← VCE Cyber Security

Vigenere cipher

A cipher that uses a keyword, where each keyword letter sets a different shift for the matching letter of the message.

Part of: VCE Applied Computing - U2O2 - Cyber Security — Module 46, VCE Data Analytics - U4O2 - Cyber Security: Data Security — Module 66, ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 61, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 114, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 16, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 111

Understanding Vigenere cipher in VCE Cyber Security

What it does: Multiple shifts from the keyword make it harder to spot a repeating pattern than a single shift would. Only the same keyword, applied the same way, correctly reverses the encryption. The keyword's letters set different shifts across the message. The keyword cycles back to its start and repeats to cover the whole message. The keyword is the key feature that makes a cipher a Vigenere cipher.

Common mistakes: Line 1 says it uses one fixed shift, but a Vigenere cipher uses a keyword instead of a single shift. Line 2 says every letter shifts the same, but each keyword letter sets a different shift.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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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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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning
Duo
Does a Vigenere cipher use a keyword instead of one fixed shift?
YesNo
Matches
Using a different keyword to decrypt
↓
This won't work - the message stays scrambled
Applying
Meanings
Term
Vigenere cipher
Meaning
A cipher that uses a keyword, where each keyword letter sets a different shift for the matching letter of the message.
Practice
keyword = "KEY"
Testing
Gaps
A Vigenere cipher uses a
Check Answer
Spots
1Uses one fixed shift
2Uses a keyword instead

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Quick Questions

1. Does a Vigenere cipher use a keyword instead of one fixed shift?
A. Yes ✓
B. No
The keyword's letters set different shifts across the message.
2. Do all letters in the message get shifted by the same amount?
A. Yes
B. No ✓
Each keyword letter sets a different shift for the matching message letter.
3. Does the keyword repeat if it's shorter than the message?
A. Yes ✓
B. No
The keyword cycles back to its start and repeats to cover the whole message.
4. Must the same keyword be used to decrypt as was used to encrypt?
A. No
B. Yes ✓
A different keyword won't correctly restore the original message.
5. Does using a different keyword than the one used to encrypt still decrypt the message correctly?
A. No ✓
B. Yes
Only the matching keyword correctly reverses the encryption.

Match the Pairs

A keyword → What a Vigenere cipher uses instead of one fixed shift
Each keyword letter shifts a different message letter → What happens as the keyword repeats
A Vigenere cipher is harder to break than a single fixed shift → Using a keyword with multiple shifts
Decrypting needs the exact same keyword used to encrypt → Matching the keyword to reverse the cipher
Using a different keyword to decrypt → This won't work - the message stays scrambled
Using only one shift number for the whole message → This isn't a Vigenere cipher - it's just a single fixed shift

Key Term

Vigenere cipher
A cipher that uses a keyword, where each keyword letter sets a different shift for the matching letter of the message.

Fill in the Blanks

1. A Vigenere cipher uses a keyword instead of one shift.
Hint: Type the word for the word used instead of a single fixed shift.
2. Each keyword letter sets a different shift.
Hint: Type the word for how far a letter in the message moves.
3. The keyword repeats if it's shorter than the message.
Hint: Type the word for what the keyword does again and again to cover the whole message.
4. You need the same keyword to decrypt the message.
Hint: Type the word for what must match in order to decrypt correctly.
5. This makes the message harder to break than one shift.
Hint: Type the word for how much more difficult this cipher is to crack.

Spot the Error

Question 1
A Uses one fixed shift
B Uses a keyword instead
C Keyword sets different shifts
Line A — Line 1 says it uses one fixed shift, but a Vigenere cipher uses a keyword instead of a single shift.
Question 2
A Keyword sets different shifts
B Every letter shifts the same
C Keyword repeats if too short
Line B — Line 2 says every letter shifts the same, but each keyword letter sets a different shift.
Question 3
A Keyword repeats if too short
B Keyword never repeats
C Same keyword decrypts it
Line B — Line 2 says the keyword never repeats, but the keyword repeats if it's shorter than the message.
Question 4
A Same keyword decrypts it
B Different keyword decrypts it
C Keyword sets different shifts
D Keyword repeats if too short
Line B — Line 2 says a different keyword can decrypt it, but only the same keyword used to encrypt will work.
Question 5
A Harder to break than one shift
B Uses a keyword to shift letters
C Easier to break than one shift
Line C — Line 3 says it's easier to break than one shift, but a Vigenere cipher is actually harder to break.

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