The document provides an overview of network security essentials, including: 1. Symmetric encryption uses a shared secret key between sender and receiver to encrypt and decrypt messages. Common symmetric algorithms include the Shift Cipher and Caesar Cipher. 2. The Caesar Cipher replaces each letter with the letter three positions further down the alphabet, encrypting "ET TU BRUTUS" to "HW WX EUXWXV". 3. Network security concepts like confidentiality, integrity, and availability are discussed alongside security attacks, services, and mechanisms defined in the X.800 standard. Models for providing network and access security are also introduced.

1. Network Security
Essentials
Lecture 1
By
Usman Zia

2. The art of war teaches us to rely not on the
likelihood of the enemy's not coming, but
on our own readiness to receive him; not
on the chance of his not attacking, but
rather on the fact that we have made our
position unassailable.
—The Art of War, Sun Tzu

3.  The combination of space, time, and
strength that must be considered as the
basic elements of this theory of defense
makes this a fairly complicated matter.
Consequently, it is not easy to find a fixed
point of departure.
— On War, Carl Von Clausewitz

4. Computer Security
 The protection afforded to an automated
information system in order to attain the
applicable objectives of preserving the
integrity, availability and confidentiality of
information system resources (includes
hardware, software, firmware,
information/data, and telecommunications)
[NIST 1995]

5. Key Security Concepts

6. Three Key Objectives
 Confidentiality
 Data confidentiality
 Privacy
 Integrity
 Data integrity
 System integrity
 Availability
 Additional concepts
 Authenticity
 Accountability

7. Levels of Impact
 3 levels of impact from a security breach
 Low
 Moderate
 High

8. Examples of Security
Requirements
 confidentiality – student grades
 integrity – patient information
 availability – authentication service

9. Computer Security Challenges
1. not simple
2. must consider potential attacks
3. procedures used counter-intuitive
4. involve algorithms and secret info
5. must decide where to deploy mechanisms
6. battle of wits between attacker / admin
7. not perceived on benefit until fails
8. requires regular monitoring
9. too often an after-thought
10. regarded as impediment to using system

10. OSI Security Architecture
 ITU-T X.800 “Security Architecture for OSI”
 defines a systematic way of defining and
providing security requirements
 for us it provides a useful, if abstract,
overview of concepts we will study

11. Aspects of Security
 3 aspects of information security:
 security attack
 security mechanism: detect, prevent,
recover
 security service
 terms
 threat – a potential for violation of security
 attack – an assault on system security, a
deliberate attempt to evade security services

12. Passive Attacks (1)
Release of Message Contents

13. Passive Attacks (2)
Traffic Analysis

14.  Passive attacks do not affect system resources
 Eavesdropping, monitoring
 Two types of passive attacks
 Release of message contents
 Traffic analysis
 Passive attacks are very difficult to detect
 Message transmission apparently normal
• No alteration of the data
 Emphasis on prevention rather than detection
• By means of encryption

15. Active Attacks (1)
Masquerade

16. Active Attacks (2)
Replay

17. Active Attacks (3)
Modification of Messages

18. Active Attacks (4)
Denial of Service

19.  Active attacks try to alter system resources or
affect their operation
 Modification of data, or creation of false data
 Four categories
 Masquerade
 Replay
 Modification of messages
 Denial of service: preventing normal use
• A specific target or entire network
 Difficult to prevent
 The goal is to detect and recover

20. Security Service
 enhance security of data processing systems
and information transfers of an organization
 intended to counter security attacks
 using one or more security mechanisms
 often replicates functions normally associated
with physical documents
• which, for example, have signatures, dates; need
protection from disclosure, tampering, or
destruction; be notarized or witnessed; be
recorded or licensed

21. Security Services
 X.800:
“a service provided by a protocol layer of
communicating open systems, which ensures
adequate security of the systems or of data
transfers”
 RFC 2828:
“a processing or communication service
provided by a system to give a specific kind of
protection to system resources”

