Vigenère Cipher
The Vigenère Cipher is a method of encrypting alphabetic text using a series of Caesar ciphers based on the letters of a keyword. It is a polyalphabetic substitution cipher, meaning the same plaintext letter can be encoded to different ciphertext letters depending on its position.
What You'll Learn
Introduction to Vigenère Cipher
The Vigenère Cipher is a method of encrypting alphabetic text using a series of Caesar ciphers based on the letters of a keyword. It is a polyalphabetic substitution cipher, meaning the same plaintext letter can be encoded to different ciphertext letters depending on its position.
Prerequisites
How to Solve Vigenère Cipher Problems
Step 1: Write the keyword repeatedly above the plaintext (or ciphertext for decoding)
Step 2: Convert each plaintext letter and key letter to positions (A=0 to Z=25)
Step 3: For encoding: cipher_position = (plain_position + key_position) mod 26
Step 4: For decoding: plain_position = (cipher_position - key_position) mod 26
Step 5: Convert resulting positions back to letters
Step 6: The Vigenère square (tabula recta) can be used as an alternative to arithmetic
Example Problem
Example: Decode 'LXFOPVEFRNHR' using keyword 'LEMON'. Solution: Step 1: Keyword repeated: LEMONLEMONLE (ciphertext length 12) Step 2: Convert: L(11),E(4),M(12),O(14),N(13),L(11),E(4),M(12),O(14),N(13),L(11),E(4) Step 3: Ciphertext: L(11),X(23),F(5),O(14),P(15),V(21),E(4),F(5),R(17),N(13),H(7),R(17) Step 4: Plain_position = (cipher - key) mod 26: (11-11)=0→A, (23-4)=19→T, (5-12)=-7+26=19→T, (14-14)=0→A, (15-13)=2→C, (21-11)=10→K, (4-4)=0→A, (5-12)=-7+26=19→T, (17-14)=3→D, (13-13)=0→A, (7-11)=-4+26=22→W, (17-4)=13→N Step 5: Decoded: ATTACKATDAWN Answer: ATTACKATDAWN
Pro Tips & Tricks
- The keyword determines the shift for each position (A=0 shift, B=1 shift, ..., Z=25 shift)
- Vigenère square: rows = key letters, columns = plaintext letters, intersection = ciphertext
- To decode, find ciphertext letter in the key letter's row, column header is plaintext
- Keyword length is the period of the cipher
- Longer keywords make the cipher more secure
- A keyword of length 1 is equivalent to a Caesar cipher
Shortcut Methods to Solve Faster
Common Mistakes to Avoid
Practice Worksheets
Practice makes perfect! Work through these worksheets to master Vigenère Cipher. Each worksheet contains 20 questions with detailed explanations. Start from Worksheet 1 and progress through increasing difficulty levels.
Exam Importance
Vigenère Cipher is an important topic for various competitive exams. Here's how frequently it appears:
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Start with Worksheet 1 and work your way up to expert level! Each worksheet includes: