to c o
3 C.
Cu
J2
200 STEGANOGRAPHY
Tu steganography gdc Hy lap cd nghia Id "0iu thu" kdt hpp til hai ttt steganos nghia Id "in nip dd bio vd" vd giapheui nghla Id "vidt". Thudt ngft steganography dupc Jphannes Trithemius su dung lin diu ndm 1499 ttong cudn sdeh Steganogiaphia, mdt ludn vdn vd mdt ma vd ky thudt 0 i u thu dupc nguy ttang du0 dang mdt cudn sach ma thudt.
Ky thuit 0 i u tin steganography Id mdt ky thudt
"0iu" thdng tin vdo mdt "vdt mang tin" ndo dd md khdng hi phdt hidn.
Cd nludu qui ludt 0 i u thdng tin khdc nhau eOng nhu vdt mang tin cflng rit da dang. Mdt vi du cd 0dn ttong Unh vuc steganography Id thdng 0dp bl mdt dupc giii bm Herodotus ndm 440 trade cdng nguydn khi dng dang bi 0am 0fl rit cin mdt. Ong eao diu vd xdm ehft ttdn da diu mdt nd Id tin ein ciia minh, rdi chd cho tde cua nd Id dd mpc ldn, nhd viy md qua mdt dupc nhflng ngu0 Unh gdc. Mdt vi du Wide hay dupc nhic ddn nhidu hon Id dimg nudc xd phdng vd vidt
sd lit khd bi phdt hidn. Mudn xuit hidn ddng ehft ttd 10, ehi vide nhung td 0 i y dd vdo nudc. Ldm hodn todn tuong ta vai nudc chanh (thay nudc xd phdng) cflng sd dupc mdt cdch nguy tiang khdo ldo ddng ehft tidn 0iy. Dd ldm hidn 10 thdng tin dd vidt ttdn 0 i y thi ldm ndng td 0 i y bing each ho trdn lua.
Cd thd ndi, ky thudt an tin da dupc phat hidn vd su dung idng idi tft xa xua. Tuy nhidn, trade nhiing ndm 1990, ky thudt steganography ndy chua thdt su
Hinh 1: Si/phdt tri&i manh me vd s6 lujng bai bdo nghi&i oju vd steganography dujc xudt bdn trong tdp chi EEE trong nhijig nam qua (1 ]
CONG NGHE - G I A I PHAP
dupe coi Id mdt ngdnh khoa hpc cflng nhu thu hut dupe nhidu nghidn cuu ttdn thd 0di. Nd c 0 thue su ldn manh vd cd nhiing thdnh tuu vurt bde kd ttt khi ed su chuydn ddi ttt cdng nghd tiiong tii sang cdng nghd sd. Chu dd vd steganography dupc dua vac chuong ttinh hdi thao hang ndm eua IEEE nhu tm hdi n g 0 ldn SPIE hay ACM Multimedia and Security (HUih 1). Cd tiid thiy, sd lupng b 0 bdo nghidn cuu vd ky thudt steganography tdng gip ddi hdng ndm.
Tuy ky ttiudt i n Un (steganogiaphy) vd ky thudt md hda (ciyptography) khd 0dng nhau vi ciing Id nhftng ky thudt traydn thdng 0dp mdt each an todn 0fta ngudi gm vd ngudi nhdn md khdng hi ngu0 khde phdt hidn. Tuy nhidn, ciyptography cd thd ldm eho ndi dung eiia thdng 0dp 0 che diu khdng thd dpe dupe du0 dang hi bidn ddi nhung vin ed thd nhdn ra dupe la ed su hidn huu cua thdng didp bi che diu nay. Cdn steganogiaphy Id che diu su tdn tai/hidn hftu ciia thdng 0dp-
2 . s a DO HE THONG T H L T C H I E N G I A U T I N STEGANOGRAPHY VA PHAN LOAI
Hd thdng steganography dupc chia thdnh: pUa ngu0 gm vd p0a ngudi nhdn. Tiong dd, d pWa ngudi
StegoKayK
Covw C
EmbwJdKllnte E
StegoKtyK
•ncod*r(lk)
Stego Channel
Sender Receiver
Hinh 2: Cdc thanh phdn cdu thanh ten h$ thfing steganography
nhdn bao gdm ede khdi chttc ndng:
DOT lieu can m a hoa ( E m b e d d e d i n f o ) : Id
thdng tin cin i n giiu vd gui 0 md khdng bi phdt hidn.
