HC VIN K THUT MT M N TT NGHIP
HC VIN K THUT MT M N TT NGHIP
HC VIN K THUT MT M N TT NGHIP ti: Nghin cu tm hiu nh gi cht lng
mt s lc kha trong m khiH NI 2010
HC VIN K THUT MT M
N TT NGHIP
ti: Nghin cu tm hiu nh gi cht lng mt s lc kha trong m
khiNgnh:Chuyn ngnh:Kha:Tin hc (m s 01.02.10)An ton thng tin02 (2005
2010)
Cn b hng dn khoa hc :Sinh vin thc hin:TS. Trn Vn TrngNguyn Vn
Thnh
H NI 2010
MC LC
LI NI U...6Chng 1: M U V M KHI81.1 Gii thiu chung v m khi81.2 an
ton ca cc h m khi101.2.1. Cc kiu tn cng111.2.2 an ton v iu kin v an
ton tnh ton111.2.3 phc tp x l v phc tp d liu ca mt tn cng c
th131.2.4 Cc tham s ca m khi141.2.4.1 di khi m141.2.4.2 di kha k v
c kha ng kt151.3 Cc ch hot ng ca m khi151.3.1 Vector khi to
IV151.3.2 Ch ECB161.3.3 Ch CBC171.3.4 Ch CFB181.3.5 Ch OFB191.3.6
Ch CTR201.4 Nguyn l thit k m khi211.4.1 Nguyn l thit k chung v an
ton211.4.2 Nguyn l thit k cho ng dng221.5 Cc cu trc m khi c
bn221.5.1 Cu trc m Feistel221.5.2 Cu trc cng-nhn24Chng 2: LC KHA CA
M KHI V MT S LC C TH252.1 Phn loi cc lc kho ca cc h m khi252.2 Mt s
lc kho mnh282.3 Chun m d liu X vit (GOST)292.4 Thut ton m d liu quc
t IDEA302.4.1 Lc kho ca IDEA322.4.1.1 Tnh tuyn tnh trong cc php ton
s hc MUL322.4.1.2 Lp kho yu bao hm yu t tuyn tnh342.4.1.3 Lp kho yu
vi sai372.4.1.3.1 Lp kho yu c c trng xc sut 1372.5 Chun m d liu
DES392.5.1 M t DES392.5.2 Mt s kin tho lun v lc kha ca DES512.5.3
Cc c ch hot ng ca DES53Chng 3 : CHUN M HA NNG CAO AES563.1 Tng quan
v chun m ha nng cao v thut ton Rjdael563.2 cc chc nng bn trong ca
mt m khi rijndael593.2.1 Cc byte nh cc a thc593.2.1.1 Cc biu din ca
byte593.2.1.2 Php cng 2 byte593.2.1.3 Php nhn 2 byte603.2.1.4. Php
dch chuyn vng ca t vo603.2.2. Cc chc nng bn trong ca mt m
Rijndael633.2.2.1. Chc nng thay th cc byte: SubBytes
(State)653.2.2.2. Chc nng dch chuyn cc dng:
ShiftRows(State)663.2.2.3. Chc nng xo trn cc ct: MixColumns
(State)663.2.2.4. Chc nng cng kha: AddRoundKey(State,
RoundKey)673.2.2.5 Chc nng m rng kha: KeyExpansion(CipherKey,
ExpandedKey)683.2.2.6. Chc nng to cc hng: Rcon[i] (Round keys and
constants)693.2.2.7. Chc nng M ha Rijndael703.2.2.8. Chc nng gii
m703.3 Tm tt vai tr cc chc nng bn trong Rijndael713.4 Thc hin nhanh
v an ton723.5 Mt vi ch v AES trong mt m ng dng733.6 Lc kha ca
AES733.6.1 Tnh tuyn tnh ca lc kha AES vi di kha 128 bt763.6.2 M t
ng n783.6.3 Kt lun v kha AES 128 bit813.7 Tng sc mnh cho lc kha
AES823.7.1 Cc lc kha ca m khi823.7.1.1 Hm 1 chiu83 3.7.1.2 Thng tin
tng h ti thiu83 3.7.1.3 Ci t hiu qu833.7.2 Lc kha ca AES843.7.2.1 M
t lc kha843.7.2.2 Thm m trc 853.7.2.3 Phn tch ca cc tc gi bi
bo853.7.2.4 Ci t883.7.3 Mt xut lc kha mi cho AES893.7.3.1 xut lc
kha 128-bit893.7.3.2 Hiu qu ci t ca lc kha xut913.7.3.3 Phn tch tnh
an ton ca lc kha xut933.7.4 Kt lun v vic lm mnh lc kha ca AES94KT
LUN....................................................................................................96TI
LIU THAM
KHO..............................................................................97
LI NI UNg y nay vi s pht trin mnh m ca Internet v cc ng dngca
n,nhu cu bo v thng tin trong cc h thng v ng dng cng c quan tm v c
ngha ht sc quan trng, v vy cc ng dng m ha v bo mt thng tin ang c s
dng ngy cng ph bin trong cc lnh vc khc nhau trn th gii, t lnh vc an
ninh, qun s, quc phng, cho n cc lnh vc dn s nh thng mi in t, ngn
hng, v tt c cc h thng thng tin thng dng khc.Cng vi s pht trin ca
khoa hc my tnh v Internet, cc nghin cu v ng dng ca mt m hc ngy cng
tr nn a dng hn, m ra nhiu hng nghin cu chuyn su vo tng lnh vc ng
dng c th vi nhng c trng ring,ng dng ca khoa hc mt m khng ch n thun
l m ha v gi m thng tin m cn bao gm nhiu vn khc nhau cn c nghin cu v
gii quyt, v d nh ch k in t, xc thc ngi dng ....Cc h mt hin nay c
