Computer Security:
Principles and Practice
DR TAGWA ISMAIL KHALIFA
Chapter 1 overview

Computer Security Concepts

Threats, Attacks, and Assets

Security Functional Requirements

Fundamental Security Design Principles

Attack Surfaces and Attack Trees

Computer Security Strategy
Learning Objectives
1.
Describe the key security requirements: CIA Triad.
2.
Discuss types of security threats and attacks.
3.
Summarize functional requirements and design principles.
4.
Explain the concepts of attack surfaces and attack trees.
5.
Understand aspects of a comprehensive security strategy
A definition of computer security

Computer security: Computer Security is defined as the
measures
and
controls
used
to
ensure
the
confidentiality, integrity, and availability of information
system assets. This includes protecting hardware,
software, firmware, and information while it is being
processed, stored, or communicated
NIST 1995
Three key objectives (the CIA triad)

Confidentiality
 Data confidentiality: Assures that confidential
information is not disclosed to unauthorized
individuals
 Privacy: Assures that individual control or influence
what information may be collected and stored

Integrity

Data integrity: assures that information and programs
are changed only in a specified and authorized
manner

System integrity: Assures that a system performs its
operations in unimpaired manner

Availability: assure that systems works promptly and
service is not denied to authorized users
Other concepts to a complete
security picture

Authenticity: the property of being genuine and being
able to be verified and trusted; confident in the validity of
a transmission, or a message, or its originator

Accountability: generates the requirement for
actions of an entity to be traced uniquely to that
individual to support nonrepudiation, deference,
fault isolation, etc

Non-repudiation: Ensuring that a sender cannot
deny sending a message.
Levels of security breach impact
1.Low: the loss will have a limited impact, e.g., a
degradation in mission or minor damage or minor
financial loss or minor harm
2.Moderate: the loss has a serious effect, e.g.,
significance degradation on mission or significant
harm to individuals but no loss of life.
3. High: the loss has severe or catastrophic adverse effect
on operations, organizational assets or on individuals (e.g.,
loss of life)
Examples of security requirements: Confidentiality
1.Access Control:

Only authorized users can view sensitive data.

Example: Only HR staff can access employee salaries.
2.User Authentication:

Users must verify their identity before accessing data.

Example: Login using username and password or biometrics.
3.Data Encryption:

Protect data by converting it into unreadable form.

Example: Encrypt data using protocols like SSL/TLS when
transmitting.
4.Data Classification:

Classify data based on sensitivity levels.

Example: Public, Internal, Confidential, Top Secret.
5.Secure Communication:

Ensure data is protected during transmission.

Example: Use HTTPS instead of HTTP.
6.Session Management:

Automatically log out inactive users.

Example: Banking apps log out after a few minutes of inactivity.
7.Least Privilege Principle:

Users get only the minimum access they need.

Example: A student can view grades but not access system settings.
8.Data Masking:

Hide sensitive parts of data when displayed.

Example: Show credit card as **** **** **** 1234.
Examples of security requirements: Integrity
1. Data Modification Control:

Only authorized users can modify data.

Example: Only doctors can update patient medical records.
2. Input Validation:

All user inputs must be validated before processing.

Example: Prevent entering invalid or malicious data into forms.
3.Data Consistency:

Data must remain consistent across all systems and databases.

Example: A bank balance should match in all related tables.
4.Audit Trails:

All changes to critical data must be logged.

Example: Record who changed a file, when, and what was changed.
5.Error Detection Mechanisms:

Use checks like hashes or checksums to detect data corruption.

Example: File integrity verification using hash values.
6.Transaction Management:

Ensure operations are completed fully or not at all (Atomicity).

Example: In banking, money transfer must either fully succeed or
fail completely.
7.Access Control:
 Restrict write/edit permissions based on user roles.
 Example: Students can view grades but cannot modify them.
8.Backup and Recovery:
 Regular backups to restore correct data if corruption occurs.
 Example: Restore database after accidental or malicious
modification.
Examples of security requirements:
Availability
1.System Uptime:

The system must be available most of the time (e.g., 99.9% uptime).

Example: E-learning platforms should be accessible 24/7.
2.Backup and Recovery:

Regular backups to restore data in case of failure.

Example: Restore system quickly after a crash or data loss.
3.Redundancy:

Use duplicate systems/components to avoid single points of
failure.

Example: Multiple servers handling the same service.
4.Disaster Recovery :

PlanA clear plan to recover from major failures.

Example: Recover services after fire, flood, or cyberattack.
5.Load Balancing:

Distribute traffic across multiple servers.

Example: Prevent system crash during high user demand.
6.Fault Tolerance:

System continues working even if part of it fails.

