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Prof. Jk LEE/security1 암호학 (Cryptology) Bob Alice 공격자 암호문 평문

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Prof. Jk LEE/security2 비자카드 번호 확인 st 16th Select odd numbers Select even numbers After * 2, if 9 then = 70 *10

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Prof. Jk LEE/security3 암호학 (Cryptology) 이란 ? 암호화 기법과 암호분석기법에 관한 원리, 수단, 방법 을 연구하는 학문 평문의 해독 불가하도록 하는 방법과 해독 불가능한 메시지를 해독 가능하도록 형태를 바꾸는 방법으로 구성

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Prof. Jk LEE/security4 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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Prof. Jk LEE/security5 S T R sender access receiver O interceptor/intruder - block - intercept - modify - fabricate

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Prof. Jk LEE/security6 Terminology Encryption Decryption Cryptosystem: system for encryption and decryption Plaintext Ciphertext

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Prof. Jk LEE/security7 Encryption Algorithms Encryption encryption decryption plaintextciphertext Original plaintext

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Prof. Jk LEE/security8 encryption decryption plaintextciphertext Original plaintext key Symmetric cryptosystem encryption decryption plaintextciphertext Original plaintext Encryption Key:K E Decryption Key:K D Asymmetric cryptosystem

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Prof. Jk LEE/security9 Cryptanalysis Cryptography: hidden writing cryptanalyst: studies encryption,encryption message cryptology: research of encryption and decryption

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Prof. Jk LEE/security10 - attempt to break a single message - attempt to recognize patterns in encrypted message - attempt to find general weaknesses in an encryption algorithm Cryptanalyst’s chore: break an encryption !

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Prof. Jk LEE/security11 암호시스템의 설계요건 난이도가 클 것 키의 크기가 작을 것 암. 복호화 여건의 간결성과 처리속도의 효율성 에러 전파율이 적을 것

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Prof. Jk LEE/security12 암호시스템의 분류 시대별 분류 : 고전 암호시스템 :19 세기이전 근대 암호시스템 :1,2 차 대전 현대암호화 시스템 :1950 년이후 평문의 암호화 단위분류 : 블록 암호시스템 스트림 암호시스템 암호화 형식에 의한 분류 : 비밀키 ( 대칭형 ) 암호시스템 : 비밀키 공개키 ( 비대칭형 ) 암호시스템 : 공개키와 비공개키

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Prof. Jk LEE/security13 스트림 암호 : stream cipher encryption decryption plaintextciphertext Original plaintext 키 생성 알고리즘 비밀키 기밀성과 무결성이 보장되는 채널 암호화의 속도가 빠르다 오류의 영향이 적다 비트가 독립적인 관계로 각각의 비트를 암호의 개별적인 개체로 취급이 가능 암호키에 대한 엄격한 동기화 요구 Synchronization! 키 생성 알고리즘이 중요 ! 평문 길이 최소 단위 : 한 개 단위의 비트나 문자

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Prof. Jk LEE/security14 블록 암호 :Block cipher encryption decryption plaintext Bolck ciphertext Original plaintext 비밀키 기밀성과 무결성이 보장되는 채널 평문의 길이가 한개이상 DES,RSA 등 암호화, 블럭화에 대한 처리 시간이 요구 오류시 다른 비트등에 영향 스트림 암호와 대칭성

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Prof. Jk LEE/security15 관용암호시스템 :conventional cryptosystem 대칭형암호시스템 :symmetric cryptosystem Ex) DES 송수신자간에 대칭키 ( 비밀 키 ) 공유 or 암호화, 복호 화 키가 동 일

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Prof. Jk LEE/security16 공개키암호시스템 :Public-key cryptosystem 비대칭형암호시스템 :Asymmetric cryptosystem Ex) 디지털 서명, 개인신분확인등에 활용 송신자 : 공개키, 수신자 : 개인키 or 암호화, 복호 화 키가 다 를 경우

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Prof. Jk LEE/security17 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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Prof. Jk LEE/security18 Monoalphabetic ciphers The Caesar cipher: C i =E(p i ) =p i +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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Prof. Jk LEE/security19 Example L FDPH L VDZ L FRQTXHUHG I I+3 L I ?? I CAME I SAW I CONQUERED

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Prof. Jk LEE/security20 P: I DO NOT LIKE BRUTUS C: L GR QRW OLNH BUXWXV F(m) = (m+?) mod 26

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Prof. Jk LEE/security21 Advantage/disadvantage of the Caesar cipher Quite simple cipher obvious pattern is major weakness 암호화 : C = E k (m) = (m + k) mode 26 복호화 : m = D k (c) = (c - k) mode 26

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Prof. Jk LEE/security22 Ex) UZQSOVUOHXMOPVGPOPEVSGZWSZOPFPESXUDBMETSX AIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWTMXUZUHSX EPTEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ Frequency distributions

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Prof. Jk LEE/security23 Frequency distributions Cipher’s frequency : P Z S 8.33 U 8.33 O 7.50 M 6.67 H 5.83 D 5.00 E 5.00 V 4.17 X 4.17 F 3.33 W 3.33 Q 2.50 T 2.50 A 1.67 B 1.67 G 1.67 Y 1.67 I 0.83 J 0.83 C 0 K 0 L 0 N 0 R 0

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Prof. Jk LEE/security24 E T 9.25 R 8.50 N 7.75 I 7.75 O 7.50 A 7.25 S 6.00 D 4.25 L 3.75 H 3.50 C 3.50 F 3.00 U 3.00 M 2.75 P 2.75 Y 2.25 G 2.00 W 1.50 V1.50 B 1.25 K 0.50 X 0.50 Q 0.50 J 0.25 Z 0.25 Frequencies of English letters