chia thnh hai loi: h mt kha b mt v h mt kha cng khai. Trong h mt
kha b mt thng c chia thnh cc h m khi v h m dng. Cc h m khi c s dng
ph bin hn v d dng chun ha v do cc n v x l thng tin hin nay thng c
dng khi nh byte hoc words.Mc d m khi c s dng rng ri v kh an ton tuy
nhin vn c nhiu loi tn cng nhm vo bn thn c ch m cng nh thnh phn quan
trng nht y l lc kha ca m khi. V d nh tn cng kha quan h v tn cng trt
kha ca Binham, tn cng ni suy ca Ferguson...V vy mc tiu ca ti Nghin
cu tm hiu nh gi cht lng mt s lc kha trong m khi do TS. Trn Vn Trng
hng dn l nhm nghin cu lc kha v phng php lm mnh cc lc kha ca m khi c
th chng li cc kiu tn cng nhm vo lc kha.B cc ca ti gm c :Li m u Nu l
do s dng m khi , tm quan trng ca m khi trong vn m ha bo mt thng tin
t y a ra mc ch ca ti, li cm n Chng I: M u v m khi Trong chng ny gii
thiu qua v m khi, an ton, c kiu tn cng , c ch hot ng v nguyn l thit
k m khi.Chng II: Lc kha ca m khi v mt s lc kha c th Trong chng ny
gii thiu cc loi lc kha trong m khi v tm hiu mt s lc kha c th nh
GOST,IDEA,DESChng III: Chun m ha nng cao AES Trong chng ny gii thiu
tng quan v AES, chc nng cch thc hot ng,lc kha v nghin cu cc bi bo v
tnh tuyn tnh v lm mnh cho lc kha ca AES. Kt lun Trong ny nu ra nhng
kt qu nghin cu t c trong ti cng nh nhng vic cha lm c v phng hng pht
trin ca ti.Do thi gian c hn nn ni dung ca ti cn s si, kt qu dt c
cng cha nhiu rt mong s ng gp kin v nhn xt t pha cc thy ,c v cc bn.
Trong thi gian lm ti ca mnh, ti nhn c s hng dn v gip tn tnh ca thy
gio ,TS Trn Vn Trng.Xin gi li cm n chn thnh ti thy gio .
Chng I: M U V M KHI
1.1 Gii thiu chung v m khiNgy nay vi s pht trin ln mnh ca nn cng
ngh thng tin trn ton th gii, hu ht cc ban nghnh , t chc, cng ty u s
dng h thng thng tin trong hot ng ca mnh. V vy mt khi lng ln cc thng
tin c truyn trn cc knh thng tin v mng my tnh hin nay ang ngy cng
gia tng c bit i hi cn phi c bo v khi cc d r khng mong mun, tc l m
bo tnh b mt, ng thi cng cn phi c bo v trnh s gi mo v s t chi trch
nhim, tc l m bo tnh xc thc. K thut mt m c pht trin v vn dng m bo c
tnh b mt v tnh xc thc .Cc h mt hin nay c chia thnh hai loi: h mt
kha b mt v h mt kha cng khai. Trong h mt kha b mt, nhng ngi s dng
hp php (ngi gi v ngi nhn) phi chia s mt kha b mt chung v kha khng c
bit i vi thm m i phng. Trong h mt kha cng khai, ngi s dng hp php ch
cn cc thng tin trung thc cng khai no . Mc d cc h mt kha cng khai t
ra l l tng i vi nhiu ng dng mt m, nhng tc thp v gi thnh cao ngn cn
vic s dng chng trong nhiu trng hp. Trong phn ny chng ta ch tho lun
v cc h mt kha b mt.Chng ta s s dng m hnh h mt ca Shannon trong Hnh
1.1.Trong m hnh ny, kha b mt Z c phn phi ti ngi gi v ngi nhn theo
mt knh an ton. Kha ny sau c s dng m ha bn r X thnh bn m Y bi ngi gi
v c dng gii m bn m Y thnh bn r X bi ngi nhn. Bn m c truyn trn knh
khng an ton, v chng ta gi thit l thm m i phng lun c th truy nhp nhn
c cc bn m. Tt nhin thm m khng th truy nhp c ti kha b mt. H mt kha b
mt nh th c gi l h mt i xng phn bit vi h mt kha cng khai khng i xng
trong cc kha khc nhau c s dng bi ngi m v ngi dch. Ch rng X, Y, v Z
trong m hnh ny l cc bin ngu nhin. Trong m hnh ny chng ta cng lun gi
thit bn r X v kha Z l c lp thng k.Cc h mt kha b mt thng c chia thnh
cc h m khi v h m dng. i vi m khi bn r c dng cc khi "ln" (chng hn
128-bit) v dy cc khi u c m bi cng mt hm m ha, tc l b m ha l mt hm
khng nh. Trong m dng, bn r thng l dy cc khi "nh" (thng l 1-bit) v c
bin i bi mt b m ha c nh.Cc h m khi c u im l chng c th c chun ha mt
cch d dng, bi v cc n v x l thng tin hin ny thng c dng block nh
bytes hoc words. Ngoi ra trong k thut ng b, vic mt mt block m cng
khng nh hng ti chnh xc ca vic gii m ca cc khi tip sau, cng l mt u
im khc ca m khi.
thm m
ngun rni nhnB gii mDK(.) B m haEK(.) X Y X
Z Zknh an ton
ngun kha
Hnh 1.1: M hnh h mt kha b mtNhc im ln nht ca m khi l php m ha
khng che du c cc mu d liu: cc khi m ging nhau s suy ra cc khi r cng
ging nhau. Tuy nhin nhc im ny c th c khc phc bng cch a vo mt lng nh
c nh trong qu trnh m ha, tc l bng cch s dng cch thc mc xch khi m
(CBC-Cipher Block Channing mode) trong hm m ha khng nh c p vo tng
XOR ca block r v block m trc . Php m lc ny c kiu cch k thut nh m
dng p dng i vi cc khi "ln".Gi s F2 l trng Galois hai phn t. K hiu
F2m l khng gian vc t cc b m-tuples cc phn t ca F2. Trong phn ny
chng ta gi thit khng mt tng qut rng, bn r X, bn m Y ly cc gi tr
trong khng gian vc t F2m, cn kha Z ly gi tr trong khng gian vc t
F2k. Nh vy m-l di bt ca cc khi r v m, cn k-l di bit ca kha b mt.nh
ngha 1.1. H m khi kha b mt l mt nh x E: F2m x Sz F2m, sao cho vi mi
z Sz, E(., z) l mt nh x c ngc t F2m vo F2m.Hm c ngc E(., z) c gi l
hm m ha tng ng vi kha z. nh x nghch o ca E(., z) c gi l hm gii m
tng ng vi kha z v s c k hiu l D(., z). Chng ta vit Y = E(X, Z) i vi
mt m khi c ngha l bn m Y c xc nh bi bn r X v kha b mt Z theo nh x
E. Tham s m c gi l di khi cn tham s k c gi l di kha ca h m khi . C
kha ng ca h m khi c xc nh bi s kt = log2 (#(Sz)) bit. Nh vy di kha
s bng c kha ng nu v ch nu Sz = F2k, tc l mi b k-bit nh phn u l mt
kha c hiu lc. Chng hn i vi chun m d liu DES, di kha l k = 64 bit,
trong khi c kha ng ca n l kt = 56 bit. Ch rng y ta xem xt cc m khi
c di khi m bng di khi r.1.2 an ton ca cc h m khiNh ni trn, mt m khi
c s dng nhm bo v chng s d d khng mong mun ca bn r. Nhim v ca thm m
i phng l ph h m ny theo ngha anh ta c th m ra c cc bn r t cc bn m
chn bt c. Mt h m l b ph hon ton nu nh thm m c th xc nh c kha b mt
ang s dng v t anh ta c th c c tt c cc thng bo mt cch d dng nh l mt
ngi dng hp php. Mt h m l b ph thc t nu thm m c th thng xuyn m ra c
cc bn r t cc bn m nhn c, nhng vn cha tm ra c kha. an ton lun gn vi
cc e da tn cng. Nh ni trn, chng ta gi s rng k tn cng lun c th truy
nhp ti mi th c truyn thng qua knh khng an ton. Tuy nhin, c th c cc
thng tin khc i vi thm m. Kh nng tnh ton ca thm m phi lun c xem xt
trc khi xem xt an ton ca mt m c th b truy nhp.1.2.1. Cc kiu tn
cngMt gi thit c chp nhn ph bin nht trong mt m l thm m i phng lun c
th truy nhp hon ton ti cc bn m c truyn trn knh khng an ton. Mt gi
thit c chp nhn khc na l:Gi thit Kerckhoff: Thm m i phng l c bit ton
b chi tit ca qu trnh m ha v gii m ch tr gi tr kha b mt.Gi thit
Kerckhoff suy ra rng an ton ca mt h mt kha b mt ch cn ph thuc vo
chnh kha mt m thi. Di gi thit Kerckhoff, cc tn cng c th c phn loi
theo cc tri thc ca thm m nh sau:- Tn cng ch bit bn m: thm m i phng
khng bit thm t thng tin g ngoi bn m nhn c.- Tn cng bn r bit: Thm m
i phnng bit thm mt vi cp R/M i vi kha ang dng.- Tn cng bn r la chn:
Thm m i phnng c th t c cc bn m tng ng vi cc bn r n nh c bit bt k i
vi kha ang dng.Tn cng bn r la chn l tn cng mnh nht trong cc tn cng
trn. Nu mt h m l an ton chng li tn cng bn r la chn th n cng an ton
trc cc tn cng khc. Trong thc t, ta nn dng h m c an ton chng li tn
cng bn r la chn, ngay c khi thm m i phng him c c hi thu lm c thng
tin g hn so vi tn cng ch bit bn m.1.2.2 an ton v iu kin v an ton
tnh ton an ton ca mt h mt ph thuc rt ln vo kh nng tnh ton ca thm m
i phng. Mt h mt c gi l an ton v iu kin nu n an ton chng li thm m i
phng c kh nng tnh ton v hn. an ton v iu kin cng c gi l an ton l
thuyt lin quan ti tnh khng th ph c ca mt h mt. Mt h mt l an ton
chng li i phng c kh nng tnh ton b hn ch no c gi l an ton tnh ton.
