cryptographic techniques

Web Security: Theory & ApplicationsSchool of Software, Sun Yat-sen University

Module I. Fundamentals of Information Security

Chapter 2Cryptographic Techniques

Web Security : Theory And Applications 2 / 41

Chapter 2 - Outline

2.1 Cryptology Introduction Introduction History Concepts & Items

2.2 Symmetric Key Cryptographic Algorithms Introduction Types & Modes Data Encryption Standard (DES) Advanced Encryption Standard (AES)


2.3 Asymmetric Key Cryptographic Algorithms Introduction The RSA Algorithm Digital Signatures

2.4 Hashing Algorithms Introduction Message-Digest Algorithm (MD5)

2.5 Typical Applications MD5 and Passwords AES and Wi-Fi Protected Access RSA and E-Business

Chapter 2 - Outline


Chapter 2 - Outline

2.1 Cryptology Introduction Introduction History Concepts & Items

2.2 Symmetric Key Cryptographic Algorithms2.3 Asymmetric Key Cryptographic Algorithms2.4 Hashing Algorithms2.5 Typical Applications


2.1.1 Introduction Definition

Cryptography (or cryptology; from Greek krypts, "hidden, secret"; and grphein, "writing", or -logia, "study", respectively) is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation.

2.1 Cryptology Introduction


Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering.

Applications of cryptography include ATM cards, computer passwords, and electronic commerce.



2.1.2 History The Manual ERA The Mechanical ERA The Modern ERA


German Lorenz cipher machine,

used in World War II to encrypt

very-high-level general staff

messages


2.1.3 Concepts & Items Plain Text and Cipher Text

Key and Key Space

Cryptosystem Services

Confidentiality, Integrity, Authenticity, Non-repudiation, Access Control

Cryptographic Methods

Symmetric, Asymmetric Attributes of Strong Encryption

Confusion, Diffusion



Chapter 2 - Outline

2.1 Cryptology Introduction 2.2 Symmetric Key Cryptographic Algorithms

Introduction Types & Modes Data Encryption Standard (DES) Advanced Encryption Standard (AES)

2.3 Asymmetric Key Cryptographic Algorithms 2.4 Hashing Algorithms 2.5 Typical Applications


2.2.1 Introduction Symmetric-key cryptography is sometimes called secret-

key cryptography. It is a kind of encryption system in which the sender and receiver of a message share a single, common key that is used to encrypt and decrypt the message.

Symmetric-key systems are simpler and faster. The two parties must somehow exchange the key in a secure way.

The most popular symmetric-key system is the DES,Data Encryption Standard.

2.2 Symmetric Key Crypt. Algorithms


2.2.2 Algorithm Types and Modes Algorithm Types

Block CipherE(M,K)=E(M1,K)E(M2,K) ..E(Mn,K)

MM1M2 Mn

Stream Cipher



Algorithm Modes

Electronic Code Book (ECB) Mode



Source ECB mode non-ECB mode



Cipher Block Chaining (CBC) Mode (IBM,1976) CBC - Encryption



CBC - Decryption



Cipher Feedback (CFB) Mode CFB - Encryption



CFB - Decryption



Output Feedback (OFB) Mode OFB - encryption



OFB - Decryption



2.2.3 Data Encryption StandardDES Background and History

How DES Works

Feistel function



2.2.4 Advanced Encryption StandardAES Introduction

2001, FIPS PUB 197 Rijndael - Joan Daemen & Vincent Rijmen

Properties Block length = 128 bit Key length = 128/192/256 bit

Steps AddRoundKey SubBytes ShiftRows MixColumns



Chapter 2 - Outline

2.1 Cryptology Introduction 2.2 Symmetric Key Cryptographic Algorithms 2.3 Asymmetric Key Cryptographic Algorithms

Introduction The RSA Algorithm Digital Signatures

2.4 Hashing Algorithms 2.5 Typical Applications


2.3.1 Introduction Definition

A cryptographic system that uses two keys -- a public key known to everyone and a private or secret key known only to the recipient of the message.

Scenery When Alice wants to send a secure message to Bob, she uses

Bob's public key to encrypt the message. Bob then uses his private key to decrypt it.

Property It is important that the public and private keys are related in such

a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key from the known public key.

