Highly Imperceptible and Reversible Text Steganography Using Invisible Character based Codeword

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1 Assocaton for Informaton Systms AIS Elctronc Lbrary (AISL) PACIS 2017 Procdngs Pacfc Asa Confrnc on Informaton Systms (PACIS) Summr Hghly Imprcptbl and Rvrsbl Txt Stganography Usng Invsbl Charactr basd Codword Mohammad Sadur Rahman RMIT Unvrsty, Mlbourn, Ibrahm Khall RMIT Unvrsty, Mlbourn, Xun Y RMIT Unvrsty, Mlbourn, xun.y@rmt.du.au Ha Dong RMIT Unvrsty, Mlbourn, ha.dong@rmt.du.au Follow ths and addtonal works at: Rcommndd Ctaton Rahman, Mohammad Sadur; Khall, Ibrahm; Y, Xun; and Dong, Ha, "Hghly Imprcptbl and Rvrsbl Txt Stganography Usng Invsbl Charactr basd Codword" (2017). PACIS 2017 Procdngs Ths matral s brought to you by th Pacfc Asa Confrnc on Informaton Systms (PACIS) at AIS Elctronc Lbrary (AISL). It has bn accptd for ncluson n PACIS 2017 Procdngs by an authorzd admnstrator of AIS Elctronc Lbrary (AISL). For mor nformaton, plas contact lbrary@asnt.org.

2 Hghly Imprcptbl and Rvrsbl Txt Stganography Hghly Imprcptbl and Rvrsbl Txt Stganography Usng Invsbl Charactr basd Codword Compltd Rsarch Papr Mohammad Sadur Rahman School of Scnc, RMIT Unvrsty Mlbourn, Vctora, Australa Ibrahm Khall School of Scnc, RMIT Unvrsty Mlbourn, Vctora, Australa Xun Y School of Scnc, RMIT Unvrsty Mlbourn, Vctora, Australa xun.y@rmt.du.au Ha Dong School of Scnc, RMIT Unvrsty Mlbourn, Vctora, Australa ha.dong@rmt.du.au Abstract Txt stganography mthod can b appld for protctng prvacy and authntcty of txt-basd documnts. Txt stganography s a challngng task as slght modfcaton n txt fl can b dntfd. In gnral, mprcptblty of txt stganography s vry poor. Addtonally, data hdng capacty of txt stganography s vry lss. Currnt rsarch works fal to solv both mprcptblty and capacty problms smultanously. W propos a novl data comprsson basd rvrsbl txt stganography schm addrssng both mprcptblty and capacty problms of txt stganography. Our proposd mthod mbds scrt mssag wthn txt fl as nvsbl charactr by followng a st of mbddng ruls. W prsnt xprmntal rsults that dmonstrat hghr mprcptblty and capacty of proposd mthod wthout ncrasng sz of txt fl contanng scrt nformaton. Most mportantly, our proposd mthod succssfully rtrvs scrt nformaton and rconstructs orgnal txt fl wthout any rror. Kywords: Txt stganography, Huffman codng, scurty, prvacy prsrvaton Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

3 Hghly Imprcptbl and Rvrsbl Txt Stganography Introducton Rcnt mprovmnt n Informaton and Communcaton Tchnology (ICT) producs larg volum of txt data rlatd to customrs, supplrs and opratons ownd by busnss organzatons. Mmos, busnss agrmnts, mals, rports and customr rcords ar fw xampls of txt-basd busnss data. Managng larg volum of txt-basd data by thr own s xpnsv for busnss organzatons. Cloud basd managmnt of larg volum data mnmzs th cost of data storag. Addtonally, cloud offrs scalabl storag wth ffcnt managmnt of data producd by organzatons. Hnc, busnss organzatons ar outsourcng thr bg data to thrd-party cloud for storng and managng (Lu, 2013). Data stord n th cloud ar snstv. Outsourcng data to cloud arss prvacy concrns for data ownrs (Hashm, 2015; Lu, 2014). For nstanc, stord data n th cloud can b compromsd, and ndvdual prsonal nformaton can b dsclosd (Lu, 2014). Svral