an ton tnh ton cng c gi l an ton thc t, lin quan ti tnh kh ph ca mt
h mt. Tt c cc h mt an ton v iu kin u l khng c tnh thc t v l do s c
ni di y. Tuy nhin cng khng c mt h mt thc t no l c chng minh l an
ton theo ngha tnh ton. an ton v iu kinMc d trong hu ht cc ng dng an
ton v iu kin l khng cn thit v cng l khng th thc hin c trn thc t,
nhng nghin cu v an ton v iu kin cho chng ta nhiu gi c ch cho vic
thit k v s dng cc h mt thc t. Chng hn l do c bn ca h m dng l mt hon
thin c cung cp bi h thng m mt ln "one-time-pad".nh ngha 1.2
(Shannon 1949): Mt h mt s cung cp mt hon thin nu cc khi r v cc khi
m l c lp thng k.Kh nng thc thi h mt b mt hon thin c cho thy bi
Shannon trong bi bo ca ng ta nm 1949. H "M nhm kha dng mt ln"sau y
(c m t trong v d 1) cung cp mt h mt b mt hon thin nh th. tng s dng
h thng kha dng mt ln u tin c xut bi Vernam trong nm 1926. M Vernam
thng c gi l h mt mt ln "one-time-pad". Mc d trong mt thi gian di
ngi ta tin rng h mt mt l l khng th b ph, nhng phi n cng trnh ca
Shannon mi chng minh c tnh b mt hon thin ca n.V d 1: (h m khi nhm
kha dng mt ln): Xt h m khi cho trong Hnh 1.2, y l php ton nhm nh
ngha trn tp hp F2m. H m ny c b mt hon thin nu kha c chn ngu nhin u
v c lp vi mi khi r. ..., X2, X1 ..., Y2, Y1
..., Z2, Z1
Hnh 1.2: H m khi nhm kha dng mt ln. Cc kha Zi l c chnngu nhin u
v c lp.H thng b mt hon thin thng l khng thc t, bi v Shannon cho thy
mt lng kha khng gii hn cn phi c nu nh ta cho php mt lng thng bo
khng hn ch. Tuy nhin, tng ca h mt hon thin thit lp nn mt nguyn l
bit trong thc t mt m l m bo an ton th nn thay kha mt cch thng
xuyn.
an ton tnh ton
Trong thc t khng k tn cng no c kh nng tnh ton v hn. an ton ca mt
h mt thc t ph thuc vo tnh khng th ph h m v mt l thuyt m ng hn l ph
thuc kh thc t ca cc tn cng. Mt h mt c gi l an ton tnh ton nu kh ca
tn cng ti u vt qu kh nng tnh ton ca thm m. Shannon m t kh ca tn cng
nh th (tn cng ch bit bn m) bi c trng W(n) xem nh l khi lng cng vic
i hi xc nh kha khi n-bn m l c bit. Ta cng c th xem xt W(n) i vi cc
kiu tn cng khc. Trong sut phn ny , chng ta s dng t " phc tp" m t kh
nh th. phc tp ca mt tn cng hiu mt cch chung chung l s trung bnh cc
php ton (thao tc) dng trong tn cng . Ch rng mt h m l an ton tnh ton
c ngha l phc tp ca tn cng ti u vt qu kh nng tnh ton ca thm m i
phng. chng minh mt h mt l an ton tnh ton cn phi ch ra c cn di hu ch
v phc tp ca vic gii quyt mt bi ton tnh ton no . Hin ti, iu ny l
khng th i vi tt c cc bi ton tnh ton. Do vy, trong thc t, vic nh gi
an ton ca mt h mt ph thuc vo phc tp ca tn cng tt nht cho ti hin ti.
Mt m khi thc t c xem l an ton tnh ton nu khng c tn cng bit no c th
lm tt hn so vi tn cng vt cn kha. Trong tn cng vt cn kha ch bit bn m
trn mt m khi, mi mt kha c th u c th gii m ca mt hoc hiu hncc khi m
chn bt c cho ti khi no mt kha cho kt qu khi r c th c c. phc tp ca
tn cng ny, xem nh l s cc php gii m th, v mt trung bnh s bng i vi mt
h m khi c c kha ng l kt. Tn cng vt cn kha l mt tn cng "brute-force"
n c th p vo h m khi bt k. Nh vy mt h m khi mun an ton th c kha ng
ca n l phi ln to cho tn cng vt cn kha l khng th thc hin c.1.2.3 phc
tp x l v phc tp d liu ca mt tn cng c th
phc tp ca mt tn cng c chia ra lm hai phn: phc tp d liu v phc tp
x l. phc tp d liu l lng d liu u vo cn cho tn cng trong khi phc tp x
l l lng cc tnh ton cn x l d liu nh th. Thnh phn dominant-tri hn
thng c m t nh l phc tp ca tn cng ny. Chng hn, trong tn cng vt cn
kha, lng d liu u vo cn cho tn cng ny l s cc khi m chn bt c (hoc s
cc cp r/m trong tn cng bn r bit), ni chung l mt s lng rt nh so vi s
cc php ton (trung bnh cn php gii m vi cc kha khc nhau trong vic tm
ra kha ng) cn thit ca tn cng ny. Do vy phc tp ca tn cng duyt kha
thng chnh l phc tp x l. V d khc l tn cng vi sai ca Biham v Shamir,
l kiu tn cng bn r la chn. i vi tn cng vi sai phc tp vt tri ln bi s
cc cp r/m cn trong tn cng , trong khi s cc tnh ton s dng trong tn
cng ny li tng i nh. Do phc tp ca tn cng vi sai thc cht l phc tp d
liu.
