← IGCSE Computer Science Paper 1

Ciphertext

The scrambled, unreadable data made when plaintext is encrypted. It is what gets sent across a network and looks meaningless without the key.

Part of: IGCSE Computer Science 0478 — 2.3 Encryption — Module 4, ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 48, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 79, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 100, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 3, WA Computer Science ATAR Year 11 — Cyber Security — Module 64

Understanding Ciphertext in IGCSE Computer Science Paper 1

What it does: The data transmitted is the ciphertext, not the readable plaintext, so anyone intercepting it cannot read it. The plaintext is encrypted first, so only the ciphertext travels across the network. Ciphertext looks meaningless without the key, so the interceptor cannot understand it. Xjqqt Btwqi is Hello World shifted by 5 letters (a Caesar cipher). Hello World is the readable plaintext. Encryption changes plaintext into ciphertext. Decryption changes ciphertext back into plaintext. Encryption does not stop interception. It makes the intercepted data useless to the interceptor. Ciphertext is the scrambled result of encryption, so a clear readable word is not ciphertext.

Common mistakes: Ciphertext is made by encrypting plaintext with a key. Copying plain into cipher leaves the message readable. It should be cipher ← encrypt(plain, 3). The original readable message is plaintext, so it should be stored in plain. As written, plain is never set, so encrypt(plain, key) has nothing to encrypt. It should be plain ← "PAY NOW".

Class: _____________________  ·  Name: _____________________  ·  Date: _________

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This module's key terms are also in a printable crossword (Modules 189–203) and word search (Modules 189–217).

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Learning
Duo
Which term describes the scrambled, unreadable data produced when plaintext is encrypted?
CiphertextPlaintext
Matches
send(network, plaintext)
↓
The readable plaintext is sent instead of the ciphertext, so an interceptor could read it. The encrypted ciphertext should be sent.
Applying
Meanings
Term
ciphertext
Meaning
The scrambled, unreadable data made when plaintext is encrypted. It is what gets sent across a network and looks meaningless without the key.
Practice
plaintext ← "HELLO"
Testing
Gaps
ciphertext ← (plaintext, key)
Check Answer
Spots
1plain ← "HELLO"
2cipher ← plain

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Quick Questions

1. Which term describes the scrambled, unreadable data produced when plaintext is encrypted?
A. Plaintext
B. Ciphertext ✓
Ciphertext is the output of encryption. Plaintext is the original readable data that goes in.
2. Which form of the data is sent across the network when encryption is used?
A. Ciphertext ✓
B. Plaintext
The plaintext is encrypted first, so only the ciphertext travels across the network.
3. An interceptor captures data that was encrypted. Without the key, what does it look like?
A. Meaningless scrambled characters ✓
B. The original readable message
Ciphertext looks meaningless without the key, so the interceptor cannot understand it.
4. Which of these is an example of ciphertext?
A. Xjqqt Btwqi ✓
B. Hello World
Xjqqt Btwqi is Hello World shifted by 5 letters (a Caesar cipher). Hello World is the readable plaintext.
5. Which statement is correct?
A. Encryption turns ciphertext into plaintext
B. Encryption turns plaintext into ciphertext ✓
Encryption changes plaintext into ciphertext. Decryption changes ciphertext back into plaintext.

Match the Pairs

ciphertext → The scrambled, unreadable data produced when plaintext is encrypted
ciphertext = encrypt(plaintext, key) → Plaintext is turned into ciphertext using an encryption key
Ciphertext is what is sent across the network, so an interceptor only sees scrambled data → send(network, ciphertext)
Plaintext and ciphertext side by side: readable message versus scrambled output → HELLO -> KHOOR
send(network, plaintext) → The readable plaintext is sent instead of the ciphertext, so an interceptor could read it. The encrypted ciphertext should be sent.
ciphertext = "HELLO" → HELLO is readable plaintext, not ciphertext. Ciphertext should look scrambled, such as KHOOR after encryption.

Key Term

ciphertext
The scrambled, unreadable data made when plaintext is encrypted. It is what gets sent across a network and looks meaningless without the key.

Fill in the Blanks

1. ciphertext ← ENCRYPT(plaintext, key)
Hint: Type the function name that scrambles readable data into unreadable data.
2. plaintext ← DECRYPT(ciphertext, key)
Hint: Type the variable holding the scrambled data that is being turned back into readable data.
3. SEND(ciphertext, receiver)
Hint: Type the variable holding the scrambled data that travels across the network, not the readable original.
4. ciphertext ← ENCRYPT(plaintext, key)
Hint: Type the second argument, the value that controls how the data is scrambled.
5. IF key = correctKey THEN plaintext ← DECRYPT(ciphertext, key) ENDIF
Hint: Type the keyword that starts the code run when the condition is true.

Spot the Error

Question 1
A plain ← "HELLO"
B cipher ← plain
C SEND cipher
Line B — Ciphertext is made by encrypting plaintext with a key. Copying plain into cipher leaves the message readable. It should be cipher ← encrypt(plain, 3).
Question 2
A cipher ← "PAY NOW"
B key ← 5
C cipher ← encrypt(plain, key)
D SEND cipher
Line A — The original readable message is plaintext, so it should be stored in plain. As written, plain is never set, so encrypt(plain, key) has nothing to encrypt. It should be plain ← "PAY NOW".
Question 3
A plain ← "GO HOME"
B key ← 1
C cipher ← encrypt(plain, key)
D SEND plain
Line D — The ciphertext is what must be sent across the network. Sending plain would let an interceptor read the message. It should be SEND cipher.
Question 4
A plain = "CAT"
B cipher = "FDW"
C print("Plain:", cipher)
D print("Cipher:", cipher)
Line C — The label says Plain but the line prints the ciphertext, so the side-by-side comparison is wrong. It should print plain: print("Plain:", plain).
Question 5
A plain = "CAT"
B key = 3
C cipher = encrypt(plain, key)
D print("Cipher:" cipher)
Line D — There is a comma missing between the two items in print(). It should be print("Cipher:", cipher).

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