cryptography mthods ar bng usd for protctng prvacy and authntcty of snstv txt-basd data n cloud. Howvr, opratons ovr cryptographcally ncryptd data ar usually complx and tm consumng (Lu, 2014). Morovr, mannglss form of ncryptd data may attract th attnton of ntrudrs (Chang, 2010). Altrnatvly, nformaton hdng tchnqu such as stganography can b usd for provsonng prvacy and authntcty of scrt nformaton. Stganography s an art of nformaton hdng that provds scrt communcaton ovr publc channl that allows avodanc of unauthorzd usrs attnton (Artz, 2001). It s manly usd for sharng scrt nformaton. Howvr, authntcty of a communcaton can b nsurd as wll by applyng stganography. For xampl, dgtal sgnaturs of two busnss parts can b hddn nsd a busnss agrmnt through a txt stganography mthod to produc a dgtally sgnd busnss agrmnt. Th sgnd busnss agrmnt can b stord n cloud. Latr, th sam txt stganography mthod can b appld to chck authntcty of th stord busnss agrmnt. In gnral, stganography mthod masks a scrt mssag n a covr mdum. Th covr mdum can b any dgtal mda such as txt (Jn'An, 2008; L, 2013; Por, 2012), mag (Subhdar, 2014), sgnal (Abuadbba, 2016), audo and vdo. A stgo mdum s producd as a rsult of maskng scrt mssag n covr mdum. Th functon that s usd to hd scrt mssag n covr mdum s calld mbddng procdur. Extracton procdur rtrvs th hddn scrt mssag from stgo mdum. Important rqurmnts of a stganography mthod ar mprcptblty, robustnss and capacty (Zakr, 2012). Imprcptblty s a masur of scurty of stganography mthod that dtrmns th lvl of prcpton of xstnc of a scrt mssag n a covr fl. Robustnss dnots th lvl of ablty to rsst altraton of scrt mssag. Capacty rfrs to amount of data that can b hddn n a covr mdum. A rvrsbl or losslss stganography approach can tak out a scrt mssag from stgo mdum and rconstruct covr mdum wthout any altraton. Thrfor, th prmary goals of stganographc approachs ar hgh mbddng capacty and low dstorton of th covr mdum (L, 2013). In ths rsarch work, w dvlop a txt-basd stganography approach. Th txt-basd stganography s challngng snc a slght modfcaton n txt contnt can b asly dscovrd (L, 2013). Hnc, th mprcptblty of txt covr mdum s vry poor. Svral rsarch works ar don on txt basd stganography. Rsarch works n (Jn'An, 2008; Yu, 2008) convrts th txt fls nto dgtal mags and mbd scrt mssag by adjustng spacng of lttrs, words or lns. As a rsult, txt fl cannot b modfd latr. Th work n (Por, 2012) hds scrt mssag as nvsbl charactr n Mcrosoft Word fl. Th work s not applcabl for othr txt format. Morovr, mbddng scrt mssag ncrass sz of stgo txt fl than covr txt fl. Fw comprsson basd tchnqus ar proposd n (Chn, 2010; L, 2013; Satr, 2012; Satr, 2014) that comprss covr txt fl bfor mbddng. Imprcptblty of txt covr fl bcoms vry lss f th covr txt fl s comprssd. Th objctv of ths rsarch s to dvlop a hghly mprcptbl and rvrsbl txt stganography mthod wth hghr capacty. Our work prsnts a hghly mprcptbl and rvrsbl txt stganography mthod basd on Huffman Cod to ncod scrt mssag. W us bnary format of scrt mssag n our papr. A scrt mssag can b data ownr s dntfcaton numbr, dgtal sgnatur of a busnss organzaton, tc. Encodd scrt mssag s mbddd nto txt covr fl as nvsbl charactrs. Customr rcords, nvocs, and busnss agrmnts can b namd as txt covr mdums. Th proposd txt stganography approach conssts of thr parts: (1) gnratng a codng tabl for a st of symbols, (2) mbddng a scrt mssag n a txt covr fl basd on th codng tabl, and (3) xtractng th hddn scrt mssag from th stgo txt fl and rconstructon of covr txt fl. A codng tabl s gnratd usng Huffman Cod that contans symbols, thr corrspondng frquncy and codword. Each codword n th codng tabl has th proprty that no codword n th Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

