← IGCSE Computer Science Paper 1

Writing an exam answer about encryption

A full-mark answer uses the key terms correctly and gives the steps in order: plaintext is encrypted with a key into ciphertext. Encryption does not stop interception.

Part of: IGCSE Computer Science 0478 — 2.3 Encryption — Module 21

Understanding Writing an exam answer about encryption in IGCSE Computer Science Paper 1

What it does: A full-mark answer gives the steps in order and uses the right key at each stage: public key to encrypt, private key to decrypt. Encryption does not stop interception. It makes intercepted data unreadable. Saying it stops interception loses the mark. Encryption turns plaintext (the original data) into ciphertext (the scrambled data). Do not swap the terms. Symmetric means one shared key does both jobs. Two different keys is asymmetric encryption. The private key is kept secret by the recipient and decrypts what the public key encrypted. Describing the reverse step with the correct terms shows the examiner you understand decryption as well as encryption.

Common mistakes: Symmetric encryption uses ONE shared key for both encrypting and decrypting. The line should say the same key decrypts it. With asymmetric encryption, data encrypted with the public key can only be decrypted with the matching private key, not the public key.

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This module's key terms are also in a printable crossword (Modules 204–217) and word search (Modules 189–217).

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Learning

Duo

Which sentence is worth a mark for why data is encrypted?

So that if the data is intercepted, it cannot be understoodSo that the data cannot be intercepted

Matches

The sender encrypts the message with the private key

↓

Wrong key. In asymmetric encryption the sender encrypts with the recipient's public key. The private key is used to decrypt and is kept secret.

Applying

Meanings

Term

plaintext, key, ciphertext

Meaning

A full-mark answer uses the key terms correctly and gives the steps in order: plaintext is encrypted with a key into ciphertext. Encryption does not stop interception.

Practice

1. Plaintext is encrypted with a key into ciphertext.

Testing

Gaps

ciphertext ← ENCRYPT(, key)

Check Answer

Spots

3This produces ciphertext

4A different key decrypts it

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Quick Questions

1. Which sentence is worth a mark for why data is encrypted?

A. So that the data cannot be intercepted

B. So that if the data is intercepted, it cannot be understood ✓

Encryption does not stop interception. It makes intercepted data unreadable. Saying it stops interception loses the mark.

2. Which answer uses the key terms correctly?

A. Plaintext is encrypted using a key to produce ciphertext ✓

B. Ciphertext is encrypted using a key to produce plaintext

Encryption turns plaintext (the original data) into ciphertext (the scrambled data). Do not swap the terms.

3. Describing symmetric encryption: which is correct?

A. The same key is used to encrypt and to decrypt the data ✓

B. One key encrypts the data and a different key decrypts it

Symmetric means one shared key does both jobs. Two different keys is asymmetric encryption.

4. Describing asymmetric encryption: which is correct?

A. A public key and a private key are used as a pair ✓

B. Both people keep one identical secret key

Asymmetric encryption uses a key pair: a public key and a private key. One identical shared key is symmetric.

5. Which step is correct for sending a secure message with asymmetric encryption?

A. Encrypt with the recipient's private key

B. Encrypt with the recipient's public key ✓

A common mistake is encrypting with the private key. The sender uses the recipient's public key, and the recipient decrypts with their private key.

Match the Pairs

plaintext + key → ciphertext → Encryption in an exam answer: the original readable data is scrambled using a key to make unreadable ciphertext

ciphertext + key → plaintext → Decryption in an exam answer: the unreadable ciphertext is turned back into readable plaintext using the correct key

Asymmetric steps in order: the sender encrypts with the recipient's public key, and the recipient decrypts with their private key → encrypt(public key) → send → decrypt(private key)

Symmetric steps in order: the same secret key is used to encrypt the plaintext and to decrypt the ciphertext → plaintext + shared key → ciphertext → plaintext + shared key

The sender encrypts the message with the private key → Wrong key. In asymmetric encryption the sender encrypts with the recipient's public key. The private key is used to decrypt and is kept secret.

Encryption stops hackers from intercepting the data → Encryption does not stop interception. A hacker can still capture the data, but it is ciphertext, so it is unreadable without the key.

Key Term

plaintext, key, ciphertext

A full-mark answer uses the key terms correctly and gives the steps in order: plaintext is encrypted with a key into ciphertext. Encryption does not stop interception.

Fill in the Blanks

1. ciphertext ← ENCRYPT(plaintext, key)

Hint: Type the term for the original, readable data that goes into the encryption step.

2. ciphertext ← ENCRYPT(plaintext, key) plaintext ← DECRYPT(ciphertext, key)

Hint: Type the function name that turns ciphertext back into plaintext using the same shared key.

3. ciphertext ← ENCRYPT(plaintext, publicKey)

Hint: Type the name of the recipient's key that is used to encrypt in asymmetric encryption.

4. ciphertext ← ENCRYPT(plaintext, publicKey) plaintext ← DECRYPT(ciphertext, privateKey)

Hint: Type the name of the key that only the recipient holds and uses to decrypt.

5. Encryption does not stop interception but ciphertext cannot be read without the key

Hint: Type the word for what an attacker does to data on a network, which encryption cannot prevent.

Spot the Error

Question 1

A Symmetric encryption answer

B Plaintext is encrypted by key

C This produces ciphertext

D A different key decrypts it

Line D — Symmetric encryption uses ONE shared key for both encrypting and decrypting. The line should say the same key decrypts it.

Question 2

A Asymmetric answer

B Sender gets the public key

C Plaintext is encrypted with it

D Public key decrypts it

Line D — With asymmetric encryption, data encrypted with the public key can only be decrypted with the matching private key, not the public key.

Question 3

A Why encrypt data?

B Packets can be intercepted

C Encryption stops interception

Line C — Encryption does not stop data being intercepted. It makes the intercepted ciphertext unreadable without the key.

Question 4

A Sending a secure message

B Get the recipient's public key

C Encrypt with your private key

D They decrypt with private key

Line C — The sender must encrypt with the recipient's public key. Encrypting with a private key is a common mistake and would not keep the message secret.

Question 5

A Symmetric key problem

B Both sides need the same key

C Sharing the key is always safe

D Key thief could decrypt data

Line C — The shared key must be sent to the other person, and it could be intercepted. So sharing it is risky, not always safe.

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