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Cryptography and Cryptography and Network Security Network Security Chapter 2 Chapter 2 Classical Encryption Classical Encryption Techniques Techniques Fifth Edition Fifth Edition by William Stallings by William Stallings Lecture slides by Lawrie Lecture slides by Lawrie Brown Brown
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Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

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Page 1: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Cryptography and Cryptography and Network SecurityNetwork Security

Chapter 2Chapter 2 – – Classical EncryptionClassical Encryption

TechniquesTechniques

Fifth EditionFifth Edition

by William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Page 2: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Symmetric EncryptionSymmetric Encryption

or conventional / or conventional / private-keyprivate-key / single-key / single-key sender and recipient share a common keysender and recipient share a common key all classical encryption algorithms are all classical encryption algorithms are

private-keyprivate-key was only type prior to invention of public-was only type prior to invention of public-

key in 1970’skey in 1970’s and by far most widely usedand by far most widely used

Page 3: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Some Basic TerminologySome Basic Terminology

plaintextplaintext - original message - original message

ciphertextciphertext - coded message - coded message

ciphercipher - algorithm for transforming plaintext to ciphertext - algorithm for transforming plaintext to ciphertext

keykey - info used in cipher known only to sender/receiver - info used in cipher known only to sender/receiver

encipher (encrypt)encipher (encrypt) - converting plaintext to ciphertext - converting plaintext to ciphertext

decipher (decrypt)decipher (decrypt) - recovering ciphertext from plaintext - recovering ciphertext from plaintext

cryptographycryptography - study of encryption principles/methods - study of encryption principles/methods

cryptanalysis (codebreaking)cryptanalysis (codebreaking) - study of principles/ - study of principles/ methods of deciphering ciphertext methods of deciphering ciphertext withoutwithout knowing key knowing key

cryptologycryptology - field of both cryptography and cryptanalysis - field of both cryptography and cryptanalysis

Page 4: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Symmetric Cipher ModelSymmetric Cipher Model

Page 5: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

RequirementsRequirements

two requirements for secure use of two requirements for secure use of symmetric encryption:symmetric encryption: a strong encryption algorithma strong encryption algorithm a secret key known only to sender / receivera secret key known only to sender / receiver

mathematically have:mathematically have:Y Y = E(K, = E(K, XX)) (or (or EK(X))

X X = D(K, = D(K, YY) ) (or (or EK(X)) assume encryption algorithm is knownassume encryption algorithm is known implies a secure channel to distribute keyimplies a secure channel to distribute key

Page 6: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

CryptographyCryptography

can characterize cryptographic system by:can characterize cryptographic system by: type of encryption operations usedtype of encryption operations used

• substitutionsubstitution• transpositiontransposition• productproduct

number of keys usednumber of keys used• single-key or privatesingle-key or private• two-key or publictwo-key or public

way in which plaintext is processedway in which plaintext is processed• blockblock• streamstream

Page 7: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

CryptanalysisCryptanalysis

objective to recover key not just messageobjective to recover key not just message general approaches:general approaches:

cryptanalytic attackcryptanalytic attack brute-force attackbrute-force attack

if either succeed all key use compromisedif either succeed all key use compromised

Page 8: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Cryptanalytic AttacksCryptanalytic Attacks ciphertext onlyciphertext only

only know algorithm & ciphertext, is statistical, only know algorithm & ciphertext, is statistical, know or can identify plaintext know or can identify plaintext

known plaintextknown plaintext know/suspect plaintext & ciphertextknow/suspect plaintext & ciphertext

chosen plaintextchosen plaintext select plaintext and obtain ciphertextselect plaintext and obtain ciphertext

chosen ciphertextchosen ciphertext select ciphertext and obtain plaintextselect ciphertext and obtain plaintext

chosen textchosen text select plaintext or ciphertext to en/decryptselect plaintext or ciphertext to en/decrypt

Page 9: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

More DefinitionsMore Definitions unconditional securityunconditional security

no matter how much computer power or time no matter how much computer power or time is available, the cipher cannot be broken is available, the cipher cannot be broken since the ciphertext provides insufficient since the ciphertext provides insufficient information to uniquely determine the information to uniquely determine the corresponding plaintext corresponding plaintext

computational securitycomputational security given limited computing resources (eg time given limited computing resources (eg time

needed for calculations is greater than age of needed for calculations is greater than age of universe), the cipher cannot be broken universe), the cipher cannot be broken

