Contact Information

Cryptography 21/04/23 | pag. 2

Professor

Assistant

Office

Webpage

Course Text

Ann DoomsAnn.Dooms@vub.ac.be

Adriaan BarriAdriaan.Barri@vub.ac.be

Pleinlaan 9, 2.29

http://homepages.vub.ac.be/~andooms (Education)

• Johanes A. Buchmann. Introduction to Cryptography, Springer, 2000.• Richard Mollin, Codes: The Guide to Secrecy From Ancient to Modern Times, Chapman & Hall, 2005. • Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone Handbook of Applied Cryptography, CRC Press, 1996.

Introduction

Introduction21/04/23 | pag. 3

Cryptology

Introduction21/04/23 | pag. 4

Cryptology

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A form of communication which is primarily concerned with the secure transmission (through encryption) of a secret message over an insecure channel.

Deals with attacks on encrypted intercepted messages to recover the secret message.

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Cryptography

Cryptanalysis

Cryptosystem

Introduction21/04/23 | pag. 5

Example: Ceasar Cipher

Cryptography 21/04/23 | pag. 6

Example: Ceasar Cipher

Introduction21/04/23 | pag. 7

Hence the number of keys should be large!

Basic Concepts

Introduction21/04/23 | pag. 8

Symmetric vs AsymmetricCryptosystems

Introduction21/04/23 | pag. 9

Cryptanalysis

Introduction21/04/23 | pag. 10

Kerckhoff’s principle

Attacks

Introduction21/04/23 | pag. 11

Example: Attacks

Introduction21/04/23 | pag. 12

Alphabet

Introduction21/04/23 | pag. 13

Alphabet

Introduction21/04/23 | pag. 14

Words

Introduction21/04/23 | pag. 15

Permutations

Introduction21/04/23 | pag. 16

Permutations

Cryptography 21/04/23 | pag. 17

Symmetric Cryptosystems

Symmetric Cryptosystems21/04/23 | pag. 18

Block Ciphers

Symmetric Cryptosystems21/04/23 | pag. 19

Block Ciphers

Symmetric Cryptosystems21/04/23 | pag. 20

Block Ciphers:Permutation Cipher

Symmetric Cryptosystems21/04/23 | pag. 21

Block Ciphers:Multiple Encryption

Symmetric Cryptosystems21/04/23 | pag. 22

Block Ciphers:Modes of Use

Cryptography 21/04/23 | pag. 23

• ECB: Electronic Codebook

• CBC: Cipherblock Chaining

• CFB: Cipher Feedback

• OFB: Output Feedback

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 21/04/23 | pag. 24

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 21/04/23 | pag. 25

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 21/04/23 | pag. 26

Example:Block Ciphers – ECB

Cryptography 21/04/23 | pag. 27

Drawback:Block Ciphers – ECB

Cryptography 21/04/23 | pag. 28

Example:Block Ciphers – ECB

Cryptography 21/04/23 | pag. 29

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 21/04/23 | pag. 30

Block Ciphers – CBC:XOR

Cryptography 21/04/23 | pag. 31

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 21/04/23 | pag. 32

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 21/04/23 | pag. 33

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 21/04/23 | pag. 34

Example:Block Ciphers – CBC

Cryptography 21/04/23 | pag. 35

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 21/04/23 | pag. 36

Example:Block Ciphers – CBC

Cryptography 21/04/23 | pag. 37

Drawback:Block Ciphers – CBC

Cryptography 21/04/23 | pag. 38

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 21/04/23 | pag. 39

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 21/04/23 | pag. 40

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 21/04/23 | pag. 41

Example:Block Ciphers – CFB

Cryptography 21/04/23 | pag. 42

Drawback:Block Ciphers – CFB

Cryptography 21/04/23 | pag. 43

The smaller the block length r, the shorter the ciphertext blocks. This means on the hand that transmission is faster, but on the other hand that the block encryption function must be applied more frequently. The optimal choice of r depends on a tradeoff between transmission and computation speed.

Block Ciphers – OFB:Output Feedback Mode

Cryptography 21/04/23 | pag. 44

Block Ciphers – OFB:Output Feedback Mode

Cryptography 21/04/23 | pag. 45

Block Ciphers – OFB:Output Feedback Mode

Cryptography 21/04/23 | pag. 46

Example:Block Ciphers – OFB

Cryptography 21/04/23 | pag. 47

Drawback:Block Ciphers – OFB

Cryptography 21/04/23 | pag. 48