4 Hghly Imprcptbl and Rvrsbl Txt Stganography dctonary s a prfx of any othr codword n th dctonary (Huffman, 1952). In our approach, a n symbol s a bnary block of lngth n. Thrfor, w hav 2 possbl bnary symbols. If n 2, thn numbr of possbl symbols s Hnc, th symbols ar { 00,01,10,11}. Th codng tabl s usd to mbd th scrt mssag. Th lngth of a bnary block n s a scrt nformaton that nd to b dcdd pror to th mbddng opraton starts. Th lngth of a block nds to b passd scrtly to both mbddng and xtracton algorthm. Th gnraton procss of codng tabl s shown n Fgur 1(a). (a) (b) (c) Fgur 1. Ovrvw of Proposd Txt Stganography Approach. (a) Gnraton of Codng Tabl usng Huffman Codng Tchnqu, (b) Embddng Procdur, and (c) Extracton Procdur Th scrt mssag mbddng algorthm hds a scrt mssag nsd a txt fl usng fw stps (s Fgur 1(b)). Frst, scrt mssag s ncodd usng th codng tabl to obtan ncodd scrt mssag. Encodng scrt mssag ncrass th scurty of th stganography (Satr, 2012;Satr, 2014). Scond, mbddng algorthm dtrmns th locatons of th spac charactrs n th txts of covr txt fl. Lt, total numbr of locatons for SPACE charactrs s t. Assum that thr ar k numbr of bnary bts n th scrt mssag, whr k t. Thrd, k numbr of locatons of spac charactrs s chosn. Locaton of th spac charactrs ar slctd randomly basd on a psudo-random squnc. Th psudo-random squnc s gnratd usng a scrt ky. Th scrt ky s usd as th sd for gnratng psudo-random squnc. Sam scrt ky s usd to gnrat psudo-random squnc durng xtracton procss. Fnally, bnary bts of ncodd scrt mssag ar rplacd by nvsbl Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

5 Hghly Imprcptbl and Rvrsbl Txt Stganography Uncod charactrs basd on a rul obtanng Stgo txt fl. A stgo ky s constructd ncludng ncssary nformaton that s rqurd to xtract scrt mssag. Th xtracton algorthm has multpl stps as wll (s Fgur 1(c)). Frst, th xtracton algorthm dntfs th locatons of nvsbl charactrs. Scond, a psudo-random squnc of k lmnts s gnratd usng th sam scrt ky (.. sd) that s usd n th mbddng procss. Thrd, f an lmnt of th squnc s nvsbl charactr thn th charactr s rplacd by corrspondng bnary bt and stord as ncodd scrt bt. Th nvsbl charactrs ar rplacd by a SPACE charactr n th stgo txt fl. As a rsult, th covr txt fl s rconstructd from th stgo txt fl wth any loss of nformaton. Fnally, th xtractd ncodd scrt mssag s dcodd usng codng tabl and scrt mssag s obtand. Contrbutons of our work ar as follows: 1) Th Scrt mssag s mbddd as nvsbl charactrs. As a rsult, no alpha-numrc charactrs n stgo txt fl ar dstortd. Thrfor, mprcptblty of th stgo txt fl bcoms hghr as th prsnc of scrt mssag cannot b prcvd. 2) Our proposd mthod succssfully rconstructs th covr fl aftr xtracton of scrt mssag. Hnc, our work s fully rvrsbl or losslss. 3) Szs of covr txt fl and stgo txt fl ar sam n our proposd mthod. Thrfor, payload s zro n our mthod. 