Page 10: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Brute Force SearchBrute Force Search

always possible to simply try every key always possible to simply try every key most basic attack, proportional to key size most basic attack, proportional to key size assume either know / recognise plaintextassume either know / recognise plaintext

Key Size (bits) Number of Alternative Keys

Time required at 1 decryption/µs

Time required at 106 decryptions/µs

32 232 = 4.3 109 231 µs = 35.8 minutes 2.15 milliseconds

56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours

128 2128 = 3.4 1038 2127 µs = 5.4 1024 years 5.4 1018 years

168 2168 = 3.7 1050 2167 µs = 5.9 1036 years 5.9 1030 years

26 characters (permutation)

26! = 4 1026 2 1026 µs = 6.4 1012 years 6.4 106 years

Page 11: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Classical Substitution Classical Substitution CiphersCiphers

where where letters of plaintext are replaced by letters of plaintext are replaced by other letters or by numbers or symbolsother letters or by numbers or symbols

or if plaintext is or if plaintext is viewed as a sequence of viewed as a sequence of bits, then substitution involves replacing bits, then substitution involves replacing plaintext bit patterns with ciphertext bit plaintext bit patterns with ciphertext bit patternspatterns

Page 12: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Caesar CipherCaesar Cipher

earliest known substitution cipherearliest known substitution cipher by Julius Caesar by Julius Caesar first attested use in military affairsfirst attested use in military affairs replaces each letter by 3rd letter onreplaces each letter by 3rd letter on example:example:

meet me after the toga partymeet me after the toga partyPHHW PH DIWHU WKH WRJD SDUWBPHHW PH DIWHU WKH WRJD SDUWB

Page 13: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Caesar CipherCaesar Cipher

can define transformation as:can define transformation as: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 za 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

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 CD 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

mathematically give each letter a numbermathematically give each letter a numbera b c d e f g h i j k l m n o p q r s t u v w x y za 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

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 250 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

then have Caesar cipher as:then have Caesar cipher as:c c = E(k, = E(k, pp) = () = (p p + + kk) mod (26)) mod (26)

p p = D(k, c) = (c – = D(k, c) = (c – kk) mod (26)) mod (26)

Page 14: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Cryptanalysis of Caesar Cryptanalysis of Caesar Cipher Cipher

only have 26 possible ciphers only have 26 possible ciphers A maps to A,B,..Z A maps to A,B,..Z

could simply try each in turn could simply try each in turn a a brute force searchbrute force search given ciphertext, just try all shifts of lettersgiven ciphertext, just try all shifts of letters do need to recognize when have plaintextdo need to recognize when have plaintext eg. break ciphertext "GCUA VQ DTGCM"eg. break ciphertext "GCUA VQ DTGCM"

Page 15: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Monoalphabetic CipherMonoalphabetic Cipher

rather than just shifting the alphabet rather than just shifting the alphabet could shuffle (jumble) the letters arbitrarily could shuffle (jumble) the letters arbitrarily each plaintext letter maps to a different random each plaintext letter maps to a different random

ciphertext letter ciphertext letter hence key is 26 letters long hence key is 26 letters long

Plain: abcdefghijklmnopqrstuvwxyzPlain: abcdefghijklmnopqrstuvwxyzCipher: DKVQFIBJWPESCXHTMYAUOLRGZNCipher: DKVQFIBJWPESCXHTMYAUOLRGZN

Plaintext: ifwewishtoreplacelettersPlaintext: ifwewishtoreplacelettersCiphertext: WIRFRWAJUHYFTSDVFSFUUFYA Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

Page 16: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Monoalphabetic Cipher Monoalphabetic Cipher SecuritySecurity

now have a total of 26! = 4 x 10now have a total of 26! = 4 x 102626 keys keys with so many keys, might think is secure with so many keys, might think is secure but would be but would be !!!WRONG!!!!!!WRONG!!! problem is language characteristicsproblem is language characteristics