Dft Udu a ddy ed thd la mdt file text (doan tm), mdt file anh,v.v. Dd tdng dd an todn eiia thdng tin, ttxidc khi duoc didn vdo vdt mang tm, thdng tin ndn dupc ma hda bing cac thudt todn md hda nhu Rijindael ttong AES (Advanced Eneiyptton Standard), DES (Data Encryption Standard), Huflman ...
- vat/file mang tin (Cover): cd tod Id mdt btte anh sd, mdt doan nhae/bdi hdt sd, video v.v. Dft Udu ein md hda se dupc chdn vdo vdt mang tin ndy. Hinh 3 trinh bdy su phdn bd ede ung dung tiong thuc td sii dung ede lom vdt mang tin khdc nhau. Ta cd thd thiy 56.1% img dung dimg vdt mang tin Id hinh anh, tidp theo Id 14,8% img dung dimg vdt mang tin Id dm thanh. Cdc img diing dttng vdt mang tin khde nhu file vdn ban, dimg khdng gian 0 a cttng (disk space), dimg video v.v chidm khoang 29%.
- Khoi ma hoa (Stego encoder): Khdi ndy se thiic hidn mdt thudt todn steganography dd chdn dfl Udu cin ma hda vdo file mang tui. Thudt todn steganography phm thda man 0du Mdn Id khdng anh hudng ddn chat lupng eua file mang tin vi nhu vdy thi v0 cam nhdn bmh thudng se khdng thd phdt hidn la su thay ddi ndo ciia file mang tin sau khi da dupc ma hda. Khdi ma hda ndy cd thd ein mdt khiu (passwoid) phuc vu cho qud ttiiih chdn thdng tin dd tdng tinh bao mdt cho hd thdng. Khdi tao khda (stegoKey) tiiy 0udc vao thudt todn steganogiaphy ma cd thd cd hode khdng.
Cdn dpMa ngu0 nhdn, khoi giii ma (stego decoder)
Embeddwilnfo E
4 4 TAP CHi CNTT & TT KY 2 (9.2012)
Disk -.1
Cover type Audio Disk space
Images Network Other files
Text Video
Count 445 416 1689
39 81 255
86
sd.
Hinh 3: T;^ 1$ cdc Ong dgng dung kj thudt steganography dga trfin "vdt mang tin";
dm thanh (audio), hinh dnh (image), mang (networt<), video, file van bdn (text),...
cd chttc ndng ngupc Im v0 khdi md hda o phia ngudi gid. Khdi 0 0 md tidn hdnh xu U file mang tin thu dupc (cdng v0 mdt khiu dimg) dd 10 tac 10 thdng tin gdc.
Dua tidn "tinh bdn vftng eua dfl Udu i n giiu", cd thd phdn 1P0 cdc ky thudt steganogiaphy thdnh 2 lom sau:
- Tlnh bdn vflng thip (fragile): NhOng ky thudt md 0dng tin in 0 i u tiong file mang tin sd bi mit ndu nhu thay ddi file mang tin. Vi du nhu file mang tin Id hinh inh bode dm thanh thi dd bdn vftng ydu vi chi cin mdt thao tac sua hay thay ddi thdt nhd file mang tin ndy Uii thdng tin in diu bi phd hiiy vd khdng thd khdi phue 10.
