Top Banner

of 33

Introduction to Steganalysis Schemes

Jul 07, 2018

Download

Documents

Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
  • 8/18/2019 Introduction to Steganalysis Schemes

    1/33

  • 8/18/2019 Introduction to Steganalysis Schemes

    2/33

     

    Outline

    • Steganalysis to LSB encoding

    • Steganalysis based on JPEG compatibility

    • Some discussions

  • 8/18/2019 Introduction to Steganalysis Schemes

    3/33

     

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    4/33

     

    &atermar'ing (s# Steganography

    )idelity

    *obustness+apacity

    &atermar'ing

    Steganography

  • 8/18/2019 Introduction to Steganalysis Schemes

    5/33

     

    Steganalysis o! LSB Encoding

  • 8/18/2019 Introduction to Steganalysis Schemes

    6/33

     

    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$

  • 8/18/2019 Introduction to Steganalysis Schemes

    7/33 

     .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

  • 8/18/2019 Introduction to Steganalysis Schemes

    8/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    9/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    10/33 

    )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!

  • 8/18/2019 Introduction to Steganalysis Schemes

    11/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    12/33 

    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 

  • 8/18/2019 Introduction to Steganalysis Schemes

    13/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    14/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    15/33 

    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$

  • 8/18/2019 Introduction to Steganalysis Schemes

    16/33 

    E/perimental *esults

    6#6

     K 7 original database

    7 embedded database

  • 8/18/2019 Introduction to Steganalysis Schemes

    17/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    18/33 

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    19/33 

    E/perimental *esults

    K K

    K K

  • 8/18/2019 Introduction to Steganalysis Schemes

    20/33

     

    E/perimental *esults "cont#$

    K

    K

  • 8/18/2019 Introduction to Steganalysis Schemes

    21/33

     

    +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

  • 8/18/2019 Introduction to Steganalysis Schemes

    22/33

     

    Steganalysis Based on JPEG

    +ompatibility

  • 8/18/2019 Introduction to Steganalysis Schemes

    23/33

     

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    24/33

     

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    25/33

     

    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 

  • 8/18/2019 Introduction to Steganalysis Schemes

    26/33

     

    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

  • 8/18/2019 Introduction to Steganalysis Schemes

    27/33

     

    Obser(ations

    • I! the bloc' B has no pi/els saturated at 2or 811

     –

  • 8/18/2019 Introduction to Steganalysis Schemes

    28/33

     

    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

     –

  • 8/18/2019 Introduction to Steganalysis Schemes

    29/33

     

     .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

  • 8/18/2019 Introduction to Steganalysis Schemes

    30/33

     

     .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

  • 8/18/2019 Introduction to Steganalysis Schemes

    31/33

     

    E/tracting the Nuanti0ation Matri/

  • 8/18/2019 Introduction to Steganalysis Schemes

    32/33

     

    Some Ciscussions

  • 8/18/2019 Introduction to Steganalysis Schemes

    33/33

    *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