1. 암호학(Cryptology) 평문
암호문 평문
Bob
Alice
공격자
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2. 비자카드 번호 확인 0699 0043 1313 9642 1st 16th Select odd numbers
Select even numbers
After * 2, if  9
then -9
*10
= 70
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3. 암호학(Cryptology)이란? 암호화 기법과 암호분석기법에 관한 원리,수단,방법을 연구하는 학문
평문의 해독 불가하도록 하는 방법과 해독 불가능한 메시지를 해독 가능하도록 형태를 바꾸는 방법으로 구성
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4. Basic Encryption and Decryption
S R
sender message receiver
S T R
sender transmission medium receiver
S R
sender access receiver
O interceptor/intruder
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5. sender access receiver O interceptor/intruder
S T R
sender access receiver
O interceptor/intruder
- block
- intercept
- modify
- fabricate
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6. Terminology Encryption Decryption
Cryptosystem: system for encryption and decryption
Plaintext
Ciphertext
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7. Encryption Algorithms
decryption
plaintext
ciphertext
Original
Encryption
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8. Symmetric cryptosystem
encryption
decryption
plaintext
ciphertext
Original
key
key
Symmetric cryptosystem
encryption
decryption
plaintext
ciphertext
Original
Encryption
Key:KE
Decryption
Key:KD
Asymmetric cryptosystem
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9. Cryptanalysis Cryptography: hidden writing
cryptanalyst: studies encryption,encryption message
cryptology: research of encryption and decryption
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10. Cryptanalyst’s chore:
- attempt to break a single message
- attempt to recognize patterns in encrypted
message
- attempt to find general weaknesses in an
encryption algorithm
break an encryption !
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11. 암호시스템의 설계요건 난이도가 클 것 키의 크기가 작을 것 암.복호화 여건의 간결성과 처리속도의 효율성 에러 전파율이 적을 것
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12. 암호시스템의 분류 시대별 분류: 평문의 암호화 단위분류: 암호화 형식에 의한 분류: 고전 암호시스템:19세기이전
근대 암호시스템:1,2차 대전
현대암호화 시스템:1950년이후
평문의 암호화 단위분류:
블록 암호시스템
스트림 암호시스템
암호화 형식에 의한 분류:
비밀키(대칭형) 암호시스템:비밀키
공개키(비대칭형) 암호시스템:공개키와 비공개키
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13. 스트림 암호: stream cipher Synchronization! encryption decryption plaintext
평문 길이 최소 단위:한 개 단위의 비트나 문자
스트림 암호: stream cipher
encryption
decryption
plaintext
ciphertext
Original
키 생성 알고리즘
비밀키
기밀성과 무결성이 보장되는 채널
암호화의 속도가 빠르다
비트가 독립적인 관계로 각각의 비트를
암호의 개별적인 개체로 취급이 가능
오류의 영향이 적다
암호키에 대한 엄격한 동기화 요구
Synchronization!
키 생성 알고리즘이 중요!
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14. 블록 암호:Block cipher 스트림 암호와 대칭성 Bolck Original ciphertext plaintext
평문의 길이가 한개이상
블록 암호:Block cipher
encryption
decryption
plaintext
Bolck
ciphertext
Original
비밀키
기밀성과 무결성이 보장되는 채널
DES,RSA등
스트림 암호와 대칭성
암호화,블럭화에 대한
처리 시간이 요구
오류시 다른 비트등에 영향
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15. 관용암호시스템:conventional cryptosystem 대칭형암호시스템:symmetric cryptosystem
암호화,복호화 키가 동일
관용암호시스템:conventional cryptosystem or
대칭형암호시스템:symmetric cryptosystem
Ex) DES
송수신자간에 대칭키(비밀 키) 공유
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16. 공개키암호시스템:Public-key cryptosystem 비대칭형암호시스템:Asymmetric cryptosystem
암호화,복호화 키가 다를 경우
공개키암호시스템:Public-key cryptosystem or
비대칭형암호시스템:Asymmetric cryptosystem
Ex) 디지털 서명,개인신분확인등에 활용
송신자: 공개키, 수신자: 개인키
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17. Representation of Characters
Letter/code
A 0 B 1 C 2 D 3 E 4 F 5 G 6 H 7 I 8 J 9
K 10 L11 M12 N13 O14 P15 Q16 R17 S18 T19
U20 V21 W22 X23 Y24 Z25
A + 3 =D or K -1 = J :modular arithmetic
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18. Monoalphabetic ciphers
The Caesar cipher:
Ci =E(pi) =pi +3
Plaintext: 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
Ciphert.: D E F G H I J K L M N O P Q R s T U V W X Y Z A B C
Ex) TREATY IMPOSSIBLE
WUHDWB LPSRVVLEOH
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19. Example I CAME I SAW I CONQUERED L FDPH L VDZ L FRQTXHUHG L  I ??