- Tinh bdn vftng eao (robust): Thdng tin an giiu khdng dd ddng bi phd htty du cd su thay ddi file mang tin vi du ttong ky ttiudt tiiuy vdn (wateimaik).
Dua tidn Mdu "dfl Udu in giiu" vd Mdu "vdt mang Un" ta cd tiid chia hd thdng steganography thanh hai lo0 sau:
Steganography dfl Udu ngdn ngfl (Linguistic Steganography) - thudt giiu t n vdi ngdn ngfl Trong hd thdng niy, thdng 0dp cin md hda Id ngdn ngfl vd ddi tupng mang tin cQng Id ngdn ngft (doan vdn).
Steganogiaphy dfl Udu sd {Digital data Steganography) - thudt giiu tin vm dft Udu sd. Trong hd thdng ndy, thdng tin cin md hda vd vit mang tin ddu li dft Udu sd nhu Id file inh sd hode file dm thanh
Mdt Unh vuc hep ciia giiu tin Id thuy vdn sd (di0tal wateimaik) da phdt Oiln hodn thidn vd img diing Idng iai tiong cudc sdng. Trong ky thudt steganogiaphy thdng thudng, vdt mang tin ehi don 0an Id bttih nguy tiang vd khdng ed Udn quan 0 ddn thdng tin in 0 i u bdn trong nd.
Nhung tiong ky thudt wateimaik thi thdng tin in trong vdt mang tin sd mang thdng tin bd sung vd Udn quan ttiic tidp ddn vdt mang tm. Vi du khi mua mdt bdi hat mp3 trdn Uitemet, thdng tin vd ngudi ban vd/hodc ngudi mua (cdn gpi Id wateimmk payload) sd dupc chen vdo bdi hit mp3 dd md khdng lam thay ddi chit lupng dm thanh bdi hat.
Dua ttdn cdc thdng tm wateimaik ttong file mp3 ndy md cd thd theo ddi nhung hdnh vi copy bai hdt bit hpp phdp. Ddng thm cdc thdng ttn wateimaik ndy cdn dupe dimg dd luu ttrft thdng tin chi tidt vd bdi hat. Sau nay, thdng qua vide so sdnh wateimaik luu tiong file mp3 v0 thdng tin wateimaik payload chuan dd Mdm tia tinh nguydn ven (khdng hi thay ddi) cua ndi dung bdi hat.
Hon nfta, tiong ky thudt steganography traydn thdng, vdt mang tin ed chtta thdng ttn bi mat khdng
= - ^ = ^ g ^ ^ ? - ' - - - ^ - ^
^***'*'yttrT*iT^ ' " ' I " - 7 " ^ ' ^
Hinh 4: MOt dang don gidn aJa watermart< sii ding t i a ^ van bdn
CONG NGHE - G I A I PHAP
cd mdt diu hidu ndo khac bidt v 0 mdt vdt mang tin binh thudng khde vd khdng ai nghi ring ed thdng tin mdt i n chua tiong vdt dd. Nhung ttong watermark, su hidn didn ciia thdng tin watermark thudng dupc thdng bdo id idng vd kdm theo nhftng khuydn eao vd vide khdng dupe sao chep, phdn phdi bit hpp phdp file mang tin nay. Tiong ky thudt watermark, ehi ein mdt lupng thdng tin nhd (vi du 1 bit thdng tin watermark payload) Id cd the Mem tta dupc tinh hpp phdp vd SP hiiu (owneiship) eua bue anh sd, mdt bdi hdt sd hay mdt doan video.
Thdng tin in wateimaik ed tinh bdn vftng eao ndn thudng dupe ung diing ttong cdc Unh vuc nhu bao vd bin quydn cic tde phim sd hda (sdeh, ttanh anh, phim, nhac sd).
3. MOT s o KY THUAT G I A U T I N PHO BIEN LCNG DUNG TRONG DA PHl/ONG T I E N SO
Giiu tin tiong anh la mdt bd phdn chidm ty Id ldn nhit tiong eac ehuong trinh ung diing, cae phin mdm.
