Unit I: Overview of Computer Security [4 Hours]

This unit lays the foundation for everything in computer security. It defines core concepts, distinguishes the three security domains, maps out what attackers target and how, and establishes the principles every secure system is designed around. Expect definitions, CIA triad questions, attack classifications, and design principles to appear in both short-answer and long-answer sections.

1.1 Computer Security Concepts

Core Definition

Computer security is the protection of computer systems and the information they store, process, and transmit from unauthorized access, disclosure, alteration, disruption, or destruction, while ensuring that authorized users can access resources without interference.

Think of it as locking your digital house: you want to keep strangers out, make sure no one quietly changes your furniture, and make sure you can still get in whenever you need to.

Detailed Explanation

Computer security addresses three fundamental goals, known as the CIA Triad:

• Confidentiality — Information is only accessible to those who are authorized. Examples: encrypted files, access-controlled databases.
• Integrity — Information remains accurate and unmodified unless changed by an authorized process. Examples: checksums, digital signatures.
• Availability — Systems and data are accessible to authorized users when needed. Examples: uptime guarantees, DDoS protection.

Beyond the triad, two additional concepts appear frequently:

• Authenticity — Verifying that a user, message, or resource is genuine. Achieved through passwords, certificates, and biometrics.
• Accountability (Non-repudiation) — Ensuring that actions or transactions can be traced to a responsible party, so no one can later deny what they did.

Several foundational terms are used throughout the subject:

• Vulnerability — A weakness in a system that can be exploited.
• Threat — A potential event or actor that could exploit a vulnerability to cause harm.
• Attack — An actual attempt to exploit a vulnerability.
• Countermeasure (Control) — A safeguard put in place to reduce a vulnerability or block a threat.
• Risk — The likelihood and potential impact of a threat exploiting a vulnerability.

1.2 Computer Security, Information Security, and Network Security

Core Definition

Computer security focuses on protecting computing hardware and software systems from threats. Information security is the broader discipline of protecting information in any form (digital or physical) from unauthorized access, use, disclosure, modification, or destruction. Network security is a subset concerned specifically with protecting data as it travels across or is accessed through computer networks.

Information security is the umbrella. Computer security and network security are two specialized departments under it — one guards the machines, the other guards the connections.

Detailed Explanation

Computer Security deals with the hardware and software environment of a single system or a collection of systems. It covers:

• OS security (access controls, permissions)
• Application security (secure code, patching)
• Physical security of devices
• Malware protection

Information Security (InfoSec) is broader. Information does not have to be digital: a printed document, a verbal communication, or a written memo all fall within InfoSec scope. Its three pillars are still CIA, but applied across all media and all organizational processes. InfoSec encompasses policies, human behavior, legal compliance, and risk management in addition to technology.

Network Security focuses on the communication infrastructure: routers, switches, firewalls, VPNs, and protocols. It protects data in transit, prevents unauthorized access to network resources, and detects or blocks attacks like packet sniffing, man-in-the-middle, and denial-of-service.

1.3 Threats, Attacks and Assets

Core Definition

An asset is any resource — hardware, software, data, or people — that has value to an organization and must be protected. A threat is any circumstance or event with the potential to adversely impact organizational operations through unauthorized access, destruction, disclosure, modification, or denial of service. An attack is a deliberate action by a threat agent to exploit a vulnerability and cause harm to an asset.

Assets are what you want to protect. Threats are the dangers that might harm them. Attacks are when those dangers actually strike.

Detailed Explanation

Assets in a computer system fall into five categories:

• Hardware — Physical devices: CPUs, storage drives, servers, networking equipment. Threatened by theft, physical damage, or environmental hazards.
• Software — Operating systems, applications, utilities. Threatened by deletion, malware infection, or unauthorized modification.
• Data — The most valuable asset in most organizations: files, databases, configuration files, credentials. Threatened by unauthorized read, modification, or destruction.
• Communication Facilities — Network links, cables, wireless channels. Threatened by eavesdropping, jamming, or disruption.
• People — System administrators, users, security staff. Threatened by social engineering, insider attacks, or coercion.

Threats can be categorized by source:

• Human threats — Intentional (hackers, insiders, criminals) or unintentional (accidental deletion, misconfiguration).
• Natural threats — Floods, earthquakes, fires, lightning strikes.
• Environmental threats — Power outages, temperature extremes, hardware aging.