I  I+3
I CAME I SAW I CONQUERED
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20. C: L GR QRW OLNH BUXWXV F(m) = (m+?) mod 26 P: I DO NOT LIKE BRUTUS
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21. Advantage/disadvantage of the Caesar cipher
Quite simple cipher
obvious pattern is major weakness
암호화: C = Ek(m) = (m + k) mode 26
복호화: m = Dk(c) = (c - k) mode 26
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22. Frequency distributions
Ex)
UZQSOVUOHXMOPVGPOPEVSGZWSZOPFPESXUDBMETSXAIZ
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWTMXUZUHSX
EPTEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
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23. Frequency distributions
Cipher’s frequency:
P Z S U O M 6.67
H D E V X F 3.33
W Q T A B G 1.67
Y I J C K L 0
N R 0
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24. Frequencies of English letters
E T 9.25 R 8.50 N 7.75 I O 7.50 A 7.25
S D 4.25 L 3.75 H 3.50 C 3.50 F 3.00 U 3.00
M P 2.75 Y 2.25 G 2.00 W 1.50 V1.50 B 1.25
K X Q 0.50 J Z 0.25
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25. ZW 3times occurs: Z:t,W:h “ZWP” the : trigraph
P:e, Z:t
{S,U,O,M,H} {r,n,I,o,a,s}
{A,B,G,Y,I,J} {w,v,b,k,x,q,j,z}
digraph:2 문자 빈도: “th”
ZW times occurs: Z:t,W:h
“ZWP” the : trigraph
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26. UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
t a e e t e a t h a t e e a a
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWTMXUZUHSX
e t t a t h a e e e a e t h t a
EPTEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
e e e t a t e t h e et
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27. “ it was disclosed yesterday that several informal but
direct contacts have been made with political
representatives of the viet cong in moscow”
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28. Other monoalphabetic substitutions
Permutation: number of 1 to 10
p1 = 1,3,5,7,9,10,8,6,4,2
p2 = 10,9,8,7,6,5,4,3,2,1
ex) p1(3) = 5 or p2(7) = 4
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29. Let a1,a2,…,ak be a set of the plaintext alphabet,
p is a permutation of 1,2,..,k in a monoalphabetic substitution each ci is ap(pi).
Ex) p(l) = 25 - l then A : z, B:y and Z: a
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ke y
a bc d fg hi j l m no pq r s t u v wxz
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30. ABCDEFGHIJKLMNOPQRSTUVWXYZ sp ec t a u l r bd f g h i j kmn oq v wxyz
:spectacular
ex) ABCDEFGHIJKLMNOPQRSTUVWXYZ
a dg j
permutation: p(l) = (3* l) mod 26
p(K) = (3* 10) mod 26 =30-26=4=e
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31. Cryptanalysis of monoalphabetic ciphers
Brute-force:가능한 모든 키를 시도
Probable-word attack:추정단어공격
ex) 계좌 화일의 전송 --> 파일 머릿부분에 키워드의 존재
원시코드--> 표준화 된 위치에 키 문장
암호알고리즘의 특성:
절대 안정성
계산상 안정성:정보가치초과,유효기간초과
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32. steganography 문자 마킹(character marking) 보이지 않는 잉크(invisible ink)
핀 구멍(Pin punctures)
타자수정리본(tpewriter correction ribbon)
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33. Polyalphabetic substitution ciphers
If T --> a, or T --> b and
X --> a or X --> b:
T:high frequency X:low frequency
E1(T) = a, E2(T) = b while E1 (X)= b and E2(X)= a
combine two distributions:
odd positions
even positions
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34. Two encryption algorithms
Odd positions:
A B C D E F G H I J K L M N O P Q R
a d g j m o s v y b e h k n q t w z
S T U V W X Y Z
c f i l o r u x
: p(l) = (3* l) mod 26
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35. Even positions: A B C D E F G H I J K L M N O P Q R
n s x c h m r w bg l q v a f k p u
S T U V W X Y Z
z e j o t y d i
: p(l) = ((5* l)+ 13) mod 26
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36. example TREATY IMPOSSIBLE TREAT YIMPO SSIBL E encryption
fumnf dyvtv czysh h
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37. Vigenere tableaux “but soft what light through yonder window breaks”
juliet : key words
julie tjuli etjul ietju lietj uliet julie tjuli
BUTSO FTWHA TLIGH TTHRO UGHYO NDERW INDOW BREAK En
KOEAS YCQSI …..