Example: One server fails, others keep running.
7.Protection Against :

AttacksDefend against attacks that disrupt service.

Example: Prevent DDoS attacks that overload servers.
8.Maintenance and Updates:

Regular updates without long downtime.

Example: Schedule maintenance during low-usage times.
Challenges of computer security
1.
Computer security is not simple

At the beginning, security requirements seem easy because they are
expressed in simple terms like:

keeping data secret

These words are easy to understand, but: The methods used to achieve
them are complicated They require careful design and thinking
2.When designing any security system or algorithm, you
must always think about possible attacks.

Explanation:

Attackers don’t always attack in obvious ways.

Sometimes they think differently and find hidden
weaknesses.

A system may look secure, but there could be
unexpected vulnerabilities.

Example:A login system may be secure against
password guessing, but vulnerable to SQL Injection.
3. unexpected attacks, security solutions are often
complex and not intuitive.

Explanation:
•
Security mechanisms are not always simple or obvious.
•
Sometimes the solution looks complicated or strange,
but it exists for a reason.
•
You must consider all possible threats before deciding
how to secure a system.

Example:
Using multi-factor authentication (password + code)
may seem complicated, but it protects against
multiple attack types.
4. Security Mechanisms Placement

Designing security mechanisms is not enough We
must decide where to apply them
1.
Physical Placement:
Refers to location in the network .
Example :Firewalls at network entry points
2.
Logical Placement:
Refers to layers in system architecture
Example:
•
Security in TCP/IP layers.
•
Encryption in transport layer
•
Authentication in application layer
5.Security Mechanisms & Secret Information

Security mechanisms are not just algorithms or
protocols they also depend on secret information.

Participants must have confidential data
Example:Encryption keys.
Key Challenge

Raises important questions :
•
How are secrets created?
•
How are they distributed?
•
How are they protected?
6. A battle of wits between attacker / admin

The Attacker: Needs to find only a single weakness to
succeed.

The Designer: Must find and eliminate all weaknesses to
achieve perfect security.

This creates a fundamental imbalance in security
efforts.
7.It is not perceived on benefit until fails

Security is often invisible when it is working correctly

The value of protection only becomes apparent
after a crisis

Key Challenge: Justifying costs for a benefit that is
only "seen" during a failure
8.Requires constant monitoring


Security is not a one-time task; it requires:
•
Regular monitoring
•
Constant supervision
This is essential to detect and respond to threats in real-time.
9. Security as an Afterthought

Security should be an integral part of the entire
design process.

Proactive Integration: Addressing threats during the
initial planning and architecture stages.

The Benefit: Systems designed with security in mind
are more robust, cheaper to maintain, and harder
to breach than those with "bolted-on" security.
10.Security as an Impediment

Many users and security administrators view strong
security as an impediment
A model for computer security

Concept of a System Resource (or Asset).

Anything users and owners wish to protect.
Categories of Assets:

Hardware: devices, data processing, storage, and
communication devices.

Software: Operating systems and applications.

Data: Files, databases, and security-related data (e.g.,
password files).

Communication Facilities & Networks: LAN/WAN links, bridges,
routers, etc.
System Vulnerabilities

Definition: Weaknesses in system resources or
network assets.

The system can be corrupted

Effect: The system does the wrong thing or provides
incorrect answers.
Computer security terminology
1.Adversary (Threat Agent): An entity that attacks or
represents a threat to a system.
2.Asset (System Resource): Anything of value within the
system (Data, Hardware, Software, Services, or
Facilities) that requires protection.
3.Attack: Any kind of malicious activity that attempts to Collect
information ,Disrupt or Deny service.
4. Countermeasure: Any action, device, or procedure that reduces
a threat or vulnerability.
5.Risk: The probability that a particular threat will exploit a
vulnerability, resulting in a loss.
6. Security Policy: A set of rules and practices that regulate how an
organization protects its critical resources.
7. Threat: A potential for a security violation, a possible danger that
might exploit a vulnerability.
8.Vulnerability: weakness in a system’s design, implementation, or
management that could be exploited.
Security concepts and relationships
Real-World Example: Online Banking System
1.The Asset (What we protect):
The User's Bank Account Balance and personal data.
2. The Vulnerability (The weakness):
The user is using a very weak password (e.g., "123456").
3.The Threat (The danger):
A Hacker using a "Brute Force" tool to guess common passwords.
4.The Attack (The action):
The hacker runs the tool against the bank's login page specifically
targeting that user's account.
5.The Risk (The probability & impact):
There is a High Risk because the password is easy to guess, leading
to the potential theft of all funds in the account.
Applying a Countermeasure

To reduce the Risk, we implement a Countermeasure:
•
Action: Enforcing Multi-Factor Authentication (MFA).
•
Result: Even if the hacker guesses the password (exploits the
vulnerability), they cannot access the account without the second
code.
Types of Attacks
1.
Active Attack

Change or damage the system
Examples:
•
Modifying student grades .
•
Deleting files from a server .
•
Shutting down a website (DoS attack)
There is a change in data or system behavior
2. Passive Attack

Monitor or steal information without changing
anything.
Examples:
•
Eavesdropping on Wi-Fi .
•
Stealing passwords silently .

