Why encrypted data is useless to an interceptor

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Quick Questions

Shade in the A or B circle for each question.

1. Encryption stops a hacker from intercepting data on a network.

A False

B True

2. What does an interceptor see when they capture encrypted data?

A Ciphertext

B Plaintext

3. Which is the best description of the purpose of encryption?

A To make it impossible for anyone to intercept the data

B To make intercepted data impossible to understand without the key

4. An interceptor captures a student's encrypted password but does not have the key. What can they do?

A They have only ciphertext and cannot get the plaintext password

B They can easily read the plaintext password

5. Which item is protected by encrypting data sent across a network?

A The speed of the network connection

B Bank details sent to an online shop

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1intercepted = "x9Fq#2Lm"		AThe interceptor captures only ciphertext, which looks like random characters and cannot be read
2plaintext = decrypt(ciphertext, key)		BEncryption does not prevent interception. Data can still be captured, but it is unreadable without the key.
3Encryption does not stop interception, it only makes the intercepted data unreadable		Cinterceptor sees ciphertext only
4Without the key, the interceptor cannot get the plaintext		DIF key = unknown THEN plaintext = unreadable
5Encryption stops hackers from intercepting data		EIf the password was encrypted, the interceptor would see only ciphertext, not the plaintext password.
6interceptor reads password = "hunter2"		FOnly someone with the correct key can turn ciphertext back into readable plaintext

Definitions

unreadable without the key

Fill in the Blanks

Complete each line of code by filling in the missing parts.

__________ ← ENCRYPT(password, key)

IF hasKey = _____ THEN OUTPUT "Unreadable" ENDIF

plaintext ← DECRYPT(ciphertext, ___)

ciphertext ← _______(bankDetails, key)

IF hasKey = FALSE ____ plaintext ← "unknown" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A ciphertext ← decrypt(msg, key)

B SEND ciphertext

C OUTPUT "Sent safely"

Question 2

A password ← "hunter2"

B SEND password

C OUTPUT "Sent safely"

Question 3

A captured ← "Xk2!9"

B OUTPUT captured

C OUTPUT "Plaintext: password"

Question 4

A // Hacker sees plaintext only

B captured ← "Zq8#v1"

C OUTPUT captured

Question 5

A password ← INPUT()

B sent ← encrypt(password)

C SEND sent

Answer Key

Why encrypted data is useless to an interceptor

Section A: Quick Questions

A — False

A — Ciphertext

B — To make intercepted data impossible to understand without the key

A — They have only ciphertext and cannot get the plaintext password

B — Bank details sent to an online shop

Section B: Match the Pairs

intercepted = "x9Fq#2Lm" → The interceptor captures only ciphertext, which looks like random characters and cannot be read

plaintext = decrypt(ciphertext, key) → Only someone with the correct key can turn ciphertext back into readable plaintext

Encryption does not stop interception, it only makes the intercepted data unreadable → interceptor sees ciphertext only

Without the key, the interceptor cannot get the plaintext → IF key = unknown THEN plaintext = unreadable

Encryption stops hackers from intercepting data → Encryption does not prevent interception. Data can still be captured, but it is unreadable without the key.

interceptor reads password = "hunter2" → If the password was encrypted, the interceptor would see only ciphertext, not the plaintext password.

Section C: Definitions

unreadable without the key — Encryption cannot stop data being intercepted, but the interceptor sees only ciphertext and cannot get the plaintext without the key, so passwords and bank details stay safe.

Section D: Fill in the Blanks

ciphertext ← ENCRYPT(password, key)

IF hasKey = FALSE THEN
    OUTPUT "Unreadable"
ENDIF

plaintext ← DECRYPT(ciphertext, key)

ciphertext ← ENCRYPT(bankDetails, key)

IF hasKey = FALSE THEN
    plaintext ← "unknown"
ENDIF

Section E: Spot the Error

A — The data must be scrambled before it is sent, so this line needs encrypt(msg, key). decrypt would not turn the message into ciphertext.

B — This sends the password as plaintext, so an interceptor could read it. It should send the encrypted version: SEND encrypt(password, key).

C — An interceptor who only has the ciphertext and no key cannot work out the plaintext. The output should say the plaintext is unknown.

A — Encryption means the interceptor sees only ciphertext, not plaintext. The comment should say ciphertext.

B — Encryption needs a key as well as the data. The line should be encrypt(password, key).