← IGCSE Computer Science Paper 1

Why encrypted data is useless to an interceptor

Encryption cannot stop data being intercepted, but the interceptor sees only ciphertext and cannot get the plaintext without the key, so passwords and bank details stay safe.

Part of: IGCSE Computer Science 0478 — 2.3 Encryption — Module 8, ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 55, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 131, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 8, WA Computer Science ATAR Year 11 — Cyber Security — Module 92

Understanding Why encrypted data is useless to an interceptor in IGCSE Computer Science Paper 1

What it does: The data can still be captured, but what the interceptor gets is ciphertext, not plaintext. No key means no way to decrypt, so the original message stays hidden. The purpose of encryption is to make intercepted data unreadable without the key, not to prevent interception. Decryption needs the key, so an interceptor without it cannot recover the original data. Reading the plaintext password means it was not encrypted, which is what encryption is meant to prevent.

Common mistakes: The data must be scrambled before it is sent, so this line needs encrypt(msg, key). decrypt would not turn the message into ciphertext. This sends the password as plaintext, so an interceptor could read it. It should send the encrypted version: SEND encrypt(password, key).

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Learning
Duo
Encryption stops a hacker from intercepting data on a network.
FalseTrue
Matches
Encryption stops hackers from intercepting data
↓
Encryption does not prevent interception. Data can still be captured, but it is unreadable without the key.
Applying
Meanings
Term
unreadable without the key
Meaning
Encryption cannot stop data being intercepted, but the interceptor sees only ciphertext and cannot get the plaintext without the key, so passwords and bank details stay safe.
Practice
plaintext ← "MyPassword1"
Testing
Gaps
← ENCRYPT(password, key)
Check Answer
Spots
1ciphertext ← decrypt(msg, key)
2SEND ciphertext

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Quick Questions

1. Encryption stops a hacker from intercepting data on a network.
A. False ✓
B. True
Encryption does not stop interception. Data can still be captured, but the interceptor only sees ciphertext they cannot understand.
2. What does an interceptor see when they capture encrypted data?
A. Ciphertext ✓
B. Plaintext
Encrypted data travels as ciphertext, so the interceptor sees scrambled data, not the original plaintext.
3. Which is the best description of the purpose of encryption?
A. To make it impossible for anyone to intercept the data
B. To make intercepted data impossible to understand without the key ✓
The purpose of encryption is to make intercepted data unreadable without the key, not to prevent interception.
4. An interceptor captures a student's encrypted password but does not have the key. What can they do?
A. They have only ciphertext and cannot get the plaintext password ✓
B. They can easily read the plaintext password
Without the key, the interceptor cannot turn the ciphertext back into plaintext, so the password stays safe.
5. Which item is protected by encrypting data sent across a network?
A. The speed of the network connection
B. Bank details sent to an online shop ✓
Encryption protects sensitive data such as passwords, bank details and private messages from being understood if intercepted.

Match the Pairs

intercepted = "x9Fq#2Lm" → The interceptor captures only ciphertext, which looks like random characters and cannot be read
plaintext = decrypt(ciphertext, key) → Only someone with the correct key can turn ciphertext back into readable plaintext
Encryption does not stop interception, it only makes the intercepted data unreadable → interceptor sees ciphertext only
Without the key, the interceptor cannot get the plaintext → IF key = unknown THEN plaintext = unreadable
Encryption stops hackers from intercepting data → Encryption does not prevent interception. Data can still be captured, but it is unreadable without the key.
interceptor reads password = "hunter2" → If the password was encrypted, the interceptor would see only ciphertext, not the plaintext password.

Key Term

unreadable without the key
Encryption cannot stop data being intercepted, but the interceptor sees only ciphertext and cannot get the plaintext without the key, so passwords and bank details stay safe.

Fill in the Blanks

1. ciphertext ← ENCRYPT(password, key)
Hint: Type the variable name that stores the scrambled, unreadable result
2. IF hasKey = FALSE THEN OUTPUT "Unreadable" ENDIF
Hint: Type the Boolean value that describes an interceptor who does not have the key
3. plaintext ← DECRYPT(ciphertext, key)
Hint: Type the item needed to turn ciphertext back into plaintext
4. ciphertext ← ENCRYPT(bankDetails, key)
Hint: Type the function name that scrambles the data so an interceptor cannot read it
5. IF hasKey = FALSE THEN plaintext ← "unknown" ENDIF
Hint: Type the keyword that starts the code block run when the condition is true

Spot the Error

Question 1
A ciphertext ← decrypt(msg, key)
B SEND ciphertext
C OUTPUT "Sent safely"
Line A — The data must be scrambled before it is sent, so this line needs encrypt(msg, key). decrypt would not turn the message into ciphertext.
Question 2
A password ← "hunter2"
B SEND password
C OUTPUT "Sent safely"
Line B — This sends the password as plaintext, so an interceptor could read it. It should send the encrypted version: SEND encrypt(password, key).
Question 3
A captured ← "Xk2!9"
B OUTPUT captured
C OUTPUT "Plaintext: password"
Line C — An interceptor who only has the ciphertext and no key cannot work out the plaintext. The output should say the plaintext is unknown.
Question 4
A // Hacker sees plaintext only
B captured ← "Zq8#v1"
C OUTPUT captured
Line A — Encryption means the interceptor sees only ciphertext, not plaintext. The comment should say ciphertext.
Question 5
A password ← INPUT()
B sent ← encrypt(password)
C SEND sent
Line B — Encryption needs a key as well as the data. The line should be encrypt(password, key).

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