Computer and network security

Reference: Cryptography and Network Security, Principles and Practices, Fourth Edition, William

Lecture: 1

Chapter 1

• According to the DR - High knowledge is not needed any more, due to highly available tools that helps in attacks
• Attack can be applied to any layer

Attack:

any action that compromises the information security of a system

Types of Attack

1. Passive
   • Spy on message but no active harm
2. Active
   • Examples like masquerade or modify
   • Denial of Service

Some Terminology

Confidentiality: Secrecy Integrity: Being authentic or real Non-repudition: Evidence that it is me

Chapter 2

Data manipulation

• Blocks - Data is divided into block - (Used for files ? )
• Stream - Cipher the whole stream - (Maybe used for realtime ?)

Transformations done on data

• Substitution - Change the whole bits to smth else
• Transposition - Change the position or permutation
• Product - do the above together

A question dr asked

Why only multiplying by 5 can not be considered as a complete cipher

Mathematical representation

We say plaintext: $X=\{X_1,X_2,X_3,\dots \}$ The cipher text is also represented as $X$, an the key $K$ The operations are as follows: $\begin{array}{c}Y=E(K,X)\\ X=D(K,Y),whereD=E^{-1}\end{array}$

Attack systems to recover the key

2. Crypt-analytic attack - Use logical and mathematical methodologies to recover the key
3. Brute force - Use everything

Companies should announce their encryption algorithm, so it could be tested and research to be one

Types of ciphers

1. Ceaser Cipher
2. Mono alphabetic, every alphabet has a corresponding alphabet
   • cracked with frequency analysis
3. Hill cipher , Matrix of keys - cracked with matrix inversion, some stupid linear algebra
4. Vignere cipher, the key is considered as an alphabet shifting.
5. At & t Cipher, uses XOR, which is called Vernam cipher.
6. Rail fence cipher