Cffnr Imaga
Sicral Imaga
Sicga Object
K.y
1^
Communicatiani ChanntI Original
C m r
Hinh 5: H$ thdng ma hda (encoder) mOt buc dnh bi mat (secret image) vdo trong mOt buc dnh mang tin (cover image) c6 sir dgng mdt l<hdu l<hi chen tin (l<ey). Phia ngudi nhdn, cdn c6 c6 bp gid ma (decoder) vd mdt l<hdu dd tdch buc dnh bi mdt ra I<h6i buc anh mang tin. Thdng tin dn ddu trong hd thfing ndy c6 thd
la mOt bic dnh ho$c mfit file van bdn ddu hoat dijng dupc
he thdng 0 i u tin trong dft Udu da phuang tidn b 0 lupng thdng tin dupc trao ddi bing anh Id lit ldn. Hon ntta, giiu thdng Un trong anh cflng ddng vai tid hdt sire quan ttpng trong hiu hdt cdc img dung bao vd an todn thdng Un nhu: nhdn thttc thdng tui, xic dinh xuydn tae thdng tin, bao vd ban quydn tdc 0 i , 0du khidn tray nhdp, 0 i u thdng tin mdt [1].
a. Ky thuat giSu thong tin trong file anh Ky thudt nay 10 dung dd "du thtta" ttong inh dd nhimg cac bit thdng 0dp mdt vdo dd, do su "du thtta" ndy se khdng lam thay ddi ddn chit lupng hue anh vd v 0 mit ngudi thi khdng thd nhdn ra su thay ddi nay. Dd tdng tinh bao mdt eho thdng tin mdt, chttng ta cd thd "ma hda" btte thdng 0dp trade khi thue hidn nhung chttng vdo anh.
+ Ky thuit chdn thdng tin vio bit thip c&a 0dm inh - LSB (Least Significant Bit insertion)
Mdi buc anh sd thue chit Id mdt ma ttin cae 0dm anh gpi la pixel. Mdi pixel ed thd dupe bidu 0dn bang 8 bits (eho anh den tiing) hode 24 bits (eho mdt anh mau RGB) - gdm cd ba thdnh phin mau la do (8 bit), xanh luc (8 bit) vd xanh lam (Shit). Khi dd, mdi bit cd ttpng sd thip nhit (Least Significant Bit - LSB) ciia mdi pixel ttong btte anh se dupe thay the bing thdng tin cin 0iu, nhu minh hpa ttong Huih 6. Ky thudt ndy thudng dimg cho cac btte 0 n h mang tin dang BMP hay GIF.
+ Ky thuit che (MasMng hay Patchwoik): Phuong phdp ndy se chia buc hinh ra timg vung, vi du vttng A vd B, ldi tit ci cdc pixel inh ttong vung A va B lin lurt dupc chdn bit thdng tin cin giiu vio. Khi dd cudng dd dnh sdng cua ca vttng A hode B se sdng ldn hode tdi 0 (tiiy vdo bit thdng tin 0 hay 1 duac
4 6 TAP CHi CNTT & TT KY 2 (9.2012)
pixels color values
loiooicfo
1011011 1010011 1010011 10101010 10110010 10101110 10111111
^ 0100 0001 = 'A'
Hinh 6: (Via hda cho 'A' vao cdc bit t h ^ LSB cua thinh phdn mau xanh lam (Blue) trong cdc pixels oja buc dnh mang tin
chdn vdo), nhung su thay ddi dd tuong phan ndy lit nhd vd mdt ngu0 khdng cim nhdn dupc. Nhupc 0dm eiia phuang phdp ndy Id chi chen dupe mdt lupng nhd thdng tin ein i n 0 i u . Phuong phdp ndy thudng su diing ede dinh dang anh JPEG.
+ Ky thuit dttng thuit toin bidn ddi (Transfonnattons): Su dung bidn ddi eosui ridng, dimg cho cdc btte inh mang tin ed dinh dang JPEG.
