Computer Systems Security An Introduction (Part 1)

Jegan — Fri, 19 Dec 2025 11:32:12 +0000

What is Security

Security is the ability of systems to achieve their intended goals even in the presence of an adversary.

The Castle Analogy

A castle is secure if the treasure remains protected even when enemies are actively trying to attack it.

Goal: Keep the treasure safe
Adversary: Bad guys (hackers) trying to steal it
Security: Making sure the treasure stays safe even though attackers are trying to break in

So, in computer systems:

Goal:

Keep data confidential
Ensure data is not changed
Keep systems available for users

Adversary:

Hackers, malware, unauthorized users

Security:

Using logins, guards, walls, and alarms to still achieve the goal despite attacks

The Three Building Blocks of Security

Goal

Before you can protect something, you need to know WHAT you're protecting and WHY.

The three types of goals:

1. Confidentiality

Keep secrets SECRET
Like keeping your diary locked
Only you should read your file
Your bank password should stay secret

2. Integrity

Keep things CORRECT
Like making sure nobody changes your homework
Your grade file shouldn't be changed by students
Your bank balance shouldn't be changed by hackers

3. Availability

Keep it WORKING
Like making sure the library is open when you need it
Netflix should work when you want to watch
Your email should always be accessible

Threat Model

You can't protect against EVERYTHING! You need to decide who you're protecting against.

Think of it like this:

Against a thief? You need locks and maybe an alarm
Against a fire? You need smoke detectors and sprinklers
Against rain? A good roof is enough
Against a tank? Well, your house won't help much

In computers:

Are you protecting against random hackers on the internet?
Or against a government with unlimited money?
These need VERY different protections

Mechanism

How do you actually protect it?

This is like the actual locks and walls you use

Policy

The rules
- Only teachers can change grades
- Only you can read your email

Mechanism

The tools
- Username and password
- Encryption
- Firewalls

Why Security Is So Hard?

The big problem:

Security is negative — you have to defend against EVERYTHING

Imagine you own a toy store.

Making the door work is easy:

Put a lock on it
Give keys to employees
Open it during business hours

But keeping thieves out is HARD because they might:

Break a window
Dig under the floor
Come through the roof
Pretend to be an employee
Steal an employee's key
Break the lock
Come in during business hours disguised as a customer
Bribe an employee

You have to think of ALL possible ways in.

Forem: Jegan