4) Th scrt mssag s dsgusd usng Huffman Cod. Hnc, th lngth of scrt mssag s rducd and mbddng capacty s mprovd. Th rst of th papr s organzd as follows. W brfly dscrb rcnt rlvant work n Rlatd Work scton. In nxt Scton, w dscuss our proposd hghly mprcptbl and rvrsbl txt stganography mthod. W show th rsult of our xprmnts and dscuss th prformanc of our proposd txt stganography mthod n Exprmntal Rsults and Prformanc Analyss Scton. Fnally, Scton Concluson concluds th papr. Rlatd Work Svral txt basd stganography schms hav bn proposd tll dat. W dscrb som of th rcnt rsarch works rlatd to txt stganography approachs. Rsarch works n (Jn'An, 2008;Yu, 2008) convrts th txt fls nto dgtal mags frst. Latr, th scrt mssag s mbddd by modratly adjustng th spacng of lttrs, words, or lns. Th dsadvantag of convrtng txt fls nto mags s that th txts n covr fl cannot b modfd furthr onc convrtd nto mags. A dgnratng documnt contnt basd txt stganography mthod s dscussd n (Lu, 2007). Txt sgmnts ar dgnratd by nfror wrtng. Nxt, th documnt s rvsd usng chang trackng n Mcrosoft Word documnts. Th scrt mssag s mbddd n th chang trackng and th stgo fl s snt to th rcvr. Th rcvr xtracts th scrt nformaton by usng chang trackng n Mcrosoft Word documnts. Th lmtaton of ths approach s that larg amount of dtng ruls nds to b stord. Morovr, th hdng capacty s vry low. Anothr documnt basd work n (Por, 2012) that s rfrrd as UnSpaCh. Th work mbds scrt nformaton n Mcrosoft Word documnt. UnSpaCh consdrs combnaton of ntr-word, ntr-sntnc, nd-of-ln and ntr-paragraph spacng to mbd scrt nformaton n a documnt. In UnSpaCh, a two bts combnaton of scrt nformaton s rplacd by a Uncod spac charactrs and mbddd n th covr documnt. Th mappng of bt combnatons and Uncod spac charactrs ar shard as a ky pror to th communcaton. In ordr to mprov th scurty of th stganography mthod, th scrt nformaton s ncryptd bfor data mbddng. Howvr, ths approach cannot b usd for othr txt fl format. Addtonally, mbddng scrt mssag ncrass sz of stgo txt fl than covr txt fl. Comprsson basd txt stganographc mthods ar addrssd n (Chn, 2010; L, 2013;Satr, 2012;Satr, 2014). Th work n (Chn, 2010) hds th scrt mssag n th comprsson cods usng Lmpl-Zv-Wlch (LZW) mthod. Ths schm mbds scrt data n LZW comprsson cods by rducng symbol lngth. Anothr rsarch work n (L, 2013) proposd a losslss txt stganography mthod by Huffman Comprsson Codng. A modfd Huffman Codng, namd varabl Huffman codng, s usd n ths work to gnrat codwords for symbols n th covr fl. Scrt data s thn mbddd n th codwords. Rsarch works n (Satr, 2012) and (Satr, 2014) us LZW and Huffman cod basd comprsson tchnqus, rspctvly. Emal s consdrd as covr mdum. Scrt mssag s mbddd n txts of mal body from th prvously constructd txt bas. Latr, mal s Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

6 Hghly Imprcptbl and Rvrsbl Txt Stganography comprssd to produc stgo txt. Th aformntond comprsson basd stganography mthods comprss th covr fl to ncras mbddng capacty and rduc transmsson cost. Howvr, th dsadvantag of comprssng covr fl s that th mprcptblty of stgo fl s vry poor. As a rsult, stgo fl bcoms mor pron to attack. Morovr, codwords nd to b rcalculatd f th contnt of th covr fl s changd. Proposd Rvrsbl Txt Stganography Approach W dscuss our proposd rvrsbl txt stganography approach n ths scton. W us txt fl as our covr mdum. W assum that w hav a txt fl H that has p charactrs. Formally, H can b dnotd as H h, h,, h }. A scrt mssag M contans m bnary bts and dnotd { 1 2 p by M { b1, b2,, bm }. Our proposd schm has thr man stps: (1) ncodng scrt mssag, (2) mbddng ncodd mssag nto covr txt fl, and (3) xtractng ncodd scrt mssag and rconstructon of covr txt fl. Th aformntond stps of our proposd mthod ar dscrbd n th followng subsctons. Fgur 2. Gnratd Huffman Tr for Bnary Scrt Mssag Encodng Scrt Mssag W us Huffman codng (Huffman, 1952) to ncod scrt mssag M. Stps of ncodng M ar dscrbd blow: Stp-1: Idntfy st of unqu symbols S s, s,, s } n M and corrspondng Stp-2: Stp-3: { 1 2 n frquncs F f, f,, f }, whr n s th numbr of unqu symbols n M. In ordr to { 1 2 n do ths, choos a postv ntgr d that dvds th numbr of bts m n M (.., m and m 2 d 2 valu of n s d ). Hr, d dtrmns th numbr of bts n a symbol. Thrfor, maxmum 2 d. Sort lmnts s of S n ncrasng ordr accordng to thr occurrnc frquncs f n F, whr 1 n. Rmov th two last frqunt lmnts from S and mak ach lmnt a laf nod. Nxt, crat a nw nod as a parnt nod of th two laf nods wth a frquncy computd by summng th frquncs of ts two chld nods. Two bnary bts, 0 and 1, ar thn assgnd to th lft dg and rght dg of th nw nod, rspctvly. Stp-4: Insrt th nw cratd nod and ts frquncy nto st S and F, rspctvly. Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

7 Hghly Imprcptbl and Rvrsbl Txt Stganography Stp-5: Stp-6: Rpat Stps 2-4 untl th root nod s obtand. Hnc, th Huffman tr s gnratd. Each symbol s s assgnd a unqu bnary strng (codword) by travrsng th Huffman tr from th root nod to ach laf nod aftr constructng th Huffman tr. Formally, th st of codwords s dnotd by W w, w,, w }, whr n s th numbr of symbols as { 1 2 n wll as codwords. A codng Tabl T s obtand that can b xprssd as T S F, W,. Stp-7: Substtut th orgnal symbols by corrspondng codwords accordng to T obtanng an ncodd scrt mssag M { m 1, m 2,, m k }, whr m s a bt of ncodd scrt mssag( 1 k ) and k s th numbr of bts of ncodd scrt mssag ( k m ). Tabl 1. Th Codng Tabl, T Symbol Codword Frquncy Tabl 1. Th Codng Tabl, T Exampl 1: Encodng Scrt Mssag Assum that w hav th followng bnary scrt mssag for ncodng usng th aformntond stps: Th lngth of bnary scrt mssag s m mssag s d 4. Hr, d m and m 2 d 2. Lt, th numbr of bts n a symbol of th scrt. Hnc, th maxmum numbr of unqu symbols s Th st of unqu symbols n bnary scrt mssag s S = {0000,0001,001 0,001 1,01 00,01 01,01 1 0,01 1 1,1 000,1 001,1 01 0, }. Hr, th numbr of unqu symbols n gvn bnary scrt mssag s S 12. Th st of frquncs of symbols n S ar F = {4, 3, 3, 5, 5, 1, 2, 3, 1, 1, 1, 1}. Rsultng Huffman tr for th bnary scrt mssag s shown n Fgur 2. Nxt, codword s obtand for ach symbol n S from th Huffman tr and a codng tabl T s gnratd. Th codng tabl T s prsntd n Tabl 1 contanng symbols n bnary scrt mssag, thr corrspondng codwords that s obtand from th Huffman tr and frquncs. Now, ach symbol of bnary scrt mssag s rplacd by corrspondng codword statd n T. For xampl, symbol ' ' s rplacd by ' 11 1', ' 0001 ' s rplacd by ' 001 ', and so on. As a rsult, w gt th followng ncodd scrt mssag for th gvn scrt mssag: Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