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Prof. Jk LEE/security25 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 3times occurs: Z:t,W:h “ZWP” the : trigraph

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Prof. Jk LEE/security26 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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Prof. Jk LEE/security27 “ 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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Prof. Jk LEE/security28 Other monoalphabetic substitutions Permutation: number of 1 to 10 1 = 1,3,5,7,9,10,8,6,4,2 2 = 10,9,8,7,6,5,4,3,2,1 ex) 1 (3) = 5 or 2 (7) = 4

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Prof. Jk LEE/security29 Let a 1,a 2,…,a k be a set of the plaintext alphabet, is a permutation of 1,2,..,k in a monoalphabetic substitution each c i is a (pi). Ex) ( ) = 25 - 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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Prof. Jk LEE/security30 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: ( ) = (3* ) mod 26 (K) = (3* 10) mod 26 =30-26=4=e

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Prof. Jk LEE/security31 Cryptanalysis of monoalphabetic ciphers Brute-force: 가능한 모든 키를 시도 Probable-word attack : 추정단어공격 ex) 계좌 화일의 전송 --> 파일 머릿부분에 키 워드의 존재 원시코드 --> 표준화 된 위치에 키 문장 암호알고리즘의 특성 : 절대 안정성 계산상 안정성 : 정보가치초과, 유효기간초과

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Prof. Jk LEE/security32 steganography 문자 마킹 (character marking) 보이지 않는 잉크 (invisible ink) 핀 구멍 (Pin punctures) 타자수정리본 (tpewriter correction ribbon)

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Prof. Jk LEE/security33 Polyalphabetic substitution ciphers If T --> a, or T --> b and X --> a or X --> b: T:high frequency X:low frequency E 1 (T) = a, E 2 (T) = b while E 1 (X)= b and E 2 (X)= a combine two distributions: odd positions even positions

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Prof. Jk LEE/security34 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 : ( ) = (3* ) mod 26

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Prof. Jk LEE/security35 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 : ( ) = ((5* )+ 13) mod 26

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Prof. Jk LEE/security36 example TREATY IMPOSSIBLE encryption fumnf dyvtv czysh h

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Prof. Jk LEE/security37 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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Prof. Jk LEE/security38 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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Prof. Jk LEE/security39 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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Prof. Jk LEE/security40 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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Prof. Jk LEE/security41 Ic = : ( P(a i )) 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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Prof. Jk LEE/security42 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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Prof. Jk LEE/security43 The second section is: SPZPFOPXSZPDLLHNRODSDMYLPALY P-> E, Q-> F: HEOEUDEMHOESAAWCGDSHSBNAEPAN The fourth section is: VCGCUQMPPUCKFKZDUKPVCKJFJIGU U->A,V-> B: BIMIAWSVVAIQLQFJAQVBIQPLFOMA

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Prof. Jk LEE/security44 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 ………………………………………………... TAEASOKNNSAIDIXBSINTAIHDHGES ………………………………………………...

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Prof. Jk LEE/security45 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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Prof. Jk LEE/security46 EXAMPLE “STAR WARS” I KNOW ONLY THAT I KNOW NOTHING H UINF NIAP OCSO H UINF INOCHIT

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Prof. Jk LEE/security47 VERNAM CIPHER VERNAMCIPHER II Plaintext VERNAMCIPHE R Numeric Equivalent Random Number II =Sum = mod Ciphertext : tahrsp itxma

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Prof. Jk LEE/security48 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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Prof. Jk LEE/security49 USED BY VIGENERE TABLE: Machines cannot think uaopm kmkvt unhbl jmed

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Prof. Jk LEE/security50 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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Prof. Jk LEE/security51 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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Prof. Jk LEE/security52 Dual message entagement Key : disregardthismessage mess: thismessageiscrucial wpajqejvdzlqkovvmulgp

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Prof. Jk LEE/security53 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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Prof. Jk LEE/security54 example Suppose d =4, f=( ): Ptx: cryp togr aphy Cxt: pcry rtog yaph how identity? How to decipher?

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Prof. Jk LEE/security55 General monoalphbetic cipers “starw wars” --> starw STARW BCDEF GHIJK LMNOP QUVXY Z

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Prof. Jk LEE/security56 ABCDEFGHIJKLMNOPQRSTUVWXY Z SBGLQZTCHMUADINVREJOXWFKP Y

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Prof. Jk LEE/security57 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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Prof. Jk LEE/security58 “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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Prof. Jk LEE/security59 Example Columnar transpositions t hisi sames saget oshow howac olumn artra nspos I tion : tssoh oaniw haaso lrsto imghw works utpir seeoa mrook istwc nasns c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 etc.

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Prof. Jk LEE/security60 Most common Diagrams and Trigrams diagramstrigrams enent reion erand nting thive ontio infor tfour anthi orone

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Prof. Jk LEE/security61 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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Prof. Jk LEE/security62 c1 to c8, c2 to c9, …..c7 to c14. The windows of comparison shift: c1 to c9, c2 to c10….

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Prof. Jk LEE/security63

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Prof. Jk LEE/security64 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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Prof. Jk LEE/security65 Double Transposition Algorithm Involves two columnar transpositions:

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Prof. Jk LEE/security66 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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Prof. Jk LEE/security67 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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Prof. Jk LEE/security68 Y Key(optional) ISSOPMI Plaintext WDHUW…… Ciphertext Encryption Stream Encryption Example: Monoalphabetic,Polyalphabetic Ciphers

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Prof. Jk LEE/security69 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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Prof. Jk LEE/security70 Diffusion immunity to insertion slowness of encryption error propagation Example: columnar transposition

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Prof. Jk LEE/security71 GOOD ciphers? 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 Shannon Characteristics:

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Prof. Jk LEE/security72 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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