Security & Data Integrity

1. Data Integrity vs Data Privacy vs Security

Most students think these are the same thing — they are NOT. Past papers ask 1–2 mark "what is meant by…" questions, so know the difference.

	① Data Integrity	② Data Privacy	③ Security
Meaning	Is making sure that data is correct / valid. Ensures data received is same as the data sent	Keeps data confidential. Only seen by authorized personnel	Is to keep data safe. Prevention of data loss
Keyword	VALID / same as sent	AUTHORIZED only	data LOSS
e.g.	parity check	stop 3rd person reading it (encryption, passwords)	data backup

2. Security Measures to Protect Computer

Covers stand-alone computers, computers on a LAN and networks / internet. Learn all 8 measures + the other ways to protect data online.

① Encryption

• Encryption scrambles the source code
• Using an encryption key
• If the file is accessed without authorization it will be meaningless
• It requires a decryption key to unscramble the algorithm

② Data Backup

• A copy of data will have been made and stored elsewhere
• If the original data is lost, the backup can be used to restore the data

③ Disk Mirroring

• The data is stored on two disks simultaneously
• If the first data disk drive fails, the data is accessed from the second disk

	Data backup	Disk mirroring
When	copied at a fixed time later	same moment, simultaneously
Where	stored elsewhere (another medium/place)	2nd disk running alongside

④ Firewall

• Prevents unauthorized access to the data
• Monitors incoming and outgoing traffic
• Blocks transmission from unauthorized sources / websites
• Maintains an allow list
• Can be software or hardware both
• Can help to prevent hacking

⑤ User Account

• User has an username and password
• Access to resources can be limited to specific accounts
• A person cannot access the system without a valid username and password

⑥ Anti-Malware

• Scans for malicious code (= harmful code)
• Quarantines or deletes any malicious software found
• Scans can be scheduled at regular intervals

⑦ Access Rights

• Different access rights for individual / group
• To stop users from editing the program

⑧ Physical Measures

• Locked doors / keyboards
• Secure method of access

Proxy Server

A proxy server is a physical device (a server) that works like an agent / middle man (think buyer → agent → seller).

• Without a proxy: you access the internet with your own public IP ⇒ your location can easily be found.
• With a proxy: the proxy keeps your IP and talks to the internet with its own IP; replies come back to the proxy, which forwards them to you ⇒ more privacy.
• Extra (not syllabus): proxy has a cache (temporary memory) — a site you opened once is stored, so asking again saves bandwidth. Offices also use it to monitor which sites each user visited and for how long (so be careful at home!).
• VPN also changes your IP (useful if YouTube is banned).

Factor	Question to ask	e.g. (write this!)
① How often	How often should the data be backed up?	at the end of each day, as students' progress may be edited each day
② Medium	What medium should the data be backed up to?	external hard disk, as it has larger capacity (also USB, CD…)
③ Where stored	Where should the backup be stored? (not the device — where the device is kept)	off site — if the building is damaged only the original data are lost
④ What	What is backed up?	only the updated file(s)
⑤ When	When should the backup take place? (time of day)	over night (e.g. WhatsApp backs up at 2 a.m.)

3. Malware

Definition: Malware is software that is intentionally designed to cause damage to a computer or server.

Types of malware: viruses, worms, logic bombs, trojan horses, spyware

• Malicious code (= harmful code)
• that replicates / copies itself
• can cause loss of data
• can cause computer to crash
• can fill up the hard disk with data (copies take space)

• Use anti-virus · update anti-virus on regular basis
• Avoid downloads from unknown sources · avoid suspicious websites
• Use a firewall (stops viruses coming in through the network)

(1) Virus

Definition: Programs or program code that can replicate itself by inserting itself into another piece of software with the intention of deleting or corrupting files.

Problem: computer may stop working or files may get lost. Solution: run anti-virus software.

(2) Worms

Definition: A standalone piece of software which can reproduce itself automatically, does not require a host.

Problem: could corrupt user's computer ‖ delete data ‖ consume bandwidth (it keeps reproducing).

Solution: run anti-virus in the background ‖ keep OS up to date ‖ not connected to the internet.