Attacks are classified along two axes:

Axis 1 — Nature of the attack:

• Active attack — The attacker modifies, disrupts, or fabricates data. The target is affected. Examples: message modification, denial-of-service, replay attack, masquerade.
• Passive attack — The attacker observes or copies data without altering it. The target is unaware. Examples: eavesdropping, traffic analysis.

Axis 2 — Position of the attacker:

• Insider attack — Carried out by someone already inside the organization (employee, contractor). More dangerous because they already have access and knowledge.
• Outsider attack — Carried out by someone external to the organization, needing to penetrate defenses first.

Common attack types to know:

• Eavesdropping — Passively intercepting network traffic to read communications.
• Masquerade — Pretending to be an authorized user or system.
• Replay attack — Capturing a valid transmission and retransmitting it later to fool the receiver.
• Modification of messages — Altering the content of a legitimate message in transit.
• Denial of Service (DoS) — Flooding a system with requests until it cannot serve legitimate users.
• Traffic analysis — Even if messages are encrypted, analyzing communication patterns (frequency, volume, timing) to extract information.

1.4 Security Requirements

Core Definition

Security requirements are the formal set of properties a system must satisfy to be considered secure. They are most commonly expressed through the CIA Triad — Confidentiality, Integrity, and Availability — and extended with Authenticity and Non-repudiation for more comprehensive coverage.

Security requirements answer the question: "What exactly must a system guarantee to be safe?" CIA tells you the three main guarantees; the extensions fill gaps around identity and accountability.

Detailed Explanation

1. Confidentiality

Information is accessible only to those authorized to see it. Two sub-concepts:

• Data confidentiality — Private information is not disclosed to unauthorized individuals.
• Privacy — Individuals have control over what personal data is collected about them and how it is used.

Mechanisms: encryption, access controls, classification policies. Threat it counters: unauthorized disclosure.

2. Integrity

Data and systems are modified only through authorized means and processes. Two sub-concepts:

• Data integrity — Information and programs are changed only in specified, authorized ways.
• System integrity — The system performs its intended function in an unimpaired manner, free from unauthorized manipulation.

Mechanisms: checksums, hash functions, digital signatures, version control. Threat it counters: unauthorized modification.

3. Availability

Systems and data are accessible and usable upon demand by an authorized user. Covers both timely access and reliable performance.

Mechanisms: redundancy, failover, backups, DDoS mitigation. Threat it counters: disruption or denial of service.

4. Authenticity

The property of being genuine and verifiable. A system must be able to verify that a user, message, or resource is what it claims to be.

Mechanisms: passwords, certificates, biometrics, multi-factor authentication (MFA).

5. Non-repudiation (Accountability)

Actions or transactions cannot be denied after the fact. If a user sends a message or performs a transaction, there must be undeniable proof that they did so.

Mechanisms: digital signatures, audit logs, timestamps. Critical for legal and financial systems.

The inverse of CIA is sometimes called the DAD Triad: Disclosure (threatens Confidentiality), Alteration (threatens Integrity), and Destruction/Denial (threatens Availability). This framing is useful when analyzing what a given attack does to your system.

1.5 Security Design Principles

Core Definition

Security design principles are fundamental guidelines used by system designers and security engineers to build systems that are resistant to attack, minimize the impact of failures, and remain manageable over time. The most widely cited set was proposed by Saltzer and Schroeder (1975) and remains foundational in modern security engineering.

These are the rules engineers follow so they don't accidentally create a system that looks secure but breaks easily. Think of them as building codes for software.

Detailed Explanation

The eight core principles from Saltzer and Schroeder, with modern additions:

1. Economy of Mechanism (Simplicity)
Keep the security design as simple and small as possible. Complex systems have more potential failure points. If a feature is not needed, do not build it.

2. Fail-Safe Defaults
Access decisions should default to denial. A user should only gain access if explicitly granted permission — not because nothing explicitly denied them. If a system fails or encounters an unknown state, it should default to the secure (deny) position.
Example: A firewall that blocks all traffic by default, allowing only explicitly whitelisted connections.

3. Complete Mediation
Every access to every object must be checked against the access control policy, every single time — not just at login. No request should be allowed through on the assumption that it was checked before.
Example: Checking file permissions each time a file is opened, not just when the application starts.