2.3 Asymmetric Key Crypt. Algorithms


2.3.2 The RSA Algorithm Introduction

Ron Rivest, Adi Shamir & Leonard Adleman (MIT, 1977)

RSA algorithm is based on the presumed difficulty of factoring large integers, the factoring problem.



How RSA Works Generating of Public key and Private key

Select two different big prims p and q. Let N=pq. By Eulers function, the number of integers less than and

prim to N is(N) = (pq) = (p) (q) = ( p-1)(q-1)

Select an integer e which is less than and prim to (N) Compute d byed 1 (mod (N)) Destroy p and q (when N is big enough, it will be extremely

difficult to find p, q). (e,N) is used as the public key, and (d,N) the private key.

Alice will send her (e,N) to Bob, but keep her (d,N) in secret.



Decryption By applying her private (d,N), Alice can decode n from c by

Encryption Suppose that Bob want to send Alice a message m. Bob has

Alices public key (e,N) in hand. Firstly he transforms m to an

integer n less than N in a presumed method. By applying the

key (e,N), n is encrypted to c


(mod ), mode en c N or c n N Then Bob can send c to Alice through an unsafe channel.

(mod ) , modd dc n N or n c N

and recover the m from n.


Principle of RSA By Eulers function, given two prims p and q satisfied N=pq

and one integer n satisfied 0


Example of RSA For continent, we select small p,q and e. Suppose A want to sent a string key to B by RSA

encryption. Compute (e,N) and (d,N)

Let p=3, q=11, N=pq=311=33f(N)=(p-1)(q-1)=210=20

We take e=3 (3 is prim to 20) and try to get d such that ed1 mod f(n)3d1 mod 20

We get d by trial:



f(n)=(p-1)(q-1)=210=20. Take e=3 and try d.


7279

4248

1217

18186

15155

12124

993

662

331

(ed) mod f(n)edd

From the table, we find that when d=7, ed 1 mod f(n) holds.


So the public key and the private key are as

KU =(e,N)=(3,33)

and KR =(d,N)=(7,33)



Digitization


26252423222120191817161514

zyxevutdrqpon

13121110090807060504030201

mlkjihgfedcba

By the Code Table, the code values of the string key M1,M2,M3are 11,05,25

Note that the max value of any code is 26 (it can not over N=33).


Encryption

By Me= C (mod N) we have C = Me (mod N)

That is C1 = 113 mod 33, C2 = 53 mod 33, C3 = 253 mod

33

And we get C1= 11, C2= 26, C3= 16

Decryption

By Cd= M (mod N) we have M = Cd (mod N)

That is M1 = 117 mod 33, M2 = 267 mod 33, M3 = 167 mod

33

And we get M1= 11, M2= 5, M3= 25

Message recovery

By checking the Code Table, the string key is recovered.



2.3.3 Digital Signatures



Chapter 2 - Outline

2.1 Cryptology Introduction 2.2 Symmetric Key Cryptographic Algorithms 2.3 Asymmetric Key Cryptographic Algorithms 2.4 Hashing Algorithms

Introduction Message-Digest Algorithm (MD5)

2.5 Typical Applications


2.4.1.Introduction Hashing

Hashing value

Hashing algorithms

Data fingerprint

2.4 Hashing Algorithms


2.4.2.Message-Digest Algorithm (MD) Whats MD5

Message-Digest Algorithm 5 128-bits length



Assignments of Sec.2.4

Show me how MD5 works. Design an executable MD5 program in C with some

short samples.



Chapter 2 - Outline


2.2 Symmetric Key Cryptographic Algorithms

2.3 Asymmetric Key Cryptographic Algorithms


2.5 Typical Applications Assignments of Sec.2.5 Write a short thesis on one

of the three applications

MD5 and password protection. AES and Wi-Fi protected access. RSA and electronic business.


References

1. William Stallings, Cryptography and Network Security: Principles and Practice (5th Edition), Prentice Hall 2010

2. http://en.wikipedia.org/wiki/Birthday attackCryptographyData Encryption StandardDiffieHellman key exchangeDigital signatureHash functionInformation_securityMD5Public-key cryptographyRSA


Thank you!

cryptographic techniques

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applications of cryptography

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popular symmetrickey

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