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38. Cryptanalysis of polyalphabetic substitution
Kasiski method for repeated patterns:
use repetions in the ciphertext to give cluses to the dryptanalyst of the period
P : TOBEO RNOTT OBE
K: NOWNO WNOWN OWN
C: GCXRC NACPG CXR
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39. Index of coincidence(IC): introduced in 1920 by W. Friedman
measures the variation in the frequencies of the letters in a cipheretext
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40. example Dcrypt using vigenere ciper:
TSMVM MPPCW CZUGX HPECP RFAUE IOBQW
PPIMS FXIPC TSQPK SZNUL OPACR DDPKT
SLVFW ELTKR GHIZS FNIDF ARMUE NOSKR
GDIPH WSGVL EDMCM SMWKP IYOJS TLVFA
HPBJI RAQIW HLDGA IYOU
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41. split the ciper text into 5 ection gettings:
Ic = : (åP(ai))
split the ciper text into 5 ection gettings:
a->6 g->5 I->6 q->3 v->4
b->2 h->5 m->8 r->6 w->6
c->6 I->10 n->3 s->10 x->2
d->6 j->2 o->5 t->5 y->2
e->5 k->5 p->l3 u->5 z->3
f->6
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42. We split the cipher text into five sections getting:
TMCHRIPFTSODSEGFANGWESITHRHI from text positions 51, l = 0, I, ...,27.
SPZPFOPXSZPDLLHNRODSDMYLPALY from text positions 51+1, l " 0,1,...,27.
MPHEABIIQNAPVTIIMSIGMWOVBQDO from text positions 51+2, l = 0,1,...,27.
VCGCUQMPPUCKFKZDUKPVCKJFJIGU from text positions 51+3, l = 0,1,..., 27.
MWXPEWSCKLRTWRSFERHLMPSAIWA from text positions 51+4, l = 0,1,..., 27.
5i= i+1= i+2=
5I+3= I+4=
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43. SPZPFOPXSZPDLLHNRODSDMYLPALY P-> E, Q-> F:
The second section is:
SPZPFOPXSZPDLLHNRODSDMYLPALY
P-> E, Q-> F:
HEOEUDEMHOESAAWCGDSHSBNAEPAN
The fourth section is:
VCGCUQMPPUCKFKZDUKPVCKJFJIGU
U->A,V-> B:
BIMIAWSVVAIQLQFJAQVBIQPLFOMA
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44. C-> A or K-> A. Trying these gives respectively:
TAEASOKNNSAIDIXBSINTAIHDHGES
CGCEGCFFECAFAJDEAFFCADFDCGE
Of these two the first looks the most promising so we look at what we have for our five sections as rows:
………………………………………………...
HEOEUDEMHOESAAWCGDSHSBNAEPAN
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45. TMCHRIPFTSODSEGFANGWES ITHRHI HEOEUDEMHOESAAWCGDSHSBNAEPAN
M -> E, N-> F,... in the third row giving:
TMCHRIPFTSODSEGFANGWES ITHRHI
HEOEUDEMHOESAAWCGDSHSBNAEPAN
E H M WSTAAIFSHN L A AE K A YEOGN T IVG
TAEASOKNNSAIDIXBS INTAIHDHGES
Hence we decide that the plaintext is:
THE TIME HAS COME THE WALRUS SAID TO SPEAK OF
MANY THINGS OF SHOES AND SHIPS AND SEALING WAX OF
CABBAGES AND KINGS AND WHY THE SEA IS BOLLING HOT
AND WHETHER PIGS HAVE WINGS
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46. EXAMPLE “STAR WARS” I KNOW ONLY THAT I KNOW NOTHING
H UINF NIAP OCSO H UINF INOCHIT
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47. VERNAM CIPHER VERNAMCIPHER 21417130122 8157 417 II
Plaintext VERNAMCIPHE R
Numeric Equivalent
+ Random Number II
=Sum
= mod
Ciphertext : tahrsp itxma
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48. LONG SEQUENCE FROM BOOKS
“What of thinking? I am,Iexist,that is certain”
Machine cannot think
iamie xistt hatis cert
MACHI NESCA NNOTT HINK
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49. USED BY VIGENERE TABLE: Machines cannot think
uaopm kmkvt unhbl jmed
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50. High-frequency letters: A,E,O,T : 40% and N,I: 25%
a e I n o t
A a e I n o t
E e l m r s x
I I m r w x c
N n r w b c h
O o s x c d l
T t x b g h m
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51. Ci : u a o p m k m k v t Po: ? AA ? E ? E ? ? A O I I T T T
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52. Dual message entagement
Key : disregardthismessage
mess: thismessageiscrucial
wpajqejvdzlqkovvmulgp
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53. transposition plaintext --> rearrangement --> cipertext
ex) Cryptanalyst; 3 x4 matrix:column tr.