4. Open Design
The security of a mechanism should not depend on the secrecy of its design. Security through obscurity is not security. The design can be open; only the keys or credentials must be secret.
Example: AES encryption is publicly documented — its security comes from key secrecy, not algorithm secrecy.

5. Separation of Privilege
A system should require multiple conditions to be satisfied before granting access or performing a sensitive action, rather than a single condition.
Example: A bank transfer requires both a password and an OTP — two independent conditions. A nuclear launch requires two separate keys turned simultaneously.

6. Least Privilege
Every process and user should operate with only the minimum privileges necessary to complete the task at hand. Extra privileges not needed right now should not be granted.
Example: A web server process should not run as root — it only needs permission to read web files and open port 80.

7. Least Common Mechanism
Minimize the number of mechanisms shared across users or processes. Shared mechanisms create shared risk — a vulnerability in a shared component affects everyone using it.
Example: Avoid having a single authentication function shared by all applications; use separate instances where possible.

8. Psychological Acceptability
Security mechanisms must be usable. If security is too cumbersome, users will work around it, defeating its purpose. The human interface should be designed so that applying the correct security policy is the natural choice.
Example: A password manager that autofills strong passwords is more likely to be used than manually remembering 20-character random strings.

Additional modern principles:

• Isolation — Separate critical processes, data, or components so a failure in one does not cascade.
• Encapsulation — Hide implementation details; expose only what is necessary.
• Layering (Defense in Depth) — Use multiple independent layers of security so an attacker who bypasses one layer still faces others.
• Modularity — Build systems from independent components so each can be secured and updated separately.

1.6 Attack Surfaces and Attack Trees

Core Definition

An attack surface is the sum of all the different points (vectors) in a system where an unauthorized user could try to enter or extract data. The larger the attack surface, the greater the exposure. An attack tree is a conceptual diagram that represents attacks against a system in a tree structure, with the root as the attacker's goal and the leaves as the specific methods used to achieve it.

The attack surface is the "how many doors and windows does your house have?" question. Attack trees are the step-by-step plan a burglar might draw up to get inside — and a diagram the security team builds first to find those plans before the burglar does.

Detailed Explanation — Attack Surface

The attack surface includes all reachable and exploitable vulnerabilities. It is divided into three categories:

1. Network Attack Surface
Vulnerabilities exposed through network connections: open ports, accessible services (web servers, databases), network protocols, and firewalls with misconfigured rules. The internet-facing parts of any system are the largest portion of the network attack surface.

2. Software Attack Surface
Vulnerabilities in the code itself: web application bugs (SQL injection, XSS), OS vulnerabilities, poorly configured APIs, or unpatched software. Every line of code that accepts user input is potentially on the software attack surface.

3. Human Attack Surface
Vulnerabilities in people: phishing, social engineering, insider threats. Humans can be manipulated into bypassing technical controls — making the human attack surface often the most dangerous and hardest to patch.

Attack Surface Reduction
The goal is to minimize the attack surface:

• Close unused ports and disable unused services.
• Remove unused software components.
• Apply the principle of least privilege.
• Reduce code complexity.
• Train users to resist social engineering.

Detailed Explanation — Attack Trees

Attack trees, introduced by Bruce Schneier, provide a formal structure to analyze how an attacker might reach their goal. They help security teams enumerate all plausible attack paths, evaluate their likelihood and cost, and prioritize defenses.

Structure of an Attack Tree:

• Root node — The attacker's ultimate goal (e.g., "Gain unauthorized access to bank database").
• Internal nodes — Sub-goals or intermediate steps needed to reach the parent node.
• Leaf nodes — Specific, concrete attack actions (e.g., "Steal password via phishing," "Exploit SQL injection vulnerability").
• AND nodes — All child conditions must be met to satisfy the parent. (Both A and B required.)
• OR nodes — Any one child condition is sufficient to satisfy the parent. (Either A or B works.)

