Cryptography

Contact Information

Cryptography 22/04/23 | 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

Introduction22/04/23 | pag. 3

Cryptology

Introduction22/04/23 | 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

Introduction22/04/23 | pag. 5

Example: Ceasar Cipher

Cryptography 22/04/23 | pag. 6

Example: Ceasar Cipher

Introduction22/04/23 | pag. 7

Hence the number of keys should be large!

Basic Concepts

Introduction22/04/23 | pag. 8

Symmetric vs AsymmetricCryptosystems

Introduction22/04/23 | pag. 9

Cryptanalysis

Introduction22/04/23 | pag. 10

Kerckhoff’s principle

Attacks

Introduction22/04/23 | pag. 11

Example: Attacks

Introduction22/04/23 | pag. 12

Alphabet

Introduction22/04/23 | pag. 13

Alphabet

Introduction22/04/23 | pag. 14

Words

Introduction22/04/23 | pag. 15

Permutations

Introduction22/04/23 | pag. 16

Permutations

Cryptography 22/04/23 | pag. 17

Symmetric Cryptosystems

Symmetric Cryptosystems22/04/23 | pag. 18

Block Ciphers

Symmetric Cryptosystems22/04/23 | pag. 19

Block Ciphers

Symmetric Cryptosystems22/04/23 | pag. 20

Block Ciphers:Permutation Cipher

Symmetric Cryptosystems22/04/23 | pag. 21

Block Ciphers:Multiple Encryption

Symmetric Cryptosystems22/04/23 | pag. 22

Block Ciphers:Modes of Use

Cryptography 22/04/23 | pag. 23

• ECB: Electronic Codebook

• CBC: Cipherblock Chaining

• CFB: Cipher Feedback

• OFB: Output Feedback

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 22/04/23 | pag. 24

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 22/04/23 | pag. 25

Block Ciphers – ECB:Electronic Codebook Mode

Cryptography 22/04/23 | pag. 26

Example:Block Ciphers – ECB

Cryptography 22/04/23 | pag. 27

Drawback:Block Ciphers – ECB

Cryptography 22/04/23 | pag. 28

Example:Block Ciphers – ECB

Cryptography 22/04/23 | pag. 29

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 22/04/23 | pag. 30

Block Ciphers – CBC:XOR

Cryptography 22/04/23 | pag. 31

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 22/04/23 | pag. 32

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 22/04/23 | pag. 33

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 22/04/23 | pag. 34

Example:Block Ciphers – CBC

Cryptography 22/04/23 | pag. 35

Block Ciphers – CBC:Cipherblock Chaining Mode

Cryptography 22/04/23 | pag. 36

Example:Block Ciphers – CBC

Cryptography 22/04/23 | pag. 37

Drawback:Block Ciphers – CBC

Cryptography 22/04/23 | pag. 38

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 22/04/23 | pag. 39

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 22/04/23 | pag. 40

Block Ciphers – CFB:Cipher Feedback Mode

Cryptography 22/04/23 | pag. 41

Example:Block Ciphers – CFB

Cryptography 22/04/23 | pag. 42

Drawback:Block Ciphers – CFB

Cryptography 22/04/23 | 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 22/04/23 | pag. 44

Block Ciphers – OFB:Output Feedback Mode

Cryptography 22/04/23 | pag. 45

Block Ciphers – OFB:Output Feedback Mode

Cryptography 22/04/23 | pag. 46

Example:Block Ciphers – OFB

Cryptography 22/04/23 | pag. 47

Drawback:Block Ciphers – OFB

Cryptography 22/04/23 | pag. 48