number theory chapter 5: cryptology. era of electronic electronic communication electronic banking

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NUMBER THEORY Chapter 5: Cryptology

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NUMBER THEORY

Chapter 5: Cryptology

Era of Electronic

• Electronic communication• Electronic Banking

Terminology

• Cryptology: Discipline • Cryptography: Design & Implementation• Cryptoanalysis: Breaking the system

Terminology

• Plaintext: secret message • Cipher: method to alter the plaintext• Cybertext: message has been altered

Terminology

• encryption/enciphering: process• decryption/deciphering: process

Letter ↔ Integer

CAESAR CIPHERCharacter Cipher

Exercise

How to decipher?

Exercise

AFFINE TRANSFORMATIONCharacter Cipher

• Complete system of residue!

Exercise:

Secure?

• Character cipher is not safe

frequency

VIGENERE CIPHERBlock Cipher

• Ci=Pi+ki

Exercise

• Use the key “SECRET”

DIGRAPHIC CIPHERBlock Cipher

Numerical equivalent

VERNAM CIPHERStream Cipher

AUTOKEY VERNAM CIPHERStream Cipher

Exercise

• Cipher Text

• Seed– F

• Plain Text ?

EXPONENTIATION CIPHERCryptology

Deciphering

Fermat’s theorem

• p=2633, e=29

PUBLIC KEY CRYPTOLOGYCryptology

RSA Ciphering System

Deciphering

• Euclidean Algorithm

Factorizing prime?

• n=pq• What is the factor of following number?•

1522605027922533360535618378132637429718068114961380688657908494580122963258952897654000350692006139

• 37975227936943673922808872755445627854565536638199

× 40094690950920881030683735292761468389214899724061