Dinh dang inh JPEG dung ttiudt tPdn bidn ddi DCT (Discrete cosine ttansformaUons) dd ndn inh. Diu tidn, bidn ddi mdi khdi pixels anh tiong anh mang tm bing thudt todn DCT rdi thay cdc bit ed tipng sd thip nhit eua mdi hd sd DCT vdi mdi bit cua thdng 0dp cin i n 0 i u . Thuc chit phuong phdp nay Id kdt hpp ky thudt LSB vd phuong phdp bien ddi DCT Miidn eho vide phdt hidn ra thdng tin an 0 i u ttd ldn khd khdn vd phttc tap hon.
b. Ky thu$t giSiu thong tin trong file Sm thanh
+ Ky thuit ma hda bit thip (Low Bit Coding)
Cic file dm thanh sd thuc chit Id mdt chudi tin hidu sd dupc tao ra bing cich Iiy miu ttt tin hidu dm thanh tuong bf. Ky thudt ma hda bit thip cho file dm thanh thuc chit
eung 0dng nhu phuang phdp ISB eho file anh bdng cdch thay the cac bit thip cua tin hieu dm thanh bdi eac thdng tin ein in giiu. Cae bit thip ndy cd thi dupc thay ddi eac 0a Oi tft 0->l hay l->0, nhung su thay ddi ndy tde ddng lit nhd ldn tin sd dm vd tai ngudi vm tin sd ngudng nghe la 20Hz - > 20KHz rit khd phdt hidn ra. Phuong phdp ndy cd thd chdn mdt lupng ldn thdng tm vdo file dm thanh nhung ed nhupc 0dm Id tao ra nhidu (noise) eho file mang tin dm thanh ban diu.
+ Ky thuit giiu thdng ttti trong dfl Udu Echo ciia dm thanh (Echo Hiding)
Cdc miu rdi rac cua tui hidu gdc ciia file dm thanh ban diu dupe chen thdm ede khoang vang (echo) ngdn dd tao ra file mang tin dm thanh mdi. Mdt thudt todn se ma hda thdng tin mdt vdo file mang tin dm thanh mdi ndy bing cdch thay ddi thdng sd (bidn dd - ampUtade, offset hay tde dd decay rate) eua tui hidu echo ndy. Tin hidu echo ndy du ngin dd tai ngu0 khdng cam nhdn dupc.
+ Ky thuit md hda pha (Phase Coding)
Ky thudt nay dua vdo tmh tiiong ddi khdng nhay cam eua hd thdng thinh gidc d ngudi khi cd su thay ddi Phase eiia tin hidu am thanh. Phuong phdp ndy se thay thd Phase ban diu eua mdt tin hidu dm thanh, sau khi da dupc phdn tidi te midn thdi gian sang midn tin sd, bing mdt Phase khde de bidu dien dfl Udu ein
0
Hinh 7: Thfing tin dn ddu trong mfit file mang tin dang dm thanh la mfit file text. Khi nhin vao d?ig tin hidu am giua file dm thanh gfic (trai) vd file am thanh chua thfing tin dn (phdi) thi
cfi thd thdy su khdc bidt; nhung tai nguOi thi khfing cam nhan dujc sy khac bidt nSy
CONG NGHE - G I A I PHAP
in giia Ky thudt ndy khi phue tap khi thue hidn vi it ldm thay ddi tin hidu mang tin.
4 . LTNG D U N G K Y T H U A T S T E G A N O G R A P H Y T R O N G T H U ' C T I E N
Cdng nghd thdng tin vd ddc bidt la su phdt tridn ciia hd thdng mang may tinh da tao ndn mdi tmdng mp vd Id phuong tidn tiao ddi, phdn phdi tm Udu mdt cdch tidn lpi, nhanh chdng. Tuy nhidn cung ddt ra mdt vin dd vd bao vd tdi Udu, ngdn chdn vide ddnh edp vd sao chdp t 0 Udu mOt edeh bit hpp phdp. Vin dd an todn vd bao mdt thdng tin hidn nay ludn nhdn dupc su quan tdm ddc bidt eua nhidu nhd nghidn cttu tiong nhidu Unh vuc.
