← IGCSE Computer Science Paper 1

Plaintext

The original, readable data (text, file, image or message) before it is encrypted. Decrypting ciphertext with the correct key turns it back into plaintext.

Part of: IGCSE Computer Science 0478 — 2.3 Encryption — Module 3, ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 47, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 78, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 99, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 2, WA Computer Science ATAR Year 11 — Cyber Security — Module 63

Understanding Plaintext in IGCSE Computer Science Paper 1

What it does: Plaintext can be any readable data such as a file, image or message, not just typed words. Encryption takes the readable plaintext and a key and outputs unreadable ciphertext. Being sent does not make data ciphertext. Only encrypted data is ciphertext. Decrypting with the correct key restores the original plaintext. Plaintext is the data in its original form that anyone can read and understand, before any encryption is applied.

Common mistakes: The result of encryption is ciphertext, not plaintext. It should be stored in cipher: cipher ← ENCRYPT(plain, 3). The variable plain must keep the original readable data. Plaintext is produced by decrypting ciphertext with the correct key. ENCRYPT would scramble the data again. It should be plain ← DECRYPT(cipher, key).

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 statement describes plaintext?
Scrambled data that cannot be read without a keyThe original, readable data before it is encrypted
Matches
plaintext = encrypt(message, key)
↓
This shows plaintext as the output of encryption. Encryption produces ciphertext, not plaintext.
Applying
Meanings
Term
plaintext
Meaning
The original, readable data (text, file, image or message) before it is encrypted. Decrypting ciphertext with the correct key turns it back into plaintext.
Practice
plaintext ← "MEET AT NOON"
Testing
Gaps
← "photo.png"
Check Answer
Spots
1plain ← "Meet at noon"
2plain ← ENCRYPT(plain, 3)

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Quick Questions

1. Which statement describes plaintext?
A. Scrambled data that cannot be read without a key
B. The original, readable data before it is encrypted ✓
Plaintext is the data before encryption. The scrambled version is called ciphertext.
2. Plaintext can only ever be typed text, such as an email.
A. False - it can be a file, image or message ✓
B. True - only text counts as plaintext
Plaintext is any readable data before encryption, including files, images and messages, not just text.
3. A student's photo is sent over a network. Before it is encrypted, the photo is:
A. Plaintext ✓
B. Ciphertext
Any original data that has not yet been encrypted is plaintext, even if it is an image.
4. If a hacker intercepts plaintext, what can they do?
A. Read and understand it straight away ✓
B. Nothing, because it is always scrambled
Plaintext is readable, so anyone who gets it can understand it. This is why sensitive data is encrypted.
5. What is the result of decrypting ciphertext with the correct key?
A. A new encryption key
B. Plaintext ✓
Decryption reverses encryption, turning ciphertext back into the original plaintext.

Match the Pairs

plaintext → The original, readable data before it is encrypted
decrypt(ciphertext, key) = plaintext → Decrypting with the correct key turns unreadable data back into plaintext
Plaintext is not only text: an image or file that is readable before encryption is also plaintext → plaintext = photo.jpg
Plaintext is the input to encryption, and encrypting it produces ciphertext → ciphertext = encrypt(plaintext, key)
plaintext = encrypt(message, key) → This shows plaintext as the output of encryption. Encryption produces ciphertext, not plaintext.
An interceptor cannot read plaintext → Plaintext is readable by anyone who gets it, so an interceptor can read it. Only ciphertext is unreadable.

Key Term

plaintext
The original, readable data (text, file, image or message) before it is encrypted. Decrypting ciphertext with the correct key turns it back into plaintext.

Fill in the Blanks

1. plaintext ← "photo.png"
Hint: Type the variable name for the original, readable data, which can be a file or image as well as text.
2. ciphertext ← encrypt(plaintext, key)
Hint: Type the variable holding the readable data that goes into the encryption function.
3. plaintext ← decrypt(ciphertext, key)
Hint: Type the function name that turns ciphertext back into readable data.
4. ciphertext ← encrypt(plaintext, key)
Hint: Type the function name that turns readable data into unreadable data.
5. plaintext ← decrypt(ciphertext, key) OUTPUT plaintext
Hint: Type the variable that holds the readable result of decryption.

Spot the Error

Question 1
A plain ← "Meet at noon"
B plain ← ENCRYPT(plain, 3)
C OUTPUT cipher
Line B — The result of encryption is ciphertext, not plaintext. It should be stored in cipher: cipher ← ENCRYPT(plain, 3). The variable plain must keep the original readable data.
Question 2
A cipher ← "Phhw dw qrrq"
B key ← 3
C plain ← ENCRYPT(cipher, key)
D OUTPUT plain
Line C — Plaintext is produced by decrypting ciphertext with the correct key. ENCRYPT would scramble the data again. It should be plain ← DECRYPT(cipher, key).
Question 3
A image ← "photo.png"
B key ← 7
C cipher ← DECRYPT(image, key)
D OUTPUT cipher
Line C — An image is plaintext too, because it is original data before encryption. It must be encrypted, not decrypted: cipher ← ENCRYPT(image, key).
Question 4
A plain ← "Pay Sam $20"
B cipher ← ENCRYPT(plain, 5)
C OUTPUT "Seen: " + cipher
D OUTPUT "Readable: " + cipher
Line D — Plaintext is the data anyone can read and understand. Ciphertext is unreadable without the key. The readable output should use plain, not cipher.
Question 5
A message ← "See you at 8"
B cipher ← message
C OUTPUT plain
Line B — A message that has not been encrypted is plaintext. It has not been scrambled, so it should be stored as plain, not cipher: plain ← message.

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