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NETWORK SECURITY
SYMMETRIC ENCRYPTION
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Hai k thut m ha ch yu
M ha i xng
o Bn gi v bn nhn s dng chung mt kha
o Cn gi l
o M ha truyn thng
o M ha kha ring / kha n / kha b mt
o L k thut m ha duy nht trc nhng nm 70
o Hin vn cn c dng rt ph bin
M ha kha cng khai (bt i xng)o Mi bn s dng mt cp kha
o Mt kha cng khai + Mt kha ring
o Cng b chnh thc nm 1976
An ninh Mng 22
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Mt s cch phn loi khc
Theo phng thc x l
o M ha khi
o Mi ln x l mt khi nguyn bn v to ra khi bn m tng ng (chnghn 64 hay 128 bit)
o
M ha lungo X l d liu u vo lin tc (chng hn mi ln 1 bit)
Theo phng thc chuyn i
o M ha thay th
o Chuyn i mi phn t nguyn bn thnh mt phn t bn m tng ng
o M ha hon v
o B tr li v tr cc phn t trong nguyn bn
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M hnh h m ha i xng
An ninh Mng 44
Kha b mt dng chungbi bn gi v bn nhn
Kha b mt dng chungbi bn gi v bn nhn
Gii thut m ha Gii thut gii m
Nguyn bnu vo
Nguyn bnu ra
Bn m
truyn i
M ha
Y = EK(X)
Gii m
X = DK(Y)
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M hnh h m ha i xng
Gm c 5 thnh phno Nguyn bn
o Gii thut m ha
o Kha b mto Bn m
o Gii thut gii m
An ninh ph thuc vo s b mt ca kha,khng ph thuc vo s b mt ca gii thut
An ninh Mng 55
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Ph m
L n lc gii m vn bn c m hakhng bit trc kha b mt
C hai phng php ph m
o Vt cno Th tt c cc kha c th
o Thm m
o Khai thc nhng nhc im ca gii thut
o Da trn nhng c trng chung ca nguyn bn hoc mt scp nguyn bn - bn m mu
An ninh Mng 66
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Phng php ph m vt cn
V l thuyt c th th tt c cc gi tr khacho n khi tm thy nguyn bn t bn m
Da trn gi thit c th nhn bit c nguyn
bn cn tm Tnh trung bnh cn th mt na tng s cc
trng hp c th
Thc t khng kh khi nu di kha ln
An ninh Mng 77
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Thi gian tm kim trung bnh
An ninh Mng 88
Kch thckha (bit)
S lng kha Thi gian cn thit
(1 gii m/s)
Thi gian cn thit
(106 gii m/s)
32
56
128
168
26 k t
(hon v)
232 = 4,3 x 109
256 = 7,2 x1016
2128 = 3,4 x1038
2168 = 3,7 x
1050
26! = 4 x 1026
231 s = 35,8 pht
255 s = 1142 nm2127 s = 5,4 x 1024nm
2167 s = 5,9 x 1036nm
2 x 1026 s =
6,4 x 1012nm
2,15 ms
10,01 gi5,4 x 1018 nm
5,9 x 1030 nm
6,4 x 106 nm
Tui v tr : ~ 1010 nmKha DES di 56 bitKha AES di 128+ bit
Kha 3DES di 168 bit
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Cc k thut thm m
Ch c bn mo Ch bit gii thut m ha v bn m hin c
Bit nguyn bn
o
Bit thm mt s cp nguyn bn - bn m Chn nguyn bn
o Chn 1 nguyn bn, bit bn m tng ng
Chn bn m
o Chn 1 bn m, bit nguyn bn tng ng
Chn vn bn
o Kt hp chn nguyn bn v chn bn m
An ninh Mng 99
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An ninh h m ha
An ninh v iu kino Bn m khng cha thng tin xc nh duy nht nguyn
bn tng ng, bt k vi s lng bao nhiu v tc mytnh th no
o
Ch h m ha n mt ln l an ninh v iu kin An ninh tnh ton
o Tha mn mt trong hai iu kin
o Chi ph ph m vt qu gi tr thng tin
o Thi gian ph m vt qu tui th thng tin
o Thc t tha mn hai iu kin
o Khng c nhc im
o Kha c qu nhiu gi tr khng th th ht
An ninh Mng 1010
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M ha thay th c in
Cc ch ci ca nguyn bn c thay th bicc ch ci khc, hoc cc s, hoc cc khiu
Nu nguyn bn c coi nh mt chui bit ththay th cc mu bit trong nguyn bn bng ccmu bit ca bn m
An ninh Mng 1111
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H m ha Caesar
H m ha thay th xut hin sm nht & n gin nht
S dng u tin bi Julius Caesar vo mc ch quns
Dch chuyn xoay vng theo th t ch ci
o Kha k l s bc dch chuyn
o Vi mi ch ci ca vn bn
o t p = 0 nu ch ci l a, p = 1 nu ch ci l b,...