Ni chung i vi mt m khi di khi m-bit v c kha ng l kt-bit, phc tp
d liu ca tn cng bn r bit (hoc bn r la chon) c th c o bi s cc cp r/m
bit (hay la chn) cn cho tn cng ny, nhiu nht l 2m l s ton b cc cp nh
th i vi mt kha c nh. phc tp x l c th b chn trn bi s php m ha do c
tnh ca tn cng vt cn kha v do ni chung thao tc m ha l c tnh ton
nhanh, hiu qu. Nh vy chng ta c th ni rng mt h mt l an ton tnh ton
nu nh khng c tn cng no trn h mt c phc tp d liu nh hn ng k 2m php m
v phc tp x l nh hn ng k php m ha. Mt h mt c gi l an ton thc t chng
li mt tn cng c th nu vi tn cng ny, phc tp d liu vo khong 2m cp r/m
hoc phc tp x l l vo khong php m ha. i vi thm m, phc tp d liu l loi
phc tp b ng, anh ta phi ch ngi s dng to ra cc ccp r /m cho anh ta.
Mt khc, phc tp x l li l kiu phc tp ch ng v c th khc phc ni chung
bng cch s dng nhiu my tnh mnh.1.2.4 Cc tham s ca m khi1.2.4.1 di
khi m mt h m khi l an ton, di khi m ca n phi ln ngn cn cc tn cng
phn tch thng k, tc l khng cho i phng thu c thng tin c ch no v khi r
no thng xut hin nhiu hn cc khi r khc. Ngoi ra di khi m cng phi c
chn sao cho s cc cp r/m m i phng c th thu nhn c trong thc t phi nh
hn rt nhiu so vi 2m.Khi di khi ca h m tr nn ln th phc tp ca ng dng
cng tng theo. D rng phc tp trong ng dng chn ngu nhin hm c ngc l tng
theo c m so vi di khi, nhng ch c hm n gin mi xut hin ngu nhin, iu
ny to c hi phc v hm m ha thc t khi di khi m l ln. Tuy nhin, Shannon
ch ra rng s d dng trong tnh ton cc hm m ha E(., z) v hm gii m D(.,
z) vi mi z khng suy ra c vic gii tm kha z t cc phng trnh y = E(x,
z) v x = D(y, z) s l d dng khi bit x v y.1.2.4.2 di kha k v c kha
ng kt
h m khi an ton chng li tn cng vt cn kha, c kha ng cn phi ln sao
cho php m ha cn cho tn cng ny l vt xa kh nng ca thm m. Mt khc, di
kha k cng cn nh mc no sao cho vic to, phn phi v lu tr kha c th thc
hin c hiu qu v an ton. Chng hn, DES c di kha l 64 bt, cn c kha ng l
56 bit. Tn cng vt cn kha l khng th nhng cng khng l qu xa vi. Nhiu
gi mun tng c kha ng ca DES. Chng hn, m rng c kha dng ca DES ti 128
bit bng php m bi ba dng hai kha xem l mt cch thc chun s dng DES.1.3
Cc ch hot ng ca m khi Trong mt m , m khi hot ng da trn cc khi c
chiu di c nh, thng l 64 hoc 128 bit. Do cac thng bo u vo c chiu di
bt k v vic m ha vi cng mt bn r vi cng mt kha c nh lun to ra cng mt
bn m, mt vi ch hot ng ca m khi c a ra cho php cc m khi cung cp tnh
b mt cho cc thng bo c chiu di bt k. Cc ch c bit n sm nht ch cung cp
tnh b mt ca thng bo nhng khng cung cp tnh ton vn ca ni dung thng bo
nh ECB, CBC,OFB v CFB. Mt vi ch hot ng khc c thit k m bo c tnh b mt
v tnh ton vn ca ni dung thng bo nh: CCM,EAX v OCB. Cc ch LRW, CMC v
EME c thit k m ha cc Sector ca cc thit b lu tr (a cng) . Trong phn
ny chng ta xt n 5 ch ng dng dng m khi thng gp nht trong cc h thng
mt m bo v thng tin. l cc ch :Sch m in t (ECB Electronic Code Book),
Mc xch khi m (CBC Cipher Block Channing), Phn hi khi m (CFB Cipher
FeedBack), Phn hi u ra (OFB Output FeedBack), B m (CTR -
Counter).1.3.1 Vector khi to IVHu ht cc ch hot ng (tr ECB) ca m khi
u yu cu mt vector khi to khi to cho vic x l khi d liu u tin v thng
c to mt cch ngu nhin. Khng cn thit phi gi b mt gi tr ca IV nhng
khng bao gi c dng li gi tr IV vi cng mt kha b mt. Vi ch CBC v CFB
vic dng li IV l d g mt s thng tin v khi bn r u tin v mt s thng tin
c chia s trc bi hai thng bo. Vi ch OFB v CTR vic dng li IV gy ph hy
tnh an ton. Trong ch CBC, IV cn thit v phi c sinh ngu nhin ti thi
im m ha.1.3.2 Ch ECBy l ch hot ng n gin nht ca m khi , bn r u vo c
chia nh thnh cc khi v mi khi c x l m ha ring bit. im bt li chnh ca
ch ny l vic cc khi bn r c x l c lp to ra cc khi bn m tng ng, v vy n
khng cung cp tnh ton vn ca ton b ni dung thng bo v n cng khng c ngh
s dng cho hu ht cc giao thc mt m.