Nhflng tdi Udu quan ttpng, nhttng thdng tin vd tdi khoan tin dung, mdt khiu hay nhimg sd Udu vd boat ddng Mnh dpanh ciia cdng ty... dupc luu 0fl ttdn mdy tinh cua ban ddu Id nhflng thdng tin ein 0fl kin vd ban hcdn tPdn khdng mudn bi lot vdo tay nhflng kd xiu ed y dinh chidm doat dd khai thdc vd sii dung.
Cdc phin mdm su dung ky thudt steganography (vi du ttivisible Secrets) sd ldm cho nhung thdng tm bi mdt ndy tid ndn "vd hinh" bing nhflng thudt todn eao eip.
I
oHinh 8: Sd lupng phdn mdm ing d^ng ky thudt steganography sdn xudt hang nam tang nhanh chfing
giiu tap tin hay thu muc vdo mdt tim hinh, mdt ban nhac hay mdt tiang web, tao cdc gdi dfl Udu ed khi ndng te 0m ma bdng mat khiu dd d§ ddng trao ddi qua email
Bdn canh nhflng img dung tich cue eua ky thuit giiu tin, ede td chuc tdi pham vd khiing bd ettag sii dung ky thudt ndy tiong vide Udn lae tiao ddi thdng tin vm nhau. Theo Chet Hosmei, chuydn 0a vi tinh dang ldm vide cho Cue Didu tia Udn bang (FBI), hiu hdt eic td chtte tdi pham hay khiing bd ttdn thd 0di ddu stt dung phuong thttc gm thdng 0dp Mn bing phuong phdp steganography. Ky thudt ndy 0up 0 i u vdn bin hay so dd ttong file hinh anh hay dm thanh dang MP3 nhd mdt phin mdm chuydn dung.
Trong tuong lat 1^ thudt giiu tin se phdt ttidn phin mdm dd ciu tao nhflng ldi vdn tuy rit dd dpc, nhung ehua dung nhflng thdng 0$p thdt bdn ttpng, bing eddi phdi hpp vd xii diing lihting ngfl vung la, quen vd nhflng ngdn te mo hd. Tuy nhidn, cdc chuydn gia chua thd bidt chdc chdn mdy tinh od kha ndng tao ndn nhflng 10 vdn y n0iia va in nhimg thdng 0dp that cua chiing ta vdo nhflng ldi vdn do khdng.
Hien nay ngodi mdt md hpc, steganogiaphy vd watermark dang phit tridn rit manh ttdn thd gidi. 6 ttong nudc thi ddy la mdt Unh vuc m0 dupc nghidn cttu ttong nhftng ndm gin day cua thd ky 21, vd cittig mdi dupc quan tdm chu ydu o mdt sd vidn nghidn cim khoa hpe, vd mdt sd tiudng Dm hpe ldn nhu Vidn (3dng nghe thdng tin, tiudng Dm hpe Cdng nghd thudc Dm hpe (Judc Gia Hd Ndi va TP. HCM, Dm hpc Da Ndng.
Tai lif u tham khao
[1]. JESSICA FRDRCH/'S/^gancgr^y in digital media — principles aigoritim and application", Camtiridge University Press, 2010.
[2]. "Msble Secrets-est gSunhuig tttng tin bim$t tren may thh", Nhdt Tan, ht1p-y/www.khoahocphothang.com.vn/
[3]. "Bi}nl<her^t$cc6 thd nation thdng tin trong cac file inh", H.F. (theo ABC), vwvw.vnexpress.net
4 8 I TAP CHi CNTT & TT KY 2 ( 9 . 2 0 1 2 )