The attacker only observes, no changes are made
Classification Based on Source
1.Inside Attack

Comes from someone inside the system (has access)
Example:
•
An employee misusing confidential data.

Dangerous because the attacker already has permission.
2. Outside Attack

Comes from outside the system (no authorized access).
Examples:
•
A hacker on the internet Cybercriminals or hostile organizations
Threat and attacks
Type of threat consequences

Unauthorized disclosure: threat to confidentiality .

Deception: threat to integrity.

Disruption: threat to integrity and availability.

Usurpation: threat to integrity.
Theats and Assist
Examples of threats
Security functional requirements (Technical measures)
1.Access control

identification & authentication.

system & communication protection.

system & information integrity.
2.Awareness & training

Ensuring managers and users are aware of security risks and laws.

Training personnel to carry out their security-related duties.
3.Audit and Accountability:

Creating and retaining audit records to monitor and investigate
unauthorized activity.

Tracing actions to specific users so they can be held
accountable.
4. Security Assessments:

Periodically assessing security controls to determine effectiveness.

Implementing action plans to correct deficiencies and reduce
vulnerabilities.
5. Configuration Management:

Establishing baseline configurations (Hardware, Software, Firmware).

Enforcing security settings throughout the system development life
cycle.
5.Contingency Planning:

Establishing plans for emergency response and backup operations.

Ensuring Availability and continuity of operations after a disaster.
Fundamental security design principles

Despite years of research, it is still difficult to design systems that
comprehensively prevent security flaws

But good practices for good design have been documented
(analogous to software engineering)
good practices
1.
Economy of mechanism.
2.
fail-safe defaults.
3.
complete mediation.
4.
open design.
5.
separation of privileges.
6.
lease privilege.
7.
least common mechanism.
8.
psychological accountability.
9.
Isolation.
10. Encapsulation.
11.
layering.
12.
least astonishment
good practices

Economy of mechanism: the design of security measures should be
as simple as possible

Simpler to implement and to verify

Fewer vulnerabilities

Fail-safe default: access decisions should be based on permissions;
i.e., the default is lack of access

Complete mediation: every access should checked against an
access control system

Open design: the design should be open rather than secret (e.g.,
encryption algorithms)
good practices (con)

Isolation

Public access should be isolated from critical resources (no connection
between public and critical information)

Users files should be isolated from one another (except when desired)

Security mechanism should be isolated (i.e., preventing access to those
mechanisms)

Encapsulation: similar to object concepts (hide internal structures)

Modularity: modular structure
good practices (con)

Layering (defense in depth): use of multiple, overlapping protection
approaches

Least astonishment: a program or interface should always respond in
a way that is least likely to astonish a user
good practices (con)

Separation of privilege: multiple privileges should be needed to do
achieve access (or complete a task)

Least privilege: every user (process) should have the least privilege to
perform a task

Least common mechanism: a design should minimize the function
shared by different users (providing mutual security; reduce
deadlock)

Psychological acceptability: security mechanisms should not
interfere unduly with the work of users
Attack surfaces



Attack surface: the reachable and exploitable vulnerabilities in a system

Open ports

Services outside a firewall

An employee with access to sensitive info

…
Three categories

Network attack surface (i.e., network vulnerability)

Software attack surface (i.e., software vulnerabilities)

Human attack surface (e.g., social engineering)
Attack analysis: assessing the scale and severity of threats
Attack trees

A branching, hierarchical data structure that represents a set of
potential vulnerabilities

Objective: to effectively exploit the info available on attack
patterns

published on CERT or similar forums

Security analysts can use the tree to guide design and strengthen
coiuntermeasures
An attack tree
Computer security strategy

An overall strategy for providing security



Policy (specs): what security schemes are supposed to do

Assets and their values

Potential threats

Ease of use vs security

Cost of security vs cost of failure/recovery
Implementation/mechanism: how to enforce

Prevention

Detection

Response

Recovery
Correctness/assurance: does it really work (validation/review)
Computer Security Losses
Security Technologies Used
Summary

Security concepts

Terminology

Functional requirements

Security design principles

Security strategy