COMPUTER AND NETWORK SECURITY First of all, an introduction to the field of computer security and some of relevant issues are presented. Then, a background to cryptography is illustrated, beginning with a discussion of general purpose cryptographic techniques and then the cryptanalysis including: frequency distribution, index of coincidence, and kasiski method is discussed. Various authentication schemes and examining their implementation advantages and drawbacks are illustrated. Different access control systems with advantages and disadvantages are examined. Viruses, logic bomb, Trojan horse, and worms are examined as well as methods to address each of these threats. Finally, the topic of network security and how it differs from single host security are presented. Securing network is not just a matter of extending the practices used from one machine to many but also entails additional elements which are not concerned at the single host.

References : Charles P. Pfleeger, "Security in Computing", Prentice-Hall International, Inc, Eric A. Frischand Gregory B. White, "Secure computers and Networks, Analysis, Design, and Implementation", CRC Press LLC, William Stalling, "Cryptography and Network Security: Principle and Practice", Fourth Edition, Prentice Hall Pub Date : November 16, Bruce Schneier, "Applied Cryptography", Prentice-Hall, Inc Noor Dhia, “ Black- Box Attack using Fuzzy Neuro- Identifier”, Baghdad, د. وسيم عبد الامير الحمداني, ” انظمة التشفير التقليدية “, 1995.

Syllabus 1- Fundamental of Computer Security Computer Security Definition Objective of Computer Security Kinds of Security Breaches 2- Cryptography Cryptographic Terminology Substitution Cipher - Monoalphabetic - Homophonic - Polyalphabetic - Polygram Transposition Cipher - Fixed Period - Columnar

Syllabus 3-Cryptanalysis Symmetric Cryptography ( classical cipher system ) Asymmetric Cryptography 4- Information Hiding 5- User Authentication 6- Malicious Code - Virus - Trojan Hours - Worm 7- Network Security

COMPUTER AND NETWORK SECURITY INTRODUCTION: Cryptograph: Is the science and study of secret writing. Cipher: Is a secret method of writing. Where by plaintext (or clear text) is transformed in to ciphertext (cryptogram). Encipherment (encryption): The process of transforming plaintext in to ciphertext. Decipherment(Decryption): Is the reverse process of encipherment,of transforming ciphertext into plaintext. Cryptanalysis: The art and science of breaking ciphertext.

Types of Key-based algorithems : 1- Symmetric algorithms (privet key):- are algorithms where the encryption key can be calculated from the decryption key and vice versa. In most symmetric algorithms, the encryption key and the decryption key are the same. Symmetric algorithms can be divided in to two categories: Stream cipher: which operator on the plaintext a single bit at a time. Block cipher: which operator on the plaintext in a group of bits called blocks at a time. 2- Asymmetric algorithms (public key):- Are designed so that the key used for encryption is different from the key used from decryption and cant'nt easily calculated from each others.

plaintext Encipher Decipher Key ciphertext Figure -1 Encryption and Decryption symmetric Process plaintextciphertext Encipher Decipher K1 K2 Figure -2 Encryption and Decryption Asymmetric Process

Cryptology CryptographyCryptanalysis AsymmetricSymmetric Block cipherStream cipherBlock cipher Cipher system ModernConventional AsymmetricSymmetric Block cipherStream cipherBlock cipher Figure-3 Cryptography classification Figure-4 Cipher system classification

CLASSICAL CIPHER SYSTEMS Classical ciphers have been used since ancient Egypt to exchange classified messages between authorized persons. Since then, different methods and techniques are used in order to increase security level of such information. Most of theses methods and techniques are based on the idea that each natural language has its own distribution characteristics. The encryption process aims to uniformly flatten the statistical characteristics of language and obscure any correlation and dependencies between plaintext and ciphertext by diffusion and confusion. Cryptographic systems have been classified into two systems: Symmetric (one-key) cryptosystems: In symmetric or one-key systems, the enciphering and deciphering key are the same. Asymmetric ( two-key) cryptosystem: In asymmetric or two-key systems, enciphering and deciphering keys are different in such away that at least one key is computationally infeasible to determine from the other.

1- TRNSPOSITION CIPHERS: Transposition ciphers rearrange characters according to some scheme. This is done classically by some geometric figure, the figure is 2-dimentional array, and often called "permutation". Example: Encrypt the word “RENAISSANCE" using 3x4 figure, using [ ] scheme Ciphertext: ESCAARINNSE. Many transposition ciphers permute characters of the plaintext with fixed period d. let Zd be the integers 1….d, and f : Zd → Zd be a permutation over Zd, then successive blocks of d characters are encrypted by permuting the characters according to f. M = m1 …md,md+1,…...m2d,…..is encrypted as Ek(M) = mf(1)….mf(d), md+f(1) … md+f(d) RENA ISSA NCE

1- TRNSPOSITION CIPHERS: Example: suppose d = 4 and fE =[ ] and FD = [ ],thus: I1234 F(i)2413 M = RENA ISSA NCE Ex(M) = EARN SAIS CNE Dx(C) = RENA ISSA NCE

SIMPLE SUBSTITUTION CIPHERS: In simple substitution (or mono alphabetic) ciphers, each character of the plaintext replaced with a corresponding character of ciphertext. A single one-to-one mapping function (f) from plaintext to ciphertext character is used to encrypt the entire message using the same key (k); such that: Ek(M) = F(m1) F(m2) …..F(mN) =C Where: N : is the length of the message. M : is plaintext message given by M = ( m1, m2, …..,mN). C : is ciphertext message given by C = (c1,c2,….., cN). There are many types of simple substitution ciphers according to its equations used to encryption, they: Shifted alphabet (Caesar cipher): F(a) = (a + k) mod n Where k : is the number of positions to be shifted. a : is a single character of the alphabet> n : is the size of the alphabet.

SIMPLE SUBSTITUTION CIPHERS: Example: If k =3 then we can encrypt the following message as: M = R E N A I S S A N C E Ek(M) = U H Q D L V V D Q F H