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Outline
• Steganalysis to LSB encoding
• Steganalysis based on JPEG compatibility
• Some discussions
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Introduction
• Steganography
– he art o! secret communication
– Stego content "e#g# images$ should notcontain any easily detectable arti!acts due
to message embedding
– he less in!ormation is embedded% the
smaller the probability o! introducingdetectable arti!acts
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&atermar'ing (s# Steganography
)idelity
*obustness+apacity
&atermar'ing
Steganography
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Steganalysis o! LSB Encoding
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Goal
• o inspect one or possibly more images!or statistical arti!acts due to messageembedding in color images using theLSB method – o !ind out ,hich images are li'ely to
contain secret messages
– o estimate the reliability o! decisions• ype I error "!alse-alarm$ and ype II error
"Miss$
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.pplication Scenarios
Internet
.utomatic
+hec'ing
Internet node ,ith a special !ilter
)orensics E/pert
Images in
Sei0ed computer Images sent
to a certain address
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LSB Encoding
• *eplacing the LSB o! e(ery gray-le(el o!
color channel ,ith message bits
– On a(erage 123 o! the LSB are changed
– Logic behind this scheme• LSB in scanned or camera-ta'en images are
essentially random
• Encrypted "randomi0ed$ message are random
• 4o statistical arti!acts ,ill be introduced
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Important Obser(ation
• 4umber o! uni5ue colors in co(er images – ypically smaller than the number o! pi/els in the images
• 678 !or high 5uality scans in BMP !ormat
• 679 or lo,er !or JPEG images or (ideo
• Many true-color images ha(e a relati(ely small :palette;
• .!ter LSB embedding% ne, color palette ,ill ha(e adistinct !eature
– Many pairs o! close colors – .n e(idence o! LSB encoding-based steganography
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)ormulations
• U 7 number o! uni5ue colors in an image
• P 7 number o! close color pairs
– Two colors (R 1,G1,B1 ) and (R 2 ,G2 ,B2 ) are
close i!
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he Proposed Scheme
• .!ter embedding% @ ,ill be increased to @%and ,e can e(aluate the number o! uni5ue
pairs o! P#
• he (alue o! * !or an image that does notha(e a message ,ill be smaller than that
o! an image that already has a message
already embedded in it
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he Proposed Scheme "cont#$
• It is impossible to !ind a threshold o! * !or allimages – Cue to a large (ariation o! @
• Obser(ations !or reliable distinguishing – )or an image already contains a large message
• Embedding another message in it does not modi!y *signi!icantly
– )or an image not containing a message
• * increases signi!icantly – @se the relati(e comparison o! * as the decision
criterion
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Cetection .lgorithm
• o !ind out ,hether or not an image has a secretmessage
6# +alculate *>P?+"@% 8$
8# @sing LSB embedding in randomly selected pi/els – Si0e o! the test message7 D a M 4 "!or M by 4 color images$‧ ‧ ‧
D# +alculate *>P?+"@%8$
# Cecide ,hether an image is embedded
– *F>* the image already had a large message hidden
– **the image did not ha(e a message in it
*?*7 the separating statistics
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Limitations
• I! the secret message si0e is too small
– the t,o ratio ,ill be (ery close to each other
• &e cannot distinguish images ,ith and ,ithout
messages
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E/periments
• @sing an image database o! D22 color images – D12/812 pi/els
– JPEG compressed
– +apacity !or each image7 D8#H' bits "D12/812D?H$
• . message o! length 82B "8?D o! ma/imal capacity$ ,asembedded into each image to !orm a ne, database o!images ,ith messages
• he detection algorithm is run !or both database and themessage presence is tested by embedding a test
message o! si0e 6B "a>6?D2$
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E/perimental *esults
6#6
K 7 original database
7 embedded database
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Parameter Optimi0ation
• Model the density !unctions as Gaussiandistributions – N(μ, σ) and N(μ s, σ s )
• Ci!!erent si0e o! secret messages%denoted as s% and test messages aretested – Secret messages7 63 to 123 – est messages7 a>2#26 – 2#1
• *esults – μ>μ s !or all s
– s decreases N(μ s, σ s ) become !lat andthe pea' mo(es right
– s increases N(μ s, σ s ) become narro,er
and the pea' mo(es le!t• Easier to separate the t,o pea's !or largersecret message si0es
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hreshold Selection
ype I Error > ype II Error
"e5uals minimi0ing o(erall error$
+hange the threshold h to adust !or the importance o! not
missing an image ,ith a secret message at the e/pense o!