o
M ha : C = E(p) = (p + k) mod 26o Gii m : p = D(C) = (C - k) mod 26
V d : M ha "meet me after class" vi k = 3
An ninh Mng 1212
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Ph m h m ha Caesar
Phng php vt cno Kha ch l mt ch ci (hay mt s gia 1 v 25)
o Th tt c 25 kha c th
o D dng thc hin
Ba yu t quan trng
o Bit trc cc gii thut m ha v gii m
o Ch c 25 kha th
o
Bit v c th d dng nhn ra c ngn ng ca nguyn bn V d : Ph m "GCUA VQ DTGCM"
An ninh Mng 1313
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H m ha n bng
Thay mt ch ci ny bng mt ch ci khc theo trtt bt k sao cho mi ch ci ch c mt thay th duynht v ngc li
Kha di 26 ch ci
V d
o Kha
a b c d e f g h i j k l m n o p q r s t u v w x y z
M N B V C X Z A S D F G H J K L P O I U Y T R E W Qo Nguyn bn
i love you
An ninh Mng 1414
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Ph m h m ha n bng
Phng php vt cno Kha di 26 k t
o S lng kha c th = 26! = 4 x 1026
o Rt kh thc hin
Khai thc nhng nhc im ca gii thut
o Bit r tn s cc ch ci ting Anh
o C th suy ra cc cp ch ci nguyn bn - ch ci bn m
o
V d : ch ci xut hin nhiu nht c th tng ng vi 'e'o C th nhn ra cc b i v b ba ch ci
o V d b i : 'th', 'an', 'ed'
o V d b ba : 'ing', 'the', 'est'
An ninh Mng 1515
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Cc tn s ch ci ting Anh
Nguyn i Th An ninh Mng 1616
Tnstngi(%)
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V d ph m h n bng
Cho bn mUZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX
EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
Tnh tn s ch ci tng i on P l e, Z l t
on ZW l th v ZWP l the
Tip tc on v th, cui cng c
it was disclosed yesterday that several informal but
direct contacts have been made with political
representatives of the viet cong in moscow
An ninh Mng 1717
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H m ha Playfair (1)
H m ha nhiu cho Gim bt tng quan cu trc gia bn m v
nguyn bn bng cch m ha ng thi nhiu chci ca nguyn bn
S dng 1 ma trn ch ci 5x5 xy dng trnc s 1 t kha
o in cc ch ci ca t kha (b cc ch trng)
o in nt ma trn vi cc ch khc ca bng ch cio I v J chim cng mt ca ma trn
An ninh Mng 1818
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H m ha Playfair (2)
V d ma trn vi t kha MONARCHY M O N A R
C H Y B D
E F G I/J K
L P Q S T U V W X Z
M ha 2 ch ci mt lc
o Nu 2 ch ging nhau, tch ra bi 1 ch in thm
o
Nu 2 ch nm cng hng, thay bi cc ch bn phio Nu 2 ch nm cng ct, thay bi cc ch bn di
o Cc trng hp khc, mi ch ci c thay bi ch ci khc cnghng, trn ct ch ci cng cp
An ninh Mng 1919
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Ph m h m ha Playfair
An ninh m bo hn nhiu h m ha nch
C 26 x 26 = 676 cp ch ci
o
Vic gii m tng cp kh khn hno Cn phn tch 676 tn s xut hin thay v 26
Tng c qun i Anh, M s dng rng ri
Bn m vn cn lu li nhiu cu trc canguyn bn
Vn c th ph m c v ch c vi trm cpch ci cn gii m
An ninh Mng 2020
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H m ha Vigenre
L mt h m ha a bng
o S dng nhiu bng m ha
o Kha gip chn bng tng ng vi mi ch ci
Kt hp 26 h Ceasar (bc dch chuyn 0 - 25)
o Kha K = k1k2...kd gm d ch ci s dng lp i lp li vi cc chci ca vn bn
o Ch ci th i tng ng vi h Ceasar bc chuyn i
V d
o Kha : deceptivedeceptivedeceptive
o Nguyn bn : wearediscoveredsaveyourself
o Bn m : ZICVTWQNGRZGVTWAVZHCQYGLMGJ
An ninh Mng 2121
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Ph m h m ha Vigenre
Phng php vt cn
o Kh thc hin, nht l nu kha gm nhiu ch ci
Khai thc nhng nhc im ca gii thut
o Cu trc ca nguyn bn c che y tt hn h Playfair
nhng khng hon ton bin mto Ch vic tm di kha sau ph m tng h Ceasar
o Cch tm di kha
o Nu di kha nh so vi di vn bn, c th pht hin 1 dy vnbn lp li nhiu ln
o Khong cch gia 2 dy vn bn lp l 1 bi s ca di kha
o T suy ra di kha
An ninh Mng 2222
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H m ha kha t ng
Vigenre xut t kha khng lp li m c gnvo u nguyn bn
o Nu bit t kha s gii m c cc ch ci u tin
o S dng cc ch ci ny lm kha gii m cc ch cc tip
theo,...
V d :
o Kha : deceptivewearediscoveredsav
o nguyn bn : wearediscoveredsaveyourself
o M ha : ZICVTWQNGKZEIIGASXSTSLVVWLA
Vn c th s dng k thut thng k ph m
o Kha v nguyn bn c cng tn s cc ch ci
An ninh Mng 2323
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n mt ln
L h m ha thay th khng th ph c
xut bi Joseph Mauborgne
Kha ngu nhin, di bng di vn bn, ch sdng mt ln
Gia nguyn bn v bn m khng c bt k quan hno v
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