anderson, M. G. Kuhn, “Information Hiding – A Survey”, Proceedings of the IEEE,
special issue on protection of multimedia content, 87(7):1062-1078, July 1999
Summer 2005
CS 4953 The Hidden Art of Steganography
Fields Applied to Information Hiding
 Information Theory/Coding Theory
 Digital Signal Processing
– Discrete Fourier Transform/Discrete Cosine Transform
– Image/Audio/Video Processing
 Data Compression
 Cryptographic Principles
 Discrete Math
 Cryptographic Hashing
 Data Networks
 The Human Visual System/Human Auditory System
– Capabilities and limitations
Summer 2005
CS 4953 The Hidden Art of Steganography
Basic Terminology
 The data to be hidden:
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Plaintext (from cryptography)
Secret message
Stego-message
Embedded data
 The data which will have a stego-message embedded in it:
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Covertext
Cover-Object
Cover-Image\Cover-Audio\Cover-Video
Target file
Summer 2005
CS 4953 The Hidden Art of Steganography
Basic Terminology
 The key used to make the stego-message secure
– Stego-Key
– Secret Key
– Key
 The file with the steganography-message embedded
– Stegotext (ciphertext in cryptography)
– Stego-Object
– Stego-Image\Stego-Audio\Stego-Video
Summer 2005
CS 4953 The Hidden Art of Steganography
Basic Terminology
 Alice and Bob
– Classical names given to the parties wishing to communicate
 Sometimes, you may have a Carol and a Dave
 Eve, an adversary, can listen to but not modify or forge a
message
– (think passive eavesdropping)
 Wendy the Warden, another adversary, can monitor, modify,
or forge a message
– A passive warden simply listens (like Eve)
– An active warden may modify a message
– A malicious warden may forge a fake message
Summer 2005
CS 4953 The Hidden Art of Steganography
History of Steganography
 The concept of message hiding is not new – it’s been around
for centuries
– A Greek shaved the head of a slave, wrote a message, then waited for
the hair to grow back before sending the slave to his destination
– Steganography (in the form of invisible ink) was used by Washington
in the Revolutionary War
– Prior to the Civil War, quilts were sewn with special patterns to tell
escaping slaves which direction to go and what to do
– During WWI there was a cable the read, “Father is dead.” Suspecting a
hidden meaning, the censor changed it to “Father is deceased” which
caused the reply, “Is Father dead or deceased?”
– During WWII chess by mail was banned, crossword puzzles
examined, stamps were removed and replaced by ones of equal value
– In the 1980’s, some of Margaret Thatcher’s cabinet documents were
leaked to the press. She ordered that the word processors being used
by government employees, encode their identity in the word spacing of
the documents
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganography
 Steganography literally means “covered writing”
– A stegosaurus has a covered back
 Steganography’s primary goal is to hide data within some
other data such that the hidden data cannot be detected even if
it is being sought
 Secondary goals:
– prevent extraction from the cover file without destroying the cover
– prevent destruction of the stego-message without destroying the cover
 Most frequently, steganography is applied to images, but
many other data or file types are possible
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Audio
Video
Text
Executable programs
Summer 2005
CS 4953 The Hidden Art of Steganography
Watermarking
 Watermarking is very similar to steganography in that one of its goals is to
not be detected
 However, it’s primary goal is to not be able to be extracted or destroyed (at
least not without destroying the cover too)
 Typically, watermarking is designed to protect intellectual property rights
for images, sounds, and video
– If it’s easily removed or destroyed, those rights cannot be protected
 Even if it’s not detectable, an adversary could suspect that a work (of art)
could have a watermark and so take steps to destroy it
– There is a popular program called StirMark which does just that
 For some applications watermarks may be visible
 May be used to fingerprint a particular file and detect changes
– Make it tamper proof
 Used to prove ownership
 So far, to my knowledge, no one has invented a watermarking system that
is good enough to hold up in court
Summer 2005
CS 4953 The Hidden Art of Steganography
Covert Channels
 Covert channels are communication paths that were neither
designed nor intended to transfer information
 For example, the telephone was designed to allow voice
communication
– information could be conveyed by letting it ring a certain number of
times
– The time differences between successive phone calls could be used
– You could use a mobile phone and call from different locations – the
street names convey the message
 Unused bits in the TCP/IP protocol headers can be used to
carry information
 Hiding data in an image, then sending that image to someone
else could also be considered a covert channel
Summer 2005
CS 4953 The Hidden Art of Steganography
Anonymity
 Anonymity is about concealing the sender and receiver of
messages
 This is the least studied sub-discipline of information hiding
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganalysis
 Steganalysis is the detection of data that has been hidden
 It is a cat and mouse game – as one group of researchers come
up with better ways to hide stuff, another group figures out
how to detect it or perhaps just destroy it
 In the summer course last year, one student thought he had a
hiding system that was undetectable
– His lab partner wrote a statistical analysis program that exposed the
hidden message
– The first student modified his program to defeat the attack
– His lab partner modified his analyzer and still found the message
Summer 2005
CS 4953 The Hidden Art of Steganography
Wisdom from Cryptography
 One of the main principals of cryptography applies to
steganography as well
 It is a simple concept, yet many repeatedly ignore or dismiss it
 The premise from which to measure a secure steganographic
system is to assume that the opponent knows the system being
employed, yet still cannot find any evidence of a hidden
message
– Kerchoff’s Principle: the system should not depend on secrecy and
should be able to fall into enemy hands without disadvantage
– The ONLY way to find any evidence of a message is with the key
– Many systems have relied on the “Security by Obscurity” premise and
many have failed – those that have not have been lucky
• CSS for DVD, RIAA digital watermarking, Adobe e-books, SDMI
Summer 2005
CS 4953 The Hidden Art of Steganography
Wisdom from Cryptography
 Often designers think (erroneously) that “They’ll never think
of this.” – yet, the designer did???