8 Hghly Imprcptbl and Rvrsbl Txt Stganography Th ncodd scrt mssag has 1 01 bts,.. k 1 01 usng Huffman codng rducs scrt mssag sz. Embddng Scrt Mssag Th obtand ncodd scrt mssag M. Thrfor, ncodng th bnary scrt mssag s mbddd n covr txt fl H to gnrat stgo txt fl H. Formally, H s dnotd as H h, h,, h }, whr p s th numbr of charactrs n H. Th { 1 2 p stps of mbddng scrt mssag ar statd blow: Stp-1: Copy all th charactrs of H nto H. Stp-2: Fnd a st E that contans ndxs of SPACE charactrs n H. Formally, E s dnotd as E { 1 j t,1 p, h SPACE }. Hr, t s th total numbr of SPACE charactrs n H. j j j Stp-3: A psudo-random squnc R wth k lmnts s gnratd usng a psudo-random squnc gnrator sd. Th valu of an lmnt of R must b btwn 1 and p. Formally, R s dnotd as R { r 1 k,1 r p } r { j 1 j t } ).. Hr, r rfrs to an ndx of E (.., Stp-4: Each m s mbddd at j th ndx of H such that r { j 1 j t }. Instad of mbddng a scrt bt drctly, an nvsbl charactr s mbddd. Assum that w hav two nvsbl charactrs: and. In ordr to mbd a scrt bt m, an nvsbl charactr rplacs a SPACE charactr at h j usng th followng rul: h j f f m m 0 1 (1) Fnally, th stgo txt fl H s obtand. Fgur 3. Rlatonshp among H, H, E, R and M Gnraton of Stgo Ky A stgo ky K s constructd at th nd of mbddng procdur n ordr to xtract scrt mssag and rconstruct covr txt fl. Th stgo ky ncluds codng tabl ( T ), numbr of bts n ncodd scrt mssag ( k ), sd ( ) for gnratng psudo-random squnc and valus of two nvsbl charactrs and. Formally, K s dfnd as a tupl K T, k,,,. Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

9 Hghly Imprcptbl and Rvrsbl Txt Stganography Exampl 2: Embddng Scrt Mssag Consdr that w hav a txt fl H. W plan to mbd ncodd scrt mssag numbr of bts n M s 1 01 M wthn H. Lt, k. Th contnt of H s copd to anothr fl H. Nxt, a st E s gnratd contanng th ndxs of spac charactrs n H. Assum that numbr of spacs n H s 1 85 ( t 1 85 ). Hnc, Th numbr of lmnts E s 1 85 (.., E 1 85 ). A psudo random squnc R wth 1 01 lmnts s gnratd usng a sd 119. Hr, lmnts of R rang btwn 1 and An lmnt of R rfrs to an ndx of E. For xampl, th frst lmnt of R rfrs to th 19th ndx of E, th scond lmnt of R rfrs to th 7th ndx of E, and so on. Nxt, a bt m of ncodd scrt mssag M s to b mbddd n H. Th frst bt 1 of ncodd scrt mssag s to b mbddd at th ndx dnotd by th 19th lmnt of E. Consdr, 19th lmnt of E contans a valu Thrfor, th frst ncodd scrt bt 1 should b mbddd at ndx 7 61 of H. Rlatonshp among H, H, E, R and M s llustratd n Fgur 3. Instad of mbddng a bnary bt, w rplac a bnary bt by an nvsbl charactr accordng to a rul spcfd n Equaton 1. Consdr that w slct SPACE and NULL as two nvsbl charactrs,.., SPACE and NULL. If a bt of th ncodd scrt mssag s 0, thn SPACE charactr of H at a locaton accordng to psudo random squnc s rplacd by SPACE. On th othr hand, SPACE charactr of H at a locaton accordng to psudo random squnc s rplacd by NULL If a bt of th ncodd scrt mssag s 1. Prformng th opraton for all of th ncodd scrt bts, H bcoms th stgo txt fl. Aftr gnratng th stgo fl, a stgo ky K T, k,,, s constructd as K T, 1 01,119, SPACE, NULL. Extractng Scrt Mssag and Rconstructon of Covr txt fl Th objctv of ths procdur s to xtract th scrt mssag from stgo txt fl H h, h,, h }, { 1 2 p whr p s th numbr of charactrs n H. Addtonally, covr txt fl H s rconstructd n ths procdur. Extracton of scrt mssag and rconstructon of H rqurs stgo ky K. Th stps ar dscussd blow: Stp-1: Fnd a st E that contans th locatons of nvsbl charactrs n H. Formally, E s dfnd as E { 1 j t,1 q, h h }. Hr, t s th total numbr of and. j j j j Stp-2: A psudo-random squnc R wth k lmnts s gnratd usng sd. R s dnotd as R { r 1 k,1 r q }. Hr, r rfrs to an ndx of E (.., { j 1 j t } ). Stp-3: Crat a st M { m, m,, m } of lngth 1 2 k k to hold th xtractd ncodd bts of scrt mssag. r Stp-4: Chck th j followng rul: charactr of H (.., h ) whr j r, and nsrt 0 or 1 at m basd on th j m 0 1 f f h h j j (2) Stp-5: Substtut j th charactr of H by SAPCE charactr f Stp-6: Rpat Stp 4-5 for ach txt fl. H s th rconstructd covr txt fl. h. j r R to obtan ncodd scrt mssag M and rconstruct covr Stp-7: Crat an mpty st M to hold dcodd scrt mssag bts. Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

10 Hghly Imprcptbl and Rvrsbl Txt Stganography Stp-8: Each codword s dntfd n M and dcodd usng th codng tabl T. Each dcodd symbol s addd to M. Fnally, bnary scrt mssag M s obtand. Exampl 3: Extractng Scrt Mssag and Rconstructon of Covr txt fl Assum that w hav stgo ky cratd that contans ndxs of nvsbl charactrs K T, k,,, as K T 1 01,1 1 9, SPACE, NULL,. A st E s SPACE and NULL. Th numbr of lmnts of E s E Subsquntly, a psudo-random squnc R wth k 1 01 lmnts s cratd usng sd Hr, R s smlar to th psudo-random squnc R that s gnratd durng mbddng procdur du to sam sd Elmnts of R rang btwn 1 and An lmnt of R rfrs to an ndx of E. Nxt, a st M wth k 1 01 lmnts s cratd for holdng ncodd scrt bts. Assum that frst lmnt of R s 19. Hnc, charactr at a locaton of H that s pontd by th 19th lmnt of E s chckd and mbddd bt s rtrvd usng Equaton 2. If th charactr s SPACE thn add 0 to th frst ndx of M. Add 1 to th frst ndx of M f th charactr s NULL, and rplac NULL charactr by SPACE charactr. Sam procdur nds to b followd for vry lmnt of R. As a rsult, w gt a st of ncodd scrt mssag rconstructd covr txt fl H, and M and H H. Aftrwards, an mpty st M s cratd. Nxt, ach codword n M s dntfd and dcodd usng codng tabl T (s Tabl 1). Fnally, w gt M as bnary scrt mssag. Exprmntal Rsults and Prformanc Analyss A st of xprmnts ar carrd out to valuat th prformanc of our proposd txt stganography approach n trms of thr paramtrs: mbddng capacty (EC), bt rror rat (BER) and mprcptblty. In partcular, EC masurs bt rat that s th sz of scrt mssag rlatv to sz of covr txt fl (Dsoky, 2009). EC s calculatd as follows: BER of xtractd scrt mssag s a masur that dtrmns th corrcton capablts of a stganography approach. BER s th rato (n prcntag) btwn numbr of bts that s xtractd corrctly and numbr of bts of orgnal scrt mssag (Abuadbba, 2016). BER can b dfnd as follows: Imprcptblty of a stganography mthod dtrmns th lvl of prcpton of th prsnc of scrt mssag nsd a fl (Subhdar, 2014). Th mprcptblty of a txt basd stganography s hghr f rlatv changs of charactrs btwn covr and stgo txt fl ar lowr. Exprmnts ar prformd on Intl(R) Cor(TM) GHz prsonal computr wth 8 GB RAM. Softwar for th proposd mthod s dvlopd n Java programmng languag. A larg txt fl s chosn as covr txt fl from URL: Dffrnt szs (n bts) of scrt mssag ar xamnd to valuat th prformanc of our proposd approach. In Tabl 2, mbddng capacty (EC) bfor and aftr ncodng th scrt mssag has bn dmonstratd. Th sz of our sampl covr fl s bts. W xamn th mbddng capacty of our proposd mthod for multpl scrt mssags wth dffrnt lngth. It s found that mbddng capacty s mprovd aftr ncodng th scrt mssag. Tabl 3 llustrats valus of bt rror rat (BER) of our proposd mthod for dffrnt scrt mssags. It s found that BER s 0% for ach scrt mssag. (3) (4) Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

