← IGCSE Computer Science Paper 1

Choosing symmetric or asymmetric encryption for a scenario

Use symmetric if both sides can already share a key safely; use asymmetric for strangers, such as a customer and a website. Justify the choice with 'because'.

Part of: IGCSE Computer Science 0478 — 2.3 Encryption — Module 20, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 35

Understanding Choosing symmetric or asymmetric encryption for a scenario in IGCSE Computer Science Paper 1

What it does: The answer picks symmetric and uses 'because' to give a reason based on the safe key sharing. If both people can safely share a key in advance, symmetric encryption works well. It uses one key and is simple to use. Exam answers need a reason linked to the scenario. Using 'because' and giving a specific reason earns the mark. 'The best one' is too vague. The public key is shared openly and used to encrypt. Only the website keeps the private key to decrypt. Symmetric encryption does work, but sharing the one key with a stranger is risky. An interceptor who gets the key can decrypt everything.

Common mistakes: The test is the wrong way round. People who already know each other can share a key safely, so symmetric is chosen when the value is TRUE. It should be IF knowEachOther = TRUE THEN. A customer and a website are strangers, so strangers should be TRUE. With FALSE the asymmetric choice would never run. It should be strangers ← TRUE.

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Learning
Duo
Two friends who meet daily want to send private messages. They can swap a key in person first. Which type of encryption fits best?
Symmetric, because they can already share one key safelyAsymmetric, because friends can never share a key
Matches
Use symmetric for a customer and a website because it is faster.
↓
Symmetric is the wrong choice for strangers. The customer and website cannot share a secret key safely first, so asymmetric is needed.
Applying
Meanings
Term
symmetric vs asymmetric
Meaning
Use symmetric if both sides can already share a key safely; use asymmetric for strangers, such as a customer and a website. Justify the choice with 'because'.
Practice
IF keyAlreadyShared = TRUE THEN
Testing
Gaps
choice ← ""
Check Answer
Spots
1knowEachOther ← TRUE
2IF knowEachOther = FALSE THEN

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Quick Questions

1. Two friends who meet daily want to send private messages. They can swap a key in person first. Which type of encryption fits best?
A. Symmetric, because they can already share one key safely ✓
B. Asymmetric, because friends can never share a key
If both people can safely share a key in advance, symmetric encryption works well. It uses one key and is simple to use.
2. A customer types card details into a shop website they have never used before. Which type of encryption fits best?
A. Asymmetric, because the customer and website are strangers with no safe way to share a key ✓
B. Symmetric, because the customer and website already share a secret key
Strangers cannot safely share a secret key beforehand. Asymmetric encryption lets the website publish a public key that anyone can use to encrypt.
3. Which answer justifies the choice of encryption properly?
A. Asymmetric, because it is the best one
B. Asymmetric, because the sender never needs to be given a secret key ✓
Exam answers need a reason linked to the scenario. Using 'because' and giving a specific reason earns the mark. 'The best one' is too vague.
4. Two colleagues who work in the same office want to send each other files. They can agree a key face to face. What should they choose?
A. Symmetric encryption ✓
B. Asymmetric encryption
They know each other and can share the key safely in person, so symmetric encryption is suitable.
5. A customer sends card details to a website. Which key does the customer's browser use to encrypt the data?
A. The website's private key
B. The website's public key ✓
The public key is shared openly and used to encrypt. Only the website keeps the private key to decrypt.

Match the Pairs

Choose symmetric: two friends who already know each other → Both people can share one secret key safely in advance, so the same key can encrypt and decrypt quickly.
Choose asymmetric: customer sending card details to a website → The two sides are strangers with no safe way to share a secret key, so a public and private key pair is used.
A justified answer for two classmates who already share a secret key → Use symmetric encryption because both people can already share the key safely.
A justified answer for a customer paying on a website they have never used before → Use asymmetric encryption because the customer and website are strangers who cannot share a key safely.
Use symmetric for a customer and a website because it is faster. → Symmetric is the wrong choice for strangers. The customer and website cannot share a secret key safely first, so asymmetric is needed.
Use asymmetric encryption for two friends who already share a key. → The answer has no 'because' justification. It also overlooks that friends who can already share a key safely can use symmetric encryption.

Key Term

symmetric vs asymmetric
Use symmetric if both sides can already share a key safely; use asymmetric for strangers, such as a customer and a website. Justify the choice with 'because'.

Fill in the Blanks

1. IF keyAlreadyShared = TRUE THEN choice ← "symmetric" ENDIF
Hint: Type the kind of encryption chosen when both people can already share a key safely.
2. IF parties = "strangers" THEN choice ← "asymmetric" ENDIF
Hint: Type the kind of encryption chosen when strangers must exchange data securely.
3. Customer sends card details to a website. Use asymmetric because they are strangers.
Hint: Type the word that introduces the reason when you justify your choice.
4. Customer sends card details to a website. Encrypt with the website's public key.
Hint: Type the kind of key the website shares openly with any customer.
5. Two friends already know each other. Use symmetric because they can share a key.
Hint: Type the action the two friends can safely do with the key, which makes symmetric suitable.

Spot the Error

Question 1
A knowEachOther ← TRUE
B IF knowEachOther = FALSE THEN
C choice ← "symmetric"
D ELSE
E choice ← "asymmetric"
Line B — The test is the wrong way round. People who already know each other can share a key safely, so symmetric is chosen when the value is TRUE. It should be IF knowEachOther = TRUE THEN.
Question 2
A strangers ← FALSE
B IF strangers = TRUE THEN
C choice ← "asymmetric"
D ENDIF
Line A — A customer and a website are strangers, so strangers should be TRUE. With FALSE the asymmetric choice would never run. It should be strangers ← TRUE.
Question 3
A scenario ← "card to website"
B keyShared ← FALSE
C choice ← "symmetric"
D OUTPUT choice
Line C — A customer sending card details to a website has no shared key, so asymmetric encryption is needed. It should be choice ← "asymmetric".
Question 4
A choice ← "symmetric"
B OUTPUT "Use " + choice
C OUTPUT "because friends"
D OUTPUT "cannot share a key"
Line D — Symmetric encryption is chosen because the two friends already share a key safely. Saying they cannot share a key contradicts the choice. It should be OUTPUT "already share a key".
Question 5
A choice ← "symmetric"
B reason ← "they share a key"
C OUTPUT choice + " although"
D OUTPUT reason
Line C — A justification must use the word 'because' to link the choice to its reason. 'although' does not give a reason. It should be OUTPUT choice + " because".

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