Page 17: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Language Redundancy and Language Redundancy and CryptanalysisCryptanalysis

human languages are human languages are redundantredundant eg "th lrd s m shphrd shll nt wnt" eg "th lrd s m shphrd shll nt wnt" letters are not equally commonly used letters are not equally commonly used in English E is by far the most common letter in English E is by far the most common letter

followed by T,R,N,I,O,A,S followed by T,R,N,I,O,A,S

other letters like Z,J,K,Q,X are fairly rare other letters like Z,J,K,Q,X are fairly rare have tables of single, double & triple letter have tables of single, double & triple letter

frequencies for various languagesfrequencies for various languages

Page 18: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

English Letter FrequenciesEnglish Letter Frequencies

Page 19: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Use in CryptanalysisUse in Cryptanalysis key concept - monoalphabetic substitution key concept - monoalphabetic substitution

ciphers do not change relative letter frequencies ciphers do not change relative letter frequencies discovered by Arabian scientists in 9discovered by Arabian scientists in 9 thth century century calculate letter frequencies for ciphertextcalculate letter frequencies for ciphertext compare counts/plots against known values compare counts/plots against known values if caesar cipher look for common peaks/troughs if caesar cipher look for common peaks/troughs

peaks at: A-E-I triple, NO pair, RST triplepeaks at: A-E-I triple, NO pair, RST triple troughs at: JK, X-Ztroughs at: JK, X-Z

for for monoalphabetic must identify each lettermonoalphabetic must identify each letter tables of common double/triple letters helptables of common double/triple letters help

Page 20: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Example CryptanalysisExample Cryptanalysis

given ciphertext:given ciphertext:UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZUZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

count relative letter frequencies (see text)count relative letter frequencies (see text) guess P & Z are e and tguess P & Z are e and t guess ZW is th and hence ZWP is theguess ZW is th and hence ZWP is the proceeding with trial and error finally get:proceeding with trial and error finally get:

it was disclosed yesterday that several informal butit was disclosed yesterday that several informal butdirect contacts have been made with politicaldirect contacts have been made with politicalrepresentatives of the viet cong in moscowrepresentatives of the viet cong in moscow

Page 21: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

字母頻率攻擊法

在 1對 1 代換加密法中,明文中字母出現的頻率與其對應的密文字母所出現的頻率相同,可以利用此特性來分析密文字母出現的頻率表以破解原明文。 例子 : 字母頻率攻擊法 -- 破解 1對 1 代換加密法英文字母頻率表

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

7.5

1.4

4.1

3.2

12.7

2.3

1.9

3.8

7.7

0.2

0.4

3.8

3.0

7.0

7.5

3.0

0.2

6.7

7.3

9.2

2.8

1.0

1.4

0.3

1.6

0.1

Page 22: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

字母頻率攻擊法

已知有一篇密文為 1對 1 代換加密而成,其密文字母出現頻率之前六名,依次為 T,R,Y,F,G,U若密文中有一段為 TFUR 求其明文?因為明文字母的頻率前六名為 E,T,I,A,O,S ,所以TE, RT, YI, FA, GO, US, 因此 TFUR 其明文為 EAST 。

Page 23: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Playfair Cipher not even the large number of keys in a

monoalphabetic cipher provides security one approach to improving security was to

encrypt multiple letters the Playfair Cipher is an example invented by Charles Wheatstone in 1854,

but named after his friend Baron Playfair

在國家寶藏電影裡出現的那串密碼,是所謂的波雷費 (Playfair) 密碼,最明顯的特徵就是那兩兩一組的字母,雖然目前已經因為複雜度不夠而被淘汰,不過這也是在第一次世界大戰裡面被拿來應用過的啊!根據維基百科查到的資料,它是由一位英國科學家 Charles Wheatstone 發明的,但卻是由他的朋友波雷費勛爵 (Lyon Playfair) 所推廣普及,所以後來也以後者的名字做為這種加密法的命名。底下我用簡單的圖例來解釋這個很簡單卻很經典的加密法吧 !   在傳統的密碼學中,加密演算法最重要的就是金匙 (Key) 了,在波雷費加密法中,金匙就是用來解謎的關鍵,在電影裡面是用關鍵字「 DEATH 」做為金鑰,

http://www.youtube.com/watch?v=_ZmRcqvDanc&feature=related

Page 24: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Playfair Key MatrixPlayfair Key Matrix

a 5X5 matrix of letters based on a keyword a 5X5 matrix of letters based on a keyword fill in letters of keyword (sans duplicates) fill in letters of keyword (sans duplicates) fill rest of matrix with other lettersfill rest of matrix with other letters eg. using the keyword MONARCHYeg. using the keyword MONARCHY

