02-Classical_encryption DR MOHASIN

8/6/2019 02-Classical_encryption DR MOHASIN

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CEN 448ecur y an n erne ro oco s

Chapter 2

Classical Encryption Techniques

Dr. Mostafa Hassan DahshanComputer Engineering Department

King Saud University

[email protected]

Acknowledgements

These notes use some slides from

WilliamStallings.com website made by Dr.Lawrie Brown

Imported slides have Italic Titles

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Symmetric Encryption

aka conventional, private-key / single-key sender and recipient share a common key

private-key

was only type prior to invention of public-

ke in 1970s and by far most widely used

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In redients

Plaintext

original intelligible message

performs substitutions, transformations

input: plaintext, key. output: ciphertext

Secret Ke

different keys different outputs,

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In redients

Cipher textunintelligible scrambled message

Decryption algorithm

encryption algorithm run in reverse

input: ciphertext, key. output: plaintext

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Sim lified Model

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Sim lified Model

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Requirements

two requirements for secure use of

symme r c encryp on:a strong encryption algorithm

a secret key known only to sender / receiver

mathematicall have:Y= EK(X)

= K

assume encryption algorithm is known

implies a secure channel to distribute key8


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Characterization

Type of operationsubstitution: each element of plaintext (bit,

character ma ed to another element

transposition: plaintext elements rearranged

stream cipher: element by element (bit, byte)block cipher: block transformed as a whole

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Encr tion Attacks

Cryptanalysis

exploit characteristics of algorithm todeduce laintext or encr tion ke

may use pairs of plaintext, ciphertext

-

try all possible keys on ciphertext

on average, half of possible keys tried

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Types of Encryption Security

cipher cannot be brokenno ma er ow muc compu er power or me s

available

determine the corresponding plaintext

-

computational security

information

information11

Cryptanalysis Attacks

Attempt to deduce specific plaintext or key

Rely on

some knowledge of plaintext characteristics

Examples

some file t es have common header

exploit statistics of human language

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Cryptanalysis Attacks

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Brute Force Attacks

always possible to simply try every key

most basic attack, proportional to key size

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Brute Force Attacks

Key size (bits) Number of

alternative keys

Time required at 1

decryption/sTime required at 10

decryption/s32 2 = 4.3 x 10 2 s = 35.8 minutes 2.15 milliseconds

56 256

= 7.2 x 1016

255

s = 1142 years 10.01 hours

128 2 = 3.4 x 10 2 s = 5.4 x 10 years 5.4 x 10 years

168 2168

= 3.7 x 1050

2167

s = 5.9 x 1036

years 5.9 x 1030

years

c aracters

(permutation)

= x 2 x 10 s = . x years . x years

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Substitution Techniques

Letters in plaintext is replaced by

other letters

symbols

a n ex -sequence s rep ace y a

ciphertext sequence

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Substitution Techni ues

Caesar cipher Monoalphabetic ciphers

Polyalphabetic ciphers

One-time pad

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Caesar Ci her

Ciphertext letter = plaintext letter + 3

Letters wrap around, Z is next after Aa 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 C

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Caesar Ci her

C = E(3, p) = (p + 3) mod 26 If shift is different from 3

= = + ,

p = D(k, C) = (C k) mod 26

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Brute Force Attack

Encryption and

ecryp on a gor msare known

Only 25 keys to try

Plaintext lan ua e is

known

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Monoal habetic Ci her

Arbitrary substitution of letters Number of keys 26251 = 26! (Over

26

Regularities in the language can be

exp o e

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Monoal habetic Exam le

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Monoal habetic Exam le

Frequency of letters P e, Z t

Frequency of two-

letter combinations

ZW th

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Pla fair Ci her

55 matrix of letters

Constructed using a keyword

C H Y B D

E F G I/J K

U V W X Z

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Pla fair Ci her

Plaintext encrypted two letters at a time Repeating letters separated by filler x

e.g. balloon ba lx lo on

Letters in same row are each replaced by. . .

Letters in same col are each replaced by. . .

Otherwise, letter replaced by one in its rowan co o e o er e er. s

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Pla fair Ci her

Advantages

2626 diagrams (two letter combinations)

Disadvantages

still leaves much of language structure

few 100s of ciphertext letters are enough for

cryptanalysis

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Relative Fre uenc of Letters

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Polyalphabetic Ciphers

polyalphabetic substitution ciphers

improve security using multiple cipher alphabets make cr tanal sis harder with more al habets

to guess and flatter frequency distribution

each letter of the message

repeat from start after end of key is reached

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Vigenre Cipher

simplest polyalphabetic substitution cipher effectively multiple Caesar ciphers

1 2 ... d

ith letter specifies ith alphabet to use

use each alphabet in turn

decryption simply works in reverse

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Vigenre Cipher

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Example of Vigenre Cipher

write the plaintext out write the keyword repeated above it

use each ke letter as a Caesar ci her ke

encrypt the corresponding plaintext letter

key: deceptivedeceptivedeceptive

p a ntext: weare scovere saveyourse

ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ

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One-Time Pad

if a truly random key as long as the message is

use , e c p er w e secure called a One-Time pad

is unbreakable since ciphertext bears no

statistical relationshi to the laintext

since forany plaintext & any ciphertext there

can only use the key once though

pro ems n genera on sa e s r u on o ey32


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One-Time Pad Example

Vigenre scheme with 27 characters 27th character is space

- ,

key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih

plaintext: mr mustard with the candlestick in the hall

ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS

key: mfugpmiydgaxgoufhklllmhsqdqogtewbqfgyovuhwt

laintext: miss scarlet with the knife in the librar

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Trans osition Techni ues

Perform some permutations on plaintext

letters

Rail fence cipher

ranspos t on matr x

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Rail Fence Ci her

Plaintext written as sequence of diagonals Read off as sequence of rows

MEMATRHTGPRYETEFETEOAAT

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Trans osition Matrix

Write message in rectangle, row by row

Read message off, column by column

Order of columns is the key

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Original order of letters 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

17 18 19 20 21 22 23 24 25 26 27 28

er ranspos on 03 10 17 24 04 11 18 25 02 09 16 23 01 08 15 22

Somewhat regular structure

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More than one stage of transposition

After second transposition

04 23 19 14 11 01 26 21 18 08 06 28

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