11 Hghly Imprcptbl and Rvrsbl Txt Stganography In Fgur 4, t s dmonstratd that th prsnc of scrt mssag n stgo fl cannot b prcvd. Fgur 4(a) and 4(b) rprsnt covr fl and stgo fl rspctvly. Hnc, th mprcptblty of our proposd mthod s vry hgh. Covr Fl Sz (n bts) Tabl 2. Embddng Capacty of Proposd Mthod Scrt Mssag Encodd Scrt EC (%) Sz (n bts) Mssag Sz Bfor (n Bts) Encodng Tabl 2. Embddng Capacty of Proposd Mthod EC (%) Aftr Encodng Tabl 3. Bt Error Rat (BER) of Proposd Mthod Scrt Mssag Sz (n bts) Bts Rtrvd Corrctly BER (%) Tabl 3. Bt Error Rat (BER) of Proposd Mthod (a) Fgur 4. Dmonstraton of Imprcptblty of Proposd mthod. (a) Covr Txt Fl and (b) Stgo Txt Fl (b) It s worthwhl mntonng that sz of covr fl and stgo fl n our proposd stganography mthod ar sam. Thrfor, our stganography mthod has no payload. Addtonally, th prcntag of charactrs of covr fl that ar succssfully rconstructd aftr xtracton of scrt mssags and Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

12 Hghly Imprcptbl and Rvrsbl Txt Stganography th numbr of charactrs n covr fl s always sam (s Fgur 5). Hnc, our proposd txt stganography mthod s fully rvrsbl. In Tabl 4, our proposd mthod s compard wth othr mostly rlvant rsarch works. Comparson s prformd n trms of mprcptblty and payload ncurrd. Rsarch works n (Chn, 2010;L, 2013;Satr, 2012;Satr, 2014) comprss covr fl to produc stgo fl. Hnc, mprcptblty bcoms vry poor. On th othr hand, our proposd mthod usd comprsson codng tchnqu to rduc th lngth of scrt mssag. Thrfor, mprcptblty of our proposd mthod s vry hgh. Th work n (Por, 2012) hds scrt mssag as nvsbl charactr wthn words, sntncs and paragraphs. Th sz of stgo fl s hghr than covr fl that rsults n payload. In contrast, our proposd mthod has 0% payload. Fgur 5. Rvrsblty of Proposd Mthod Tabl 4. Comparson of Imprcptblty and Payload Crtra Mthod Dscusson Imprcptblty Payload Concluson (Chn, 2010) (L, 2013) (Satr, 2012) (Satr, 2014) Our proposd mthod (Por, 2012) Our proposd mthod Stgo txt fl s producd by comprssng covr fl. Thrfor, mprcptblty s vry poor and pron to attack. Scrt mssag s ncodd usng comprsson codng tchnqu and mbddd n covr fl to produc stgo ky. Thrfor, mprcptblty s vry hgh. A block of scrt mssag s mbddd as a block of nvsbl Uncod charactrs wthn words, sntncs and paragraphs. Th sz of stgo fl s hghr than that of covr fl. A bt of ncodd scrt mssag s mbddd as an nvsbl charactr btwn words by rplacng spac charactr. Payload s 0% as th fl szs of stgo fl and covr fl ar sam. Tabl 4. Comparson of Imprcptblty and Payload Outsourcng txt-basd busnss documnts to cloud arss prvacy concrns for busnss organzatons. Svral cryptography mthods ar bng usd for protctng prvacy and authntcty of snstv txt-basd data n cloud. Howvr, opratons ovr cryptographcally ncryptd data ar usually complx and tm consumng. Morovr, mannglss form of ncryptd data may attract th attnton of ntrudrs. Altrnatvly, stganography mthods can b usd to provd prvacy and authntcty of outsourcd txt-basd busnss data. In ths papr, w hav proposd a novl txt stganography tchnqu. W us Huffman codng tchnqu to ncod scrt mssag bfor mbddng. As a rsult, sz of scrt mssag s rducd. Th scrt mssag s mbddd n th covr txt fl as nvsbl charactr wthout affctng th txts n covr fl. Thrfor, our proposd Twnty Frst Pacfc Asa Confrnc on Informaton Systms, Langkaw 2017

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