Hnh 1.3 : M ha v gii m theo m hnh ECB bn ca ch ECB chnh bng bn
ca thut ton. Tuy nhin cu trc ca bn r trong trng hp khng c giu kn.
Mi khi nh nhau ca bn r dn n s xut hin ging nhau ca bn m. Tc m ha
bng tc ca m php khi.Ch ECB cho php song hnh n gin nng cao tc m ha.V
d v vic s dng ch ECB cho vic m mt bc nh.
nh gc M ha dng ch ECBHinh 1.4 : V d v m ha theo m hnh ECBHin
nhin qua vic quan st kt qu thu c khi bc nh b m ha dng ch ECB ta vn
d dng nhn c cc thng tin ca bc nh ban u.1.3.3 Ch CBCVi ch hot ng CBC
mi khi bn r u vo c kt hp vi khi bn m trc dung php XOR, theo cch ny
mi khi bn m u ph thuc vo cc khi bn r trc . Do cn phi c mt vector
khi to (IV) cho khi bn r u tin.
Hinh 1.5: M ha v gii m theo m hnh CBCCng thc vic m ha v gii m
thng bo tin hnh nh sau: bn ca ch CBC bng bn ca m php m n da vo. Cu
trc ca bn r c che giu nh cng khi trc ca bn m vi khi k tip ca bn r.
bn m ha vn bn tng v khng th thao tc trc tip bn r ngoi cch loi tr cc
khi t u cui bn m.Tc m ha bng tc lm vic ca m php khi, nhng phng ph n
gin song hnh ca qu trnh m ha khng tn ti, cho d qu trnh dch m c th
tin hnh mt cch song song.Ch hot ng CBC c s dng rt rng ri, thng bo c
m ha tun t v i hi chiu di ca thng bo phi l bi s ca chiu di khi v do
ni dung thng bo r cn phi c s l m trc khi thc hin m ha. Ch hot ng
CBC cung cp c ch ton vn d liu, ch cn mt bit trong ni dung thng bo b
thay i s dn n thay i ton b cc khi sau bit .1.3.4 Ch CFBCh hot ng
CFB bin m khi thnh mt h m dng t ng b v c thc hin nh sau:
Hnh 1.6 : M ha v gii m theo m hnh CFB bn ca ch CFB bng bn ca m
php m n da vo,cn cu trc ca bn r c che giu nh s dng php ton cng theo
modul 2. Vic thao tc bn r bng cch loi tr cc khi t u v cui ca bn m l
khng th c. Trong ch CFB nu hai khi bn r l ng nht th kt qu m ha chng
bc tip theo cng ng nht, iu ny gy r r thng tin v bn r.Cng ging nh ch
CBC vic m ha khng th thc hin song song nhng vic giI m d liu c th
thc hin song song v vic thay i d ch l mt bt trong ni dung thng bo
cng lm nh hng n ton b cc khi pha sau.1.3.5 Ch OFB Ch ny bin m khi
thnh mt h m dng ng b , qu trnh m ha v gii m tin hnh nh sau:
Hnh 1.7 : M ha v gii m theo m hnh OFBCh OFB tng t nh ch CFB tuy
nhin n c u th hn ch CFB ch bt k cc bit li no xut hin trong qu trnh
truyn u khng nh hng n s dch m cc khi tip theo.1.3.6 Ch CTR Cng ging
nh ch OFB, ch CTR bin m khi thnh mt m dng. Gi tr IV/Nonce v b m
Counter c th c ni, cng hoc Xor vi nhau to thnh mt gi tr duy nht cho
mi khi x l.