MM OO NN AA RR

CC HH YY BB DD

EE FF GG I/JI/J KK

LL PP QQ SS TT

UU VV WW XX ZZ

Page 25: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Encrypting and Decrypting

取連續兩個明文

(1) 若為對角 => 取另兩個對角(2) 若為同一行 => 向右取兩密文(3) 若為同一列 => 向下取兩密文(4) 若相同 => 中間插入 X (X為 null letter 表示明文中不出現之字母 )(5) 若明文為奇數 , 最後加入 X

Page 26: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Encrypting and Decrypting

例 1. M= JE SU SC RI ES

C= SL LX LB AK IL

(3) (1) (1) (1) ( 1)

M= JE SU SC RI ES

加密

解密

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

Page 27: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Encrypting and Decrypting例 2. M= LETTER

C= PF SZ LK AZ

LE TX TE RX

加密

解密

LE TX TE RX

M=LETTER 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

Page 28: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Security of Playfair CipherSecurity of Playfair Cipher

security much improved over monoalphabeticsecurity much improved over monoalphabetic since have 26 x 26 = 676 digrams since have 26 x 26 = 676 digrams would need a 676 entry frequency table to would need a 676 entry frequency table to

analyse (verses 26 for a monoalphabetic) analyse (verses 26 for a monoalphabetic) and correspondingly more ciphertext and correspondingly more ciphertext was widely used for many yearswas widely used for many years

eg. by US & British military in WW1eg. by US & British military in WW1 it it cancan be broken, given a few hundred letters be broken, given a few hundred letters since still has much of plaintext structure since still has much of plaintext structure

Page 29: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Polyalphabetic CiphersPolyalphabetic Ciphers

polyalphabetic substitution cipherspolyalphabetic substitution ciphers improve security using multiple cipher alphabets improve security using multiple cipher alphabets make cryptanalysis harder with more alphabets make cryptanalysis harder with more alphabets

to guess and flatter frequency distribution to guess and flatter frequency distribution use a key to select which alphabet is used for use a key to select which alphabet is used for

each letter of the message each letter of the message use each alphabet in turn use each alphabet in turn repeat from start after end of key is reached repeat from start after end of key is reached

Page 30: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Vigenère CipherVigenère Cipher

simplest polyalphabetic substitution ciphersimplest polyalphabetic substitution cipher effectively multiple caesar ciphers effectively multiple caesar ciphers key is multiple letters long K = kkey is multiple letters long K = k11 k k22 ... k ... kdd

iithth letter specifies i letter specifies ithth alphabet to use alphabet to use use each alphabet in turn use each alphabet in turn repeat from start after d letters in messagerepeat from start after d letters in message decryption simply works in reverse decryption simply works in reverse

Page 31: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Example of Example of Vigenère CipherVigenère Cipher

write the plaintext out write the plaintext out write the keyword repeated above itwrite the keyword repeated above it use each key letter as a caesar cipher key use each key letter as a caesar cipher key encrypt the corresponding plaintext letterencrypt the corresponding plaintext letter eg using keyword eg using keyword deceptivedeceptive

key: deceptivedeceptivedeceptivekey: deceptivedeceptivedeceptive

plaintext: wearediscoveredsaveyourselfplaintext: wearediscoveredsaveyourself

ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ

Fi (x) = ( x + ki ) mod 26 平移方式 ki =0..25

相同的明文可加密成不同的密文

Page 32: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Security of Security of Vigenère CiphersVigenère Ciphers

have multiple ciphertext letters for each have multiple ciphertext letters for each plaintext letterplaintext letter

hence letter frequencies are obscuredhence letter frequencies are obscured but not totally lostbut not totally lost start with letter frequenciesstart with letter frequencies