(3) Logic Bombs

Code embedded in a program on a computer. When certain conditions are met (such as a specific date) they are activated to carry out tasks such as deleting files or sending data to a hacker.

(4) Trojan Horses

Same as that Greek horse story — soldiers hid inside a wooden horse (sent as a "gift"); when it entered the city, they came out at night and attacked.

Definition: Malicious program often disguised as legitimate software with the intent of harming the computer.

(5) Spyware

Definition: Software that gathers information by monitoring, for example key presses on user's keyboard. The information is then sent back to the person who sent the software.

Solution: Anti-spyware software. Also: avoid the physical keyboard — use an on-screen keyboard / touch screen, since a keylogger only records real key presses.

Phishing and Pharming

Two ways malware / fraud reaches you (O-level concept — revise!).

Phishing: Someone sends email to users; when clicked it takes the user to a fake website so they could obtain somebody's confidential data or to install malware.

Problem: identity fraud ‖ misuse of financial data (card number, bank info, OTP). Solution: ignore suspicious email and undergo frequent security awareness training.

Pharming: Malicious code installed on user's computer or web server. The code redirects the user to a fake website.

4. Data Integrity — Validation & Verification

Malware finished data security. Now data integrity = data is correct/valid and received same as sent. It has two parts:

Validation	Verification
Checks that the data entered is reasonable (meets a criterion)	Checks that the data entered is same as the original
e.g. age = −40 can't be accepted	e.g. typed 50 but receiver gets 51 ✗

Validation checks (exam gives a scenario → you pick the check)

Check	What it checks	e.g.
Range check	data is between a lower and upper limit	13 as month, −120 as age ✗ (months 1–12)
Format check	data entered in the agreed format	date is dd/mm/yyyy — 12-01-2006 ✗ , 12/01/2003 ✓
Length check	data has the required number of characters	phone number should contain 7 numbers; password of 7 chars
Presence check	a field is not left empty when it should contain data	verification code present; compulsory * fields (middle name optional)
Existence check	data in a file / a file name actually exists	registered name is found (student registered?)
Limit check	checks only one of the limits (upper or lower)	1.5 litre only (bottle)
Check digit	an additional digit added to a number	see next page

Check Digit — Calculating It

5. Verification

Definition: a way of preventing errors when data is entered manually using a keyboard or when data is transferred from one computer to another. Two situations, studied separately:

Verification during data entry

• Double entry: data is entered twice, using two different people, and then compared. (must be another person — not you typing twice!)
• Visual check: entered data is compared with the original document (e.g. re-read the typed passport number against the real passport, letter by letter).
• Check digit (3rd method, sir) — extra digit calculated & attached; see above.

Verification during data transfer

Three methods: ① Check sum   ② Parity check   ③ Automatic Repeat Request (ARQ) — O-level topics, but revise.

① Checksum

• Bytes sent as a block
• Bytes are added up before transmission
• Result of the addition is sent with the data block
• The same calculation is carried out at the receiver's end
• The two values are compared

② Parity Check

• The exam tells you which parity the system uses. Count only the 1's — zeros don't matter.
• A parity bit is added to the data so the number of 1's becomes even/odd. It doesn't change the data. e.g. 7-bit data with even number of 1's already → parity bit 0; odd → parity bit 1.
• If the receiver finds the wrong count (a bit flipped by e.g. a magnetic field), the byte was transmitted incorrectly.

③ Automatic Repeat Request (ARQ)

6. Digital Signature & Hashing

Public and Private keys

	Sender holds	Receiver holds
own keys	own private key + own public key	own private key + own public key
other's key	public key (of receiver)	public key (of sender)

Hashing algorithm

Definition: Hashing algorithms are a mathematical function that produces a hash key (also called the digest).

Board example: 0+5 = 5, 4+1 = 5, 9+7 = 16, 2+2 = 4 → 5+5 = 10, 16+4 = 20 → 10+20 = 30. (The email flow below uses an illustrative digest of 54.) Same input + same algorithm ⇒ same hash; change the data and the hash changes.

Quick Recap — Full Chapter Cheat-Sheet