22. Security Services (X.800)
 Authentication - assurance that communicating
entity is the one claimed
 have both peer-entity & data origin authentication
 Access Control - prevention of the
unauthorized use of a resource
 Data Confidentiality –protection of data from
unauthorized disclosure
 Data Integrity - assurance that data received is
as sent by an authorized entity
 Non-Repudiation - protection against denial by
one of the parties in a communication
 Availability – resource accessible/usable

23. Security Mechanism
 feature designed to detect, prevent, or
recover from a security attack
 no single mechanism that will support all
services required
 however one particular element underlies
many of the security mechanisms in use:
 cryptographic techniques
 hence our focus on this topic

24. Security Mechanisms (X.800)
specific security mechanisms:
 encipherment, digital signatures, access
controls, data integrity, authentication
exchange, traffic padding, routing control,
notarization
pervasive security mechanisms:
 trusted functionality, security labels, event
detection, security audit trails, security
recovery

26. Model for Network Security

27. Model for Network Security
 using this model requires us to:
1. design a suitable algorithm for the security
transformation
2. generate the secret information (keys) used
by the algorithm
3. develop methods to distribute and share the
secret information
4. specify a protocol enabling the principals to
use the transformation and secret
information for a security service

28. Model for Network Access
Security

29. Model for Network Access
Security
 using this model requires us to:
1. select appropriate gatekeeper functions to
identify users
2. implement security controls to ensure only
authorised users access designated
information or resources

30. Standards
 NIST: National Institute of Standards and
Technology
 FIPS: Federal Information Processing
Standards
 SP: Special Publications
 ISOC: Internet Society
 Home for IETF (Internet Engineering Task
Force) and IAB (Internet Architecture Board)
 RFCs: Requests for Comments

31. Summary
 topic roadmap & standards organizations
 security concepts:
 confidentiality, integrity, availability
 X.800 security architecture
 security attacks, services, mechanisms
 models for network (access) security

32. Network Security
Essentials
Lecture 2
By
Usman Zia

33. Symmetric Encryption
 or conventional / private-key / single-key
 sender and recipient share a common key
 all classical encryption algorithms are
private-key
 was only type prior to invention of public-
key in 1970’s
 and by far most widely used

34. Some Basic Terminology
 plaintext - original message
 ciphertext - coded message
 cipher - algorithm for transforming plaintext to ciphertext
 key - info used in cipher known only to sender/receiver
 encipher (encrypt) - converting plaintext to ciphertext
 decipher (decrypt) - recovering ciphertext from plaintext
 cryptography - study of encryption principles/methods
 cryptanalysis (codebreaking) - study of principles/
methods of deciphering ciphertext without knowing key
 cryptology - field of both cryptography and cryptanalysis

35. Symmetric Cipher Model

36. 
Requirements

37.  Each letter we identify with a number
 A = 0
 B = 1
 C = 2
 ...
 Z = 25
 The key k is a number in the range 0 − 25
 Encryption is add k onto each letter modulo 26.
 Use the key k = 3.
 HELLO becomes
 KHOOR
Shift Cipher

38. 
ROT-13 cipher (2)

39.  Earliest known substitution cipher and first attested
use in military affairs
 The Roman emperor Julius Caesar used to
substitute each letter in his diplomatic
communications with the letter that was three letters
further along in the alphabet.
 Replaces each letter by 3rd letter on
 p : ABCDEFGHIJKLMNOPQRSTUVWXYZ
 F(p) : DEFGHIJKLMNOPQRSTUVWXYZABC
Caesar cipher

40.  Example:
 Plaintext : ET TU BRUTUS
 Ciphertext : HW WX EUXWXV
Caesar cipher (2)

41.  Can define transformation as:
a b c d e f g h i j k l m n o p q r s t u v w x y z
D E F G H I J K L M N O P Q R S T U V W X Y Z A B
C
 Mathematically give each letter a number
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25
 Then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)
Caesar cipher (3)

42. 
Caesar cipher (4)