Worked Example — Attack Tree for "Access a bank account without authorization":

Goal: Access bank account without authorization [OR]
├── Steal credentials [OR]
│   ├── Phishing email → harvest username/password [LEAF]
│   ├── Keylogger malware installed on victim's device [LEAF]
│   └── Data breach from third-party site (credential stuffing) [LEAF]
├── Bypass authentication [AND]
│   ├── Find session token in browser storage [LEAF]
│   └── Replay captured session token before expiry [LEAF]
└── Exploit application vulnerability [OR]
    ├── SQL injection in login form [LEAF]
    └── Broken access control (access another user's URL) [LEAF]

Reading this tree: the attacker achieves the goal by any one of the three main branches (OR). The "bypass authentication" branch requires both steps (AND): find the token and then replay it.

Why attack trees are useful:

• Make invisible threats visible and structured.
• Help calculate the cost and probability of each path.
• Allow prioritized, cost-effective defense investment.
• Support communication between technical teams and management.

1.7 Computer Security Strategy

Core Definition

A computer security strategy is the comprehensive, high-level plan an organization adopts to protect its information assets. It integrates policy, technical controls, and operational practices to prevent attacks, detect incidents, and respond to breaches — structured around three phases: Prevention, Detection, and Response.

A security strategy is not a single tool or rule — it's the overall plan. Just as a city has traffic lights, police, and courts working together, a security strategy layers prevention, monitoring, and correction into a unified approach.

Detailed Explanation

A complete security strategy operates across three levels:

Level 1 — Policy
Security policy is the formal statement of rules and practices that govern how an organization manages and protects its resources. It sets the intent and authority. Without a policy, technical controls have no direction or accountability. Example: an acceptable-use policy defines what employees may and may not do on company systems.

Level 2 — Implementation (Controls)
Technical and administrative controls that enforce the policy:

• Preventive controls — Stop attacks before they happen: firewalls, antivirus, encryption, access controls, user training.
• Detective controls — Identify attacks in progress or that have occurred: intrusion detection systems (IDS), audit logs, security monitoring.
• Corrective controls — Restore normal operation after an attack: incident response procedures, backups, disaster recovery.

Level 3 — Assessment (Assurance)
Ongoing verification that the strategy is working as intended: vulnerability assessments, penetration testing, audits, and compliance checks. Security is not a one-time event — it must be continuously evaluated and updated.

The Prevention-Detection-Response Model:

• Prevention — Reduce the probability of an attack succeeding. Harden systems, apply patches, enforce least privilege, train users.
• Detection — Identify attacks that get through prevention. Use IDS, monitor logs, set up alerts. The faster an attack is detected, the less damage it causes.
• Response — Contain the damage, recover operations, and learn from the incident. Includes incident response plans, forensic investigation, and updating defenses.

Defense in Depth
No single control is perfect. Defense in depth layers multiple independent controls so that if one fails, others remain. An attacker who bypasses the firewall still faces access controls; if they compromise a user account, encryption still protects the data. Each layer buys time and reduces the probability of a full breach.

Whole Chapter Summary

1.1 Computer Security Concepts — Security protects systems using the CIA Triad (Confidentiality, Integrity, Availability). Core vocabulary: asset, vulnerability, threat, attack, countermeasure, risk.

1.2 Computer vs Information vs Network Security — Information security is the broadest domain. Computer security guards machines; network security guards data in transit. All three share CIA as the goal.

1.3 Threats, Attacks, and Assets — Assets include hardware, software, data, communication facilities, and people. Active attacks modify or disrupt; passive attacks only observe. Insider attacks are typically more dangerous than outsider attacks.

1.4 Security Requirements — CIA + Authenticity + Non-repudiation form the full requirement set. Each property addresses a specific category of harm (disclosure, modification, denial, spoofing, repudiation). DAD is the threat-side mirror of CIA.

1.5 Security Design Principles — Saltzer and Schroeder's eight principles guide secure system design. Key: least privilege, fail-safe defaults, complete mediation, open design, and separation of privilege. Psychological acceptability ensures users actually follow security rules.

1.6 Attack Surfaces and Attack Trees — The attack surface covers all exploitable vectors: network, software, human. Attack trees map attacker goals hierarchically with AND/OR logic. Blocking AND nodes is the most effective defense. Reducing attack surface directly reduces risk.

1.7 Computer Security Strategy — Strategy operates at policy, implementation, and assessment levels. The core model: prevent → detect → respond. Defense in depth layers multiple independent controls. Continuous assessment keeps strategy current as threats evolve.

Last-Minute Revision Sheet