Hinh 1.8 : M ha v gii m theo m hnh CTRu im ln nht ca ch CTR l
cho php vic m ha v gii m c th thc hin song song nn tc hot ng c ci
thin.Cc m khi chu hai tn cng quan trng l tn cng lng sai v tn cng
tuyn tnh.Tn cng lng sai (Differential Cryptanalysis) da trn xc suet
ca cc mu lng sai ca cc cp r v m hay chnh xc hn l mu lng sai ca cc
cp u ra v u vo ca cc hm phi tuyn trong m khi tm ra cc thnh phn kha
tng ng c th t tm ra ton b kha ca m khi.Cc cp bn r, bn m mun tha mn
cc mu lng sai th phi la chn thch hp.Cc cp mu lng sai ca cc cp r v
cc cp m c xc suet cao s c s dng hiu qu trong tn cng lng sai.Trong
tn cng tuyn tnh th ngi ta tm cc s ph thuc tuyn tnh vi xc sut khc
1/2 gia cc mu bt r, kha v bn m vi xc sut c li t tm ra cc bit c th
ca kha. Nhiu bit kha c tm ra bng cch ny cn cc bit kha cn li s c tm
ra bng cawsch duyt ton b.i vi m khi, an ton ca n ph thuc vo thit k
ca m khi v kha lp m. Thit k ca m khi l quan trng v phi trnh vic
tuyn tnh ha cng cao cng tt. Kha phi ln trnh tn cng nghch l ngy
sinh. Kha cng phi ch khng c dng c bit. i vi m khi th tt nht l phi
khng cha cc lp kha yu d nhn ra.1.4 Nguyn l thit k m khiMt h m khi
tt l phi "kh ph v d s dng". C hai hm m ha E(., z) v hm gii m D(.,
z) nn d dng tnh ton. Cn vic gii kha z t y = E(x, z) v x = D(y, z)
nn l bi ton kh. Nguyn l thit k cho mt h m khi c th chia thnh cc
nguyn l ng dng v cc nguyn l an ton.1.4.1 Nguyn l thit k chung v an
tonCh c hai nguyn l thit k c chp nhn chung i vi cc m an ton thc t l
cc nguyn l v mo (confusion) v khuych tn (diffusion) c gi bi
Shannon.Nguyn l v mo (confusion): S ph thuc ca kha trn bn r v bn m
nn phi phc tp sao cho n khng c ch g i vi thm m. Chng hn, phng trnh
nh phn m t m khi nn l phi tuyn v phc tp sao cho vic gii kha z t x v
y = E(x, z) l khng th.Nguyn l v khuych tn (diffusion): Vi mi kha c
th hm m ha khng nn c s ph thuc thng k no gia cc cu trc n gin trong
bn r v cc cu trc n gin trong bn m v rng khng c quan h n gin no gia
cc hm m ha khc nhau. Nguyn l khuych tn i hi, chng hn mt h m khi cn
c thit k c tnh y -hay hon thin "complete", tc l mi bit r v mi bit
kha u nh hng ti mi bit m. 1.4.2 Nguyn l thit k cho ng dngMt h m khi
c th ng dng c phn cng v phn mm. Trong ng dng cng thng c thc hin bi
cc chp VLSI c tc cao. Trong ng dng mm phi c tnh mm do v gi thnh
thp. Trn c s c tnh khc nhau ca phn cng v phn mm, cc nguyn l thit k
cho m khi cng chia thnh hai phn.Nguyn l thit k cho ng dng mmS dng
khi con: Cc thao tc m khi nn thc hin trn cc khi con c di t nhin cho
phn mm l 8, 16, 32 bit. Hon v bit l kh thc hin trong phn mm nn
trnh.S dng cc php ton n gin: Cc thao tc m trn cc khi con nn chn d
dng cho ng dng vi cc tp lnh c s ca cc b x l chun chng hn nh php
cng, php nhn, php dch ...Nguyn l thit k cho ng dng phn cngS tng t
trong php m ha v php gii m: Qu trnh m ha v gii m nn ch khc nhau cch
s dng kha mt sao cho cng mt thit b c th s dng c cho c php m ha v
php gii m.1.5 Cc cu trc m khi c bn1.5.1 Cu trc m FeistelPhn ln cc h
m khi trn th gii hin nay l da trn cu trc m-dch Feistel c cc c tnh c
bn sau:* di ca mi khi (block) r bng di ca mi khi m, v l mt s chn m=
2. L.*Bn r c chia thnh cc khi P = (x0, x1) c di 2. L, v x0 = x1 =L*
Kho k l mt tp kho con: k1, k2 , .., kn.* Mi ki c tng ng vi mt php
bin i Fi trn khi c L.* Bn r P c m ho theo n-bc nh sau:
P = (x0, x1) Bn r: Vng 1: (x0, x1) (x1, x2)Vng 2: (x1, x2) (x2,
x3)---------------------------------Vng i: (xi-1, xi) (xi,
xi+1)----------------------------------Vng n: (xn-1, xn) (xn,
xn+1)
C = (xn+1, xn) Bn m l: Trong xi+1 = xi-1 Fi(xi)Vi cu trc m ho
trn y, qu trnh dch m s rt n gin: Gi nguyn cc thao tc nh qu trnh m
ho, ch cn thay i th t s dng kho v cc hm vng tng ng: kn, kn-1, ..,
k1 Fn, Fn-1, .., F1.Nhn xt:a/- Cu trc m Feistel trn y rt thun tin
cho m dch m bo tc nhanh v tin li cho vic cng ho cc chng trnh m dch
khi. - Cc hm vng Fi c th c cu trc hon ton ging nhau, tc l Fi = F,
min sao chng l hm c tnh cht mt m tt, v do s cng thun tin cho thao
tc m dch.b/ Qua m hnh cu trc m dch Feistel trn c th thy ngay cc dng
kho coi l yu nh sau (vi gi thit Fi F): - Kho yu l cc kho c dng:kn =
k1;kn-1 = k2;kn-2 = k3;---------Tc l D(.) = E(.), hay l E2 = I. Nh
vy thm m ch cn m ho chnh bn m thu c l s c c bn r cn tm. - Cp kho na
yu l cc cp kho c dng:kn(A) = k1(B);kn-1(A) = k2(B);kn-2(A) = k3(B);
iu ny c ngha l thm m c th dng thao tc m ho ca ngi B gii m cc bn m
ca ngi A v ngc li. Tc l ta c EA = DB, v EB = DA.Tt nhin cc dng kho
trn y l khng c php s dng trong cc m hnh m khi tng ng.1.5.2 Cu trc
cng-nhnCu trc cng-nhn c th xem nh l mt trong cc kiu ht nhn cu to nn
cc hm vng, trong hon ton s dng cc php ton s hc tng i n gin v c chn
lc cn thn. Mt s cu trc bin i khc m ta lm quen nh cc hp nn, cc php
hon v, cc php dch vng, chng c s dng trong DES, trong h m d liu
Xvit... Cu trc cng-nhn c xut bi J. L. Massey v X. Lai khi h xy dng
nn mt chun m d liu mi l PES v sau c ci tin i tn thnh IDEA. Hnh 1.10
cho ta m hnh ca cu trc cng-nhn U1 U2 Z5 + + Z6
V1 V2
Hnh 1.9 : S cu trc cng-nhn (MA). Trong s trn th cc php ton v + l
cc php nhn mdulo hoc cng mdulo trn cc nhm tng ng vi khng gian u vo
ca cc hng t: U1, U2 l cc vc t u vo, V1, V2 l cc vc t u ra, Z1, Z2 l
cc kho.
