Chapter 1 Introduction Cryptography-Principles and Practice Harbin Institute of Technology School of Computer Science and Technology Zhijun Li lizhijun.

Chapter 1 Introduction

Cryptography-Principles and PracticeHarbin Institute of TechnologySchool of Computer Science and Technology

Zhijun Lihttp://cst.hit.edu.cn/~lizhijun

[email protected]

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Outline

• Cryptography Overview

• Basic Terminology

• Goals of Cryptography

• Historical Review of Cryptology

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Cryptography Overview

• Greek: “krypto” = hide

• Cryptology – science of hiding = cryptography + cryptanalysis

• Cryptography – design to hide

• Cryptanalysis – analyzing (breaking) secrets– Cryptanalysis is what attacker does– Decryption is what legitimate receiver does

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Outline

• Cryptography Overview

• Basic Terminology

• Goals of Cryptography

• Historical Review of Cryptology

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Basic Terminology

• Cryptology

• Cryptography

• Cryptanalysis

• Ciphers (Cryptosystems, Cryptoalgorithms)

• Encryption and Decryption

• Plaintext(Cleartext)

• Ciphertext(Cryptogram)

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Cryptography

• Cryptography deals with the design of algorithms, protocol, or service using some basic tools to achieve the security goals

• Tools cover encryption algorithms, one-way functions, pseudorandom functions, hashing functions, signature schemas, etc

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Cryptanalysis

• Description: Cryptanalysis is an (usually vulnerability) analysis of a cryptosystem– Using mathematical methods to prove that the design

does (or does not) achieve a security goal under some (new) attacks

• Description: Attack is an attempt to cryptanalysis

• Note: a part of cryptography

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Ciphers

Alice Encryption Decryption Bob

Secure Channel

Oscar

M C M

K

Encipher Decipher

C=EK(M); M=DK(C)

• M is plaintext (cleartext)• C is ciphertext (cryptogram)• K is key

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Formal Description

• A cryptosystems is (M,C,K,E,D)– M set of all plaintext – C set of all ciphertext – K set of all keys (keyspace)

– E set of encryption rules, ek: M C

– D set of decryption rules dk : C M

– For any kK, there exists a ek E and corresponding dk D such that

mM, dk(ek(m))=m

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Mappings or Algorithms

• Mappings – ek and dk are functions– ek is an injection and normally an one-to-one function– If M=C, ek is a permutation– The theory aspect

• Algorithms– ek and dk should be implemented and analyzed via co

mputer– The practice aspect

• Mappings and Algorithms

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• Cryptography Overview

• Basic Terminology

• Goals of Cryptography

• Historical Review of Cryptology

Outline

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• The most basic problem:

Ensure security of communication over insecure medium

• In the architecture of information security

Goals of Cryptography

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• Confidentiality(privacy,secrecy)– Ensures that information flow between the sender and

the receiver is unintelligible to outsiders (against eavesdropping)

Goal Confidentiality

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• Message integrity– Enables the receiver to verify whether the message h

as been tampered with by outsiders

Goal Message Integrity

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Goal Message Authentication

• Message authentication– Allows the receiver of messages to determine the true

identity of the sender (against impersonation)

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Goal Non-repudiation

• Non-repudiation– Prevents the sender of a message from claiming that t

hey have not sent the message

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• Security Service is something that enhances the security – Make use of one or more security mechanisms

• Security Mechanism is designed to detect, prevent, or recover from a security attack– One particular element is: cryptographic techniques

• Attacks are any action that compromises the security of information

Security Infrastructure

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Kerckhoff’s Principle

The type of Attacks

Attacks of Cryptography

• Known something– The algorithm– Cleartext or Ciphertext or others

• Want to know (Purpose)– The key– Cleartexts for some Ciphertexts– Others

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Types of Attacks

• Ciphertext-only attack (COA)– Only know some ciphertexts

• Known-plaintext attack (KPA)– Know some pairs of message and corresponding cryptogram

• Chosen-plaintext attack (CPA)– Can choose messages and read the corresponding cryptogram– Can interact the encipher

• Chosen-ciphertext attack (CCA)– Can choose cryptograms and observe the corresponding messa

ges– Can interact the decipher

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Relationships of Attacks

• Strengths of Attacks– COA<KPA<CPA<CCA

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Brute Force Attack

Key size(bits) Time (1us/test) Time(1us/106test)

32 35.8mins 2.15 msec

40 6.4days 550 msec

56 1140yeas 10.0 hours

64 ~500000yeas 107 days

128 5 1024 yeas 5 1018 years

• Key space enumeration– Ciphertext-only attacks

• Suppose the correct plaintexts can be identified

– Know one pair of message and cryptogram

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Outline

• Cryptography Overview

• Basic Terminology

• Goals of Cryptography

• Historical Review of Cryptology

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Cryptanalysts

3000BC

monoalphabetics

900

al-Kindi - frequency analysis

Alberti – first polyalphabetic cipher

1460

Vigenère

1854

Babbage breaks Vigenère;Kasiski (1863) publishes

Cryptographers

Brief History-First 4000 Years

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Cryptographers

Cryptanalysts

1854 1918

Mauborgne – one-time pad

Mechanical ciphers - Enigma

1939

Rejewski repeated message-key attack

Turing’s loop attacks, Colossus

Enigma adds rotors, stops repeated key

1945

Feistel block cipher, DES

Linear, Differential Cryptanalysis

?

1973

Public-Key

Quantum Crypto

1978

Brief History-Last 100 Years