Symmetric encryption

Using the exact same secret key to both lock and unlock a message.

Part of: VCE Applied Computing - U2O2 - Cyber Security — Module 47, VCE Software Development - U4O2 - Cyber Security: secure software development practices — Module 58, VCE Data Analytics - U4O2 - Cyber Security: Data Security — Module 67, IGCSE Computer Science 0478 — 2.3 Encryption — Module 9, ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 62, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 81, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 132, QCE Digital Solutions Unit 2 — Data and Programming Techniques — Module 115, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 20, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 103, WA Computer Science ATAR Year 11 — Cyber Security — Module 74

Understanding Symmetric encryption in VCE Cyber Security

What it does: Since it's symmetric, the same single key handles both locking and unlocking. Keeping the key secret protects the message from being unlocked by the wrong person. If the key gets out, anyone with it could unlock the message. Sharing the key openly risks it being seen and used by someone it wasn't meant for. Only the exact same key used to lock the message can correctly unlock it. Using just one key tends to make symmetric encryption quicker to use. Symmetric encryption only needs one key that does both jobs. Without the original key, the message can no longer be unlocked correctly.

Common mistakes: Line 2 says a different key locks and unlocks, but symmetric encryption uses the exact same key for both. Line 2 says the key is safe to share with everyone, but the shared key must stay secret between the two people.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 41–50) and word search (Modules 41–74).

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Learning

Duo

Does symmetric encryption use the same key to lock and unlock a message?

YesNo

Matches

Posting the shared key somewhere public

↓

This breaks security - anyone could unlock the message

Applying

Meanings

Term

Symmetric encryption

Meaning

Using the exact same secret key to both lock and unlock a message.

Practice

shared_key = "secret"

Testing

Gaps

Symmetric encryption uses the

Check Answer

Spots

1Same key locks and unlocks

2Different key locks and unlocks

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Quick Questions

1. Does symmetric encryption use the same key to lock and unlock a message?

A. No

B. Yes ✓

One shared key is used for both locking and unlocking the message.

2. Should the shared key be kept secret between both people?

A. Yes ✓

B. No

If the key gets out, anyone with it could unlock the message.

3. Is it safe to share the key openly with everyone?

A. Yes

B. No ✓

Sharing the key openly risks it being seen and used by someone it wasn't meant for.

4. Can a different key unlock a message that was locked with the shared key?

A. No ✓

B. Yes

Only the exact same key used to lock the message can correctly unlock it.

5. Which is usually faster: symmetric encryption, or using two separate keys?

A. Symmetric encryption ✓

B. Using two separate keys

Using just one key tends to make symmetric encryption quicker to use.

Match the Pairs

Lock a message → Uses the shared key

Unlock a message → Uses that exact same shared key

Symmetric encryption uses just one key for both jobs → Lock and unlock with the same key

The shared key must be kept secret → Never share it with anyone who doesn't need it

Posting the shared key somewhere public → This breaks security - anyone could unlock the message

Using a different key to unlock the message → This won't work - only the same key can unlock it

Key Term

Symmetric encryption

Using the exact same secret key to both lock and unlock a message.

Fill in the Blanks

1. Symmetric encryption uses the same key to lock and unlock.

Hint: Type the word describing how the locking key and unlocking key compare.

2. The shared key must stay secret between both people.

Hint: Type the word for how the shared key must be kept.

3. You must share the key safely with the other person.

Hint: Type the word for giving the key to the other person who needs it.

4. Using a different key to unlock will not work.

Hint: Type the word describing a key that is not the same as the one used to lock.

5. Symmetric encryption is usually faster than using two keys.

Hint: Type the word for how quickly symmetric encryption works compared to using two separate keys.

Spot the Error

Question 1

A Same key locks and unlocks

B Different key locks and unlocks

C Key must stay secret

Line B — Line 2 says a different key locks and unlocks, but symmetric encryption uses the exact same key for both.

Question 2

A Key must stay secret

B Key is safe to share with everyone

C Same key locks and unlocks

Line B — Line 2 says the key is safe to share with everyone, but the shared key must stay secret between the two people.

Question 3

A Same key locks and unlocks

B Key must be shared safely

C Key should be shared publicly

Line C — Line 3 says the key should be shared publicly, but it must only be shared safely with the other trusted person.

Question 4

A Different key unlocks it fine

B Same key unlocks it correctly

C Key must stay secret

Line A — Line 1 says a different key unlocks it fine, but only the exact same key can correctly unlock the message.

Question 5

A Same key locks and unlocks

B Key must stay secret

C Key must be shared safely

D Sharing the key openly is fine

Line D — Line 4 says sharing the key openly is fine, but the key must be shared safely and kept secret from others.

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