– A criminal would never think to look in a flower pot for the house key
• Nooooo!
 Even in military situations, equipment is captured or bought
 If the security depends on the secrecy of the algorithm, once it
is compromised, the entire system is compromised - forever
 If a key is compromised, only that message is compromised
 A secure system will not rely on keeping the algorithm secret,
just the key
 The same premise holds true for a steganography system
– Do NOT depend on the secrecy of your algorithm for security!
Summer 2005
CS 4953 The Hidden Art of Steganography
Applications of Information Hiding
 Covert military communications
 Covert police communications
– Criminals have learned that police are nearby when they hear
encrypted communications, so they abate their activity for the moment
– A steganographic system would prevent this detection
 Digital Rights Management – protecting intellectual property
such as images, music, electronic books, etc.
 Embedding textual data in medical images would better
ensure that the picture belongs to a particular patient
– This technique could apply to personal pictures, sounds, and movies
 Tamper proofing – ensuring a data file has not been changed
 Communicating in an oppressive country w/o free speech
Summer 2005
CS 4953 The Hidden Art of Steganography
Nefarious Applications of Information Hiding
 You won’t find these applications in the book
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money laundering
drug running
child pornography
spying (good or bad depending upon which side you’re on!)
terrorism
 Unfortunately, these uses are also possible
 The technology itself isn’t bad, but like many things, it can be
(and is) abused
 Since there are nefarious uses, law enforcement and the
military is also interested in understanding hiding techniques
and detecting hidden data
 There are some projects right here at UTSA that have done
research for the Air Force
Summer 2005
CS 4953 The Hidden Art of Steganography
Goals of Information Hiding - Security
 It is secure if it cannot be removed even with full knowledge
of the embedding algorithm without knowledge of the secret
key
 Can it be detected by human perception? (Invisibility)
– See distortion/noise in an image
– Hear distortion/noise in speech or music?
 Can it be detected by statistical analysis? (Undetectability)
 Does it leave easily detectable signatures?
 Levels of Failure:
– Detection - Proof of existence of message
– Extraction – removing without destroying the cover
– Destruction – destroying the message without destroying the cover
Summer 2005
CS 4953 The Hidden Art of Steganography
Goals of Information Hiding - Capacity
 How much data can a cover image hold?
– There is a physical limit (unless the cover file size is increased)
– There is a limit as to when the data will be noticeable
 Typically, as more capacity is used, the lower the security and
robustness
Summer 2005
CS 4953 The Hidden Art of Steganography
Goals of Information Hiding – Robustness
 How well does the data maintain integrity in the face of
modifications?
 The modifications we are concerned with are quite common
– Images: blurring, sharpening, scaling, cropping, contrast, gamma,
brightness, rotation, skewing, recoloring, printing/copying/scanning,
etc.
– Audio: filtering (think bass/treble), volume adjustment, stereo to
mono, etc.
– Video: any image/audio modification, add/delete frames, temporal
adjustments, frame swapping, frame averaging
– Also: lossy compression, A/D and D/A conversion, and sophisticated
attacks
 Robustness is achieved through redundant encoding of the
message which reduces the capacity
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganographic Hiding Techniques
 Substitution systems
– Put message in redundant or noisy parts of cover
 Transform domain techniques
– Embed information in the transform space
 Spread spectrum techniques
– Message is spread across frequency spectrum of cover
 Statistical methods
– Alter some statistical properties of the cover
 Distortion techniques
– Store message by distorting the cover slightly and detecting the
change from the original
 Cover generation methods
– Encode information in the way a cover is created
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganographic Techniques - Substitution
 Replace information in the cover with the stego-message
 The most common method is to replace the Least Significant
Bit (LSB)
 Each pixel in the mandrill image is composed of 8 bits and so
have a range of values from zero (black) to 255 (white)
– The “sports enthusiast” was hidden by replacing the 4 least significant
bits
– You might be tempted to think, “That’s half the information!”
– No, it is 1/16 the information – not noticeable to us
 Other images with more solid backgrounds would NOT
provide the same level of imperceptibility
– To maximize capacity while maintaining imperceptibility, you need to
analyze the cover image
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganographic Techniques - Substitution
 You can see some of the distortion caused by the mandrill
image (Again, 4 bits were used)
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganographic Techniques - Substitution
 Using a solid white or black cover is NOT very good
Summer 2005
CS 4953 The Hidden Art of Steganography
Steganographic Techniques - Substitution
 Here is a progression of hiding in the mandrill image
Summer 2005
CS 4953 The Hidden Art of Steganography
Questions and/or Comments
Summer 2005
CS 4953 The Hidden Art of Steganography