Chng II: LC KHA CA M KHI V MT S LC C TH
2.1 Phn loi cc lc kho ca cc h m khiMt vn ht sc quan trng trong
thit k m khi l xy dng lc to kho cho h m. Thng thng mt h m khi lp
thng c s vng tng i ln. Kho phin khng th c d di tu , do t kho b mt
cn thit phi xy dng mt thut ton to ra s kho con cn thit cung cp cho
cc vng lp. Kho chnh thng di t 128 bt n 512 bit, trong khi tng s bt
kho con c th ln ti hng ngn bt. Do vy vic nghin cu lc to kho l khng
th trnh khi. Lc to kho khng ch n thun cung cp cc kho con cho cc vng
lp trong h m khi m n cn ng gp vai tr quan trng trong an ton ca chnh
h m .Tuy nhin chng ta cng thy mt s lc kho c nhng im s h thm m c th
li dng, nh lc qu n gin, lc to ra cc dng kho quan h, hay c s tng t
lp li trong cc giai on to kho con. trnh cc dng tn cng xt, Knudsen a
ra mt s yu cu i vi mt lc to kho mnh l tt c cc kho phi tt nh nhau, v
khng c cc quan h n gin.nh ngha 5.1: Xt mt h m khi lp r-vng, c khi l
2m-bit vi r kho con vng, mi kho con c di l n-bit. Mt lc kho mnh phi
c cc tnh cht sau:-Cho trc bt k s-bit ca r kho con vng c thit k t mt
kho chnh cha bit, khi kh c th tm ra c rn-s bt kho cn li t s-bit kho
bit.-Cho trc mt quan h no gia hai kho chnh, khi kh c th d on c cc
quan h gia bt k cc kho con vng no c thit k t cc kho chnh . Ni mt
cch n gin hn l lc kho mnh l lc m cc hiu bit v mt kho con no khng lm
d d bt k thng tin g i vi cc kho con khc trong lc . Trong phn ny trc
ht chng ta i phn loi cc lc kho c, v sau a ra mt s xut lin quan n
vic xy dng lc kho mnh.
Cc lc kho hin ti c th c chia thnh hai kiu. Kiu 1: l kiu tri thc
v mt kho con vng s cung cp mt cch duy nht cc bt kho ca cc kho con
vng khc hay ca kho chnh. Trong :+Kiu 1A l kiu n gin nht dng kho
chnh trong mi vng m ho. +Kiu 1B, cc kho con vng c to t kho chnh
theo cch sao cho hiu bit v mt kho con vng bt k c th xc nh trc tip
cc bt kho khc trong cc kho con vng khc hay trong kho chnh. DES,
IDEA, LOKI, GOST l cc v d v kiu ny.+Kiu 1C, tri thc v mt kho con
vng c th gip xc nh mt cch khng trc tip cc bt kho khc trong cc kho
con vng khc hay trong kho chnh. Mt vi thao tc cn thit phi dc s dng
gip xc nh tm ra cc bt kho khc hay trong kho chnh. V d v kiu ny l lc
kho ca h CAST, SAFER.Trong CAST, mi mt vng trong 4 vng u tin u s
dng 16 bt ca kho chnh, chia n thnh 2 khi 8-bit, mi khi cho qua mt
S-hp c nh. Cc u ra ca mi S-hp l 32-bit, v kt qu c XOR vi nhau to nn
kho con vng . Nu bit mt kho con ny, chng ta phi th 216 bt l u vo
cho mi S-hp tm ra xu bt no cho u ra ph hp vi kho con bit. Ch rng nu
bit bt k kho con no t vng th 5 tr i n u khng th p dng cch trn y thu
c cc thng tin khc v kho.Trong SAFER, nu K = (k1,1, ...k1,8) l mt
kho chnh 8-byte, khi kho con 8-byte vng th i, Ki,j s c xc nh nh
sau:ki,j = ki-1, j