see if look monoalphabetic or notsee if look monoalphabetic or not if not, then need to determine number of if not, then need to determine number of

alphabets, since then can attach eachalphabets, since then can attach each

Page 33: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Kasiski MethodKasiski Method

method developed by Babbage / Kasiski method developed by Babbage / Kasiski repetitions in ciphertext give clues to period repetitions in ciphertext give clues to period so find same plaintext an exact period apart so find same plaintext an exact period apart which results in the same ciphertext which results in the same ciphertext of course, could also be random flukeof course, could also be random fluke eg repeated “VTW” in previous exampleeg repeated “VTW” in previous example suggests size of 3 or 9suggests size of 3 or 9 then attack each monoalphabetic cipher then attack each monoalphabetic cipher

individually using same techniques as beforeindividually using same techniques as before

Page 34: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Autokey CipherAutokey Cipher ideally want a key as long as the messageideally want a key as long as the message Vigenère proposed the Vigenère proposed the autokeyautokey cipher cipher with keyword is prefixed to message as keywith keyword is prefixed to message as key knowing keyword can recover the first few letters knowing keyword can recover the first few letters use these in turn on the rest of the messageuse these in turn on the rest of the message but still have frequency characteristics to attack but still have frequency characteristics to attack eg. given key eg. given key deceptivedeceptive

key: deceptivewearediscoveredsavkey: deceptivewearediscoveredsav

plaintext: wearediscoveredsaveyourselfplaintext: wearediscoveredsaveyourself

ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLAciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA

Page 35: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Vernam CipherVernam Cipher

ultimate defense is to use a key as long as ultimate defense is to use a key as long as the plaintextthe plaintext

with no statistical relationship to itwith no statistical relationship to it invented by AT&T engineer Gilbert invented by AT&T engineer Gilbert

Vernam in 1918Vernam in 1918 originally proposed using a very long but originally proposed using a very long but

eventually repeating keyeventually repeating key

Page 36: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

One-Time PadOne-Time Pad

if a truly random key as long as the message is if a truly random key as long as the message is used, the cipher will be secure used, the cipher will be secure

called a One-Time padcalled a One-Time pad is unbreakable since ciphertext bears no is unbreakable since ciphertext bears no

statistical relationship to the plaintextstatistical relationship to the plaintext since for since for any plaintextany plaintext & & any ciphertextany ciphertext there there

exists a key mapping one to otherexists a key mapping one to other can only use the key can only use the key onceonce though though problems in generation & safe distribution of keyproblems in generation & safe distribution of key

Page 37: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

One-Time Pad

只能用一次,其運算為 XOR M= 1 1 0

0 0 ( 加密 ) K =1 0 0

1 0 C =0 1 0

1 0 ( 解密 ) K =1 0

0 1 0 M =1 1

0 0 0

Page 38: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Transposition CiphersTransposition Ciphers

now consider classical now consider classical transpositiontransposition or or permutationpermutation ciphers ciphers

these hide the message by rearranging these hide the message by rearranging the letter order the letter order

without altering the actual letters usedwithout altering the actual letters used can recognise these since have the same can recognise these since have the same

frequency distribution as the original text frequency distribution as the original text

Page 39: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Rail Fence cipherRail Fence cipher

write message letters out diagonally over a write message letters out diagonally over a number of rows number of rows

then read off cipher row by rowthen read off cipher row by row eg. write message out as:eg. write message out as:

m e m a t r h t g p r ym e m a t r h t g p r y e t e f e t e o a a te t e f e t e o a a t

giving ciphertextgiving ciphertextMEMATRHTGPRYETEFETEOAATMEMATRHTGPRYETEFETEOAAT

Page 40: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Row Transposition CiphersRow Transposition Ciphers

is a more complex transpositionis a more complex transposition write letters of message out in rows over a write letters of message out in rows over a

specified number of columnsspecified number of columns then reorder the columns according to some then reorder the columns according to some

key before reading off the rowskey before reading off the rowsKey: Key: 43125674312567Column Out 3 4 2 1 5 6 7Column Out 3 4 2 1 5 6 7Plaintext: a t t a c k pPlaintext: a t t a c k p o s t p o n eo s t p o n e d u n t i l td u n t i l t w o a m x y zw o a m x y zCiphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZCiphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ

Page 41: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Transposition Ciphers

(1) 反轉法:加密時,明文依序寫入,然後反向讀出密文。解密時,利用密文依

序寫入,然後反向讀出明文。例如:

明文=CIPHER

反轉 加密

密文=REHPIC

反轉 解密

明文=CIPHER

Page 42: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Transposition Ciphers

(2) 幾何圖形換位法:

假設明文M為 COUNTTHECOST,以寬度 4的長方形作為圖形加密,則可

以產生 34的長方形如下:

COUN

TTHE

COST

從 此 長 方 形 中 , 利 用 1,3,2,4 行 的 次 序 , 可 以 讀 出 密 文

C=CTCUHSOTONET。在幾何圖形換位法中,長方形與讀取行的次序可以

事先協調,當作私密金匙。當接收者得到密文 C=CTCUHSOTONET 時,

建立寬度 4的長方形

1 2 3 4

CUON THTE CSOT

然後利用私密金匙,循 1,3,2,4行讀取,還原成明文M=COUNTTHECOST。

Page 43: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Transposition Ciphers(3) 循途徑換位法:加密圖形為長方形,而寫入與讀出的方式是循某一途徑。例

如明文M為 COUNTTHECOST,其產生的長度 2之長方形如下圖所示,

CUTHCS

ONTEOT

經水平讀出密文為 C =CUTHCSONTEOT。解密時,利用密文的長度 12除

以 2得到長方形的寬度 6,因此密文 6個一組可以建立原來長方形,再垂

直方式讀出明文。基本上,途徑的方式有水平(,),垂直(,),對角線

(,,,),順時針 ,逆時針 等。

Page 44: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Transposition Ciphers行換位:

Page 45: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Product CiphersProduct Ciphers

ciphers using substitutions or transpositions are ciphers using substitutions or transpositions are not secure because of language characteristicsnot secure because of language characteristics

hence consider using several ciphers in hence consider using several ciphers in succession to make harder, but: succession to make harder, but: two substitutions make a more complex substitution two substitutions make a more complex substitution two transpositions make more complex transposition two transpositions make more complex transposition but a substitution followed by a transposition makes a but a substitution followed by a transposition makes a

new much harder cipher new much harder cipher this is bridge from classical to modern ciphersthis is bridge from classical to modern ciphers

Page 46: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Rotor MachinesRotor Machines

before modern ciphers, rotor machines were before modern ciphers, rotor machines were most common complex ciphers in usemost common complex ciphers in use

widely used in WW2widely used in WW2 German Enigma, Allied Hagelin, Japanese PurpleGerman Enigma, Allied Hagelin, Japanese Purple

implemented a very complex, varying implemented a very complex, varying substitution ciphersubstitution cipher

used a series of cylinders, each giving one used a series of cylinders, each giving one substitution, which rotated and changed after substitution, which rotated and changed after each letter was encryptedeach letter was encrypted

with 3 cylinders have 26with 3 cylinders have 2633=17576 alphabets=17576 alphabets

Page 47: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Hagelin Rotor MachineHagelin Rotor Machine

Page 48: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

SteganographySteganography

an alternative to encryptionan alternative to encryption hides existence of messagehides existence of message

using only a subset of letters/words in a using only a subset of letters/words in a longer message marked in some waylonger message marked in some way

using invisible inkusing invisible ink hiding in LSB in graphic image or sound filehiding in LSB in graphic image or sound file

has drawbackshas drawbacks high overhead to hide relatively few info bitshigh overhead to hide relatively few info bits

advantage is can obscure encryption useadvantage is can obscure encryption use

Page 49: Cryptography and Network Security Chapter 2 – Classical Encryption Techniques Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

SummarySummary

have considered:have considered: classical cipher techniques and terminologyclassical cipher techniques and terminology monoalphabetic substitution ciphersmonoalphabetic substitution ciphers cryptanalysis using letter frequenciescryptanalysis using letter frequencies Playfair cipherPlayfair cipher polyalphabetic cipherspolyalphabetic ciphers transposition cipherstransposition ciphers product ciphers and rotor machinesproduct ciphers and rotor machines stenographystenography