Vigenère Cipher
Exploring a polyalphabetic substitution cipher.
Vigenère Cipher is a free Cryptology Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Cryptology Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
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Vigenère Cipher
The Vigenère Cipher is a polyalphabetic substitution cipher that improves upon the weaknesses of the Caesar Cipher by using a keyword to determine letter shifts.
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How It Works
Instead of shifting all letters by the same value, the Vigenère Cipher shifts letters based on a repeating keyword.
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Encryption Process
- Choose a keyword (e.g., 'KEY').
- Repeat the keyword to match the length of the plaintext.
- Shift each letter of the plaintext based on the corresponding letter in the keyword.
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Decryption Process
- Use the same keyword to reverse the shifts.
- Each letter in the ciphertext is shifted backward using the corresponding letter in the keyword.
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Example
Encrypting 'HELLO' with the keyword 'KEY':
- H + K → R
- E + E → I
- L + Y → J
- L + K → V
- O + E → S
The ciphertext is 'RIJVS'.
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Strengths
- Resists frequency analysis better than the Caesar Cipher.
- More secure with a longer, random keyword.
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Weaknesses
- Still vulnerable to cryptanalysis if the keyword is short or repeated.
- Can be broken using Kasiski examination or frequency analysis.
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Modern Usage
Though insecure today, the Vigenère Cipher is still used in puzzles and cryptographic learning exercises.
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Summary
- The Vigenère Cipher uses a keyword for encryption.
- It is more secure than the Caesar Cipher but still breakable.
- It is an important step in the evolution of cryptography.
Frequently asked questions
Is the “Vigenère Cipher” lesson free?
Yes — the full text of “Vigenère Cipher” is free to read here on the web, and the Cryptology Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Cryptology Academy course, upgrade to CoddyKit PRO.
What will I learn in “Vigenère Cipher”?
Exploring a polyalphabetic substitution cipher. You practise Cryptology Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Cryptology Academy?
No prior experience is required. Cryptology Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Vigenère Cipher” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Cryptology Academy lesson?
Yes. Every Cryptology Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Caesar Cipher
- Vigenère Cipher
- Transposition Ciphers