c r y p row 2,4,1,3
t a n a \ RAYPATCTLYNS
l y s t
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54. example Suppose d =4, f=(2 3 4 1): Ptx: cryp togr aphy
Cxt: pcry rtog yaph
how identity?
How to decipher?
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55. General monoalphbetic cipers
“starw wars” --> starw
STARW
BCDEF
GHIJK
LMNOP
QUVXY Z
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56. ABCDEFGHIJKLMNOPQRSTUVWXYZ
SBGLQZTCHMUADINVREJOXWFKPY
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57. EXAMPLE(report) DE : BASED ON FREQUENCY
BRYH DRL R ITEEIA IRBS TEF CIAAXA NFR NDTEA RF FGKN RGL
AOAYJNDAYA EDRE BRYH NAGE EDA IRBS NRF FMYA EK ZK TE CKIIKNAL DAY EK FXDKKI KGA LRH NDTXD NRF RZRTGFE EDA YMIAF
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58. “Mary had a little lamb its fleece was white as snow and everywhere that mary went the lamb was sure to go it followed her to school one day which was against the rules.”
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59. Example Columnar transpositions t hisi sames saget oshow howac olumn
I tion : tssoh oaniw haaso lrsto imghw
works utpir seeoa mrook istwc nasns
c1 c2 c3 c4 c5
c c7 c8 c9 c10
c c12 etc.
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60. Most common Diagrams and Trigrams
diagrams trigrams
en ent
re ion
er and
nt ing
th ive
on tio
in for
tf our
an thi
or one
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61. Cryptanalysis by Diagram Analysis
Two different strings of letters from a transposition ciphertext can represent pairs of adjacent letters from the plaintext.
Problems: to find where in the cipertext a pair of adjacent olumns lies
where the ends of the columns are
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62. The windows of comparison shift: c1 to c9, c2 to c10….
c1 to c8, c2 to c9, …..c7 to c14.
The windows of comparison shift:
c1 to c9, c2 to c10….
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63. Prof. Jk LEE/security

64. Tssoh oaniw haaso lrsto (I(m(g(h(w
(u (t (p (I (r s)e)e)o))a m)r)o)o)k istwc nasns
50ch. -> single column 10 * 5 matrix or
second column -> 8*7 matrix
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65. Double Transposition Algorithm
Involves two columnar transpositions:
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66. tno (m(I m)tssi l(g(rr)w xswr(h s)o) cxo
Result from the second column:
tno (m(I m)tssi l(g(rr)w xswr(h s)o) cxo
hs(we)o) nxhat (ue)k)ax oao(to) isxas (I(pa)sn x
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67. - speed of transformation
Stream ciphers:
convert one symbol of plaintext immediately into a symbol of ciphertext
- speed of transformation
- low error propagation
* low diffusion
* susceptibility to malicious and
modifications
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68. Example: Monoalphabetic,Polyalphabetic Ciphers
Stream Encryption Y
Key(optional)
ISSOPMI
Plaintext
WDHUW……
Ciphertext
Encryption
Example: Monoalphabetic,Polyalphabetic Ciphers
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69. Block ciphers Encrypt a group of plaintext symbols as one block key
plaintext po
xn ba
oi encryption qc
tp kb
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70. slowness of encryption error propagation
Diffusion
immunity to insertion
slowness of encryption
error propagation
Example: columnar transposition
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71. Shannon Characteristics:
GOOD ciphers?
Shannon Characteristics:
The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption
The set of keys and enciphering algorithm should be free from complexity
The implementation of the process should be as simple as possible
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72. Errors in ciphering should not propagate and cause corrupton of further information in the message
The size of the enciphered text should be no larger than the text of the original message
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