!alse-alarm
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E/perimental *esults
K K
K K
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E/perimental *esults "cont#$
K
K
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+onclusions
• he probability o! error prediction is mainly determinedby the si0e o! the secret message – he in!luence o! the test message si0e is much smaller
• he optimal test message si0e is di!!erent !or di!!erent
secret message si0e• he detection algorithm mainly targets !or images ,ithsmaller number o! uni5ue colors – he results !or high-5uality scanned and loselessly compressed
images "@2#1M4$ may be unreliable
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Steganalysis Based on JPEG
+ompatibility
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Image Steganography
• Image !ormats
– @ncompressed "BMP$
• O!!ering the highest capacity and best o(erall security
– Palette "GI)$
• Ci!!icult to pro(ide security ,ith reasonable capacity
– Lossy compressed "JPEG% JPEG 8222$
• Ci!!icult to hide message in JPEG stream in a secure
manner ,hile 'eeping the capacity practical
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Goal o! this Paper
• o sho, that images may be e/tremely poorcandidates !or co(er images i!
• Initially ac5uired as JPEG images and later decompressed to aloseless !ormat
• )or steganalysis methods% minimal amount o!distortion is to be achie(ed to reduce (isible arti!acts – he act o! message embedding ,ill not erase the
characteristic structure created by JPEG compression – .naly0ing the C+ coe!!icients o! images to reco(er e(en
the (alues o! JPEG 5uanti0ation table
• E(idence !or steganography – .n image stored in loseless !ormat that bears a strong
!ingerprinting o! JPEG compression% yet is not !ullycompatible ,ith JPEG compressed image
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JPEG +ompression
Uncompressed Image
Borig
C+
d'"i$% i>2%%9D
C'"i$>*ound "d'"i$?N"i$$
JPEG Nuanti0ation Matri/ N
ig0ag-scanu!!man coder
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JPEG Cecompression
• u!!man decoding
• NC'"i$>N"i$C'"i$
– Multiplying 5uanti0ed C+ step ,ith
5uanti0ation step• Bra,>C+-6"NC $
– In(erse C+
• B>QBra,R – rounded to integers in the range o! 2-811
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Obser(ations
• I! the bloc' B has no pi/els saturated at 2or 811
–
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he Proposed Scheme
• Nuestion – Gi(en an arbitrary H/H bloc' B o! pi/el (alues% could this bloc'
ha(e arisen through the process o! JPEG decompression ,iththe 5uanti0ation matri/ N "i! a(ailable$T
–
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.lgorithm
6# Ci(ide the images into H/H bloc's
8# .rrange the bloc's in a list% and remo(e all
saturated bloc's !rom the list
• 7 number o! remaining bloc'sD# E/tract the 5uanti0ation matri/ N !rom all
bloc's
• I! all elements o! N are 6s% the image is not
calculated
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.lgorithm "cont#$
# )or each bloc' B% calculate S1# I! S69%
B is not compatible ,ith JPEG compression#
else
Per!orm the additional chec'9# .!ter going through bloc's% i! no incompatible bloc's is
!ound% no e(idence o! steganography is a(ailable#
V# *epeat the algorithm !or di!!erent H/H di(ision !or
detecting cropped images
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E/tracting the Nuanti0ation Matri/
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Some Ciscussions
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*e!erence
• J# )ridrich% *# Cu and M# Long% :Steganalysis o!LSB encoding in color images% ; I+ME 8222%4e, Wor'% 8222
• J# )ridrich% M# Golan and *# Cu% :Steganalysis
based on JPEG compatibility%; SPIE MultimediaSystems and .pplications IX% Cen(er% 8226
• G# Goth% :Steganalysis gets past the hype% IEEECistributed Systems Online% .pril 8221