🔐 Vigenère Cipher: Encrypt and Decrypt Online

Professional polyalphabetic substitution cipher tool. Method of encrypting alphabetic text using a series of interwoven Caesar ciphers based on a keyword. Though the "chiffre indéchiffrable" is easy to understand and implement, for three centuries it resisted all attempts to break it.

📝 Plaintext / Input Text

Characters: 43 | Letters: 35

📤 Ciphertext / Output Text

Characters: 0 | Letters: 0

⚙️ Cipher Settings

📋 CIPHER VARIANT

Choose encryption method

🔑 KEY (KEYWORD/PASSPHRASE)

Min 1 character, longer is more secure

🔄 KEY MODE

How key is applied

🔤 CUSTOM ALPHABET

Default: a-z (26 letters)

📝 CASE STRATEGY

How to handle upper/lower case

🌍 FOREIGN CHARACTERS

Include non-alphabetic characters

Encrypt numbers (0-9)

🎯 Encryption Process Visualizer

Step-by-step visualization of how Vigenère cipher encrypts your message

Plaintext:

Key:

Ciphertext:

Showing first 50 characters...

📊 Vigenère Table (Tabula Recta)

Interactive polyalphabetic substitution table - the foundation of Vigenère cipher encryption

What is Vigenère Cipher? Complete Guide

Understanding Vigenère Cipher Encryption

The Vigenère cipher is a method of encrypting alphabetic text using a series of interwoven Caesar ciphers based on the letters of a keyword. It was invented by Giovan Battista Bellaso in 1553, but is named after Blaise de Vigenère, who described it in 1586. For over three centuries, it was known as "le chiffre indéchiffrable" (the indecipherable cipher) because it resisted all attempts at cryptanalysis. is a method of encrypting alphabetic text using a series of interwoven Caesar ciphers based on the letters of a keyword. It was invented by Giovan Battista Bellaso in 1553, but is named after Blaise de Vigenère, who described it in 1586. For over three centuries, it was known as "le chiffre indéchiffrable" (the indecipherable cipher) because it resisted all attempts at cryptanalysis.

How Does Vigenère Cipher Work?

The encryption process uses a keyword that is repeated throughout the plaintext. Each letter of the keyword determines the shift for the corresponding plaintext letter: that is repeated throughout the plaintext. Each letter of the keyword determines the shift for the corresponding plaintext letter:

Choose a keyword: For example, "KEY" For example, "KEY"
Align keyword with plaintext: Repeat the keyword to match the length of your message Repeat the keyword to match the length of your message
Calculate shifts: Convert each keyword letter to a number (A=0, B=1, ... Z=25) Convert each keyword letter to a number (A=0, B=1, ... Z=25)
Apply Caesar cipher: Shift each plaintext letter by the corresponding keyword letter value Shift each plaintext letter by the corresponding keyword letter value
Wrap around: Use modulo 26 to wrap from Z back to A Use modulo 26 to wrap from Z back to A

Vigenère Cipher Formula

The mathematical formula for Vigenère encryption and decryption:

Encryption: C_i = (P_i + K_i) mod 26

Decryption: P_i = (C_i - K_i + 26) mod 26

Where P = plaintext letter position, C = ciphertext letter position, K = key letter position

Vigenère Cipher Variants

Standard Vigenère

Traditional method using addition

Beaufort Cipher

Uses subtraction: C = (K - P) mod 26

Variant Beaufort

Reciprocal version: C = (P - K) mod 26

Autokey Cipher

Uses the message itself as part of the key for enhanced security

Trithemius Cipher

Progressive key where each position uses a different shift

How to Encrypt with Vigenère Cipher

Enter your plaintext message in the input field
Choose a strong keyword (longer is better)
Select your preferred cipher variant
Configure case and character handling options
Click "Encode" to encrypt your message
Copy or download the ciphertext result

How to Decrypt Vigenère Cipher

Paste the ciphertext in the input field
Enter the correct keyword that was used for encryption
Select the same cipher variant used for encryption
Click "Decode" to decrypt the message
View the original plaintext in the output field

Breaking Vigenère Cipher

While significantly stronger than simple substitution ciphers, Vigenère can be broken using:

Kasiski Examination: Finding repeated sequences to determine key length Finding repeated sequences to determine key length
Friedman Test: Statistical analysis to estimate key length Statistical analysis to estimate key length
Index of Coincidence: Measuring frequency distribution to find key length Measuring frequency distribution to find key length
Frequency Analysis: Once key length is known, treating as multiple Caesar ciphers Once key length is known, treating as multiple Caesar ciphers

Vigenère Cipher Security

Security strength depends on:

Key Length: Longer keys are exponentially more secure Longer keys are exponentially more secure
Key Randomness: Truly random keys are harder to predict Truly random keys are harder to predict
Message Length: Shorter messages are harder to cryptanalyze Shorter messages are harder to cryptanalyze
Key Reuse: Never reuse the same key for multiple messages Never reuse the same key for multiple messages

Note: While historically significant, Vigenère cipher is not secure for protecting sensitive information in modern times. Use AES or other modern encryption standards for real security needs. While historically significant, Vigenère cipher is not secure for protecting sensitive information in modern times. Use AES or other modern encryption standards for real security needs.

Modern Applications

Today, Vigenère cipher is used for:

Educational purposes and learning cryptography fundamentals
Puzzle and game creation (escape rooms, ARGs)
Historical document analysis and research
Understanding polyalphabetic substitution principles
Teaching programming and algorithm implementation
Simple obfuscation where strong security isn't required

🏛️ Historical Significance

The Vigenère cipher was invented in 1553 by Giovan Battista Bellaso and remained unbroken for 300 years. It earned the nickname "le chiffre indéchiffrable" (the indecipherable cipher) and was used extensively in diplomatic and military communications until the mid-19th century when Charles Babbage and Friedrich Kasiski independently developed methods to break it.

⚙️ How It Works

Vigenère cipher uses a repeating keyword to create multiple Caesar cipher shifts. Each letter of the keyword determines the shift for the corresponding plaintext letter. For example, with key "KEY" (K=10, E=4, Y=24), the first letter shifts by 10, the second by 4, the third by 24, then the pattern repeats.

🧮 Mathematical Foundation

Encryption: C_i = (P_i + K_i) mod 26
Decryption: P_i = (C_i - K_i + 26) mod 26
Where P = plaintext letter position (A=0...Z=25), C = ciphertext letter position, K = key letter position, mod = modulo operation C_i = (P_i + K_i) mod 26
Decryption: P_i = (C_i - K_i + 26) mod 26
Where P = plaintext letter position (A=0...Z=25), C = ciphertext letter position, K = key letter position, mod = modulo operation

🔐 Security Analysis

Strengths: Resistant to simple frequency analysis, multiple Caesar shifts increase complexity, longer keys provide better security.
Weaknesses: Vulnerable to Kasiski examination, can be broken with sufficient ciphertext, key repetition creates patterns, not suitable for modern sensitive data. Resistant to simple frequency analysis, multiple Caesar shifts increase complexity, longer keys provide better security.
Weaknesses: Vulnerable to Kasiski examination, can be broken with sufficient ciphertext, key repetition creates patterns, not suitable for modern sensitive data.

🎯 Cipher Variants Explained

Standard Vigenère: C = (P + K) mod 26 - Traditional addition-based encryption
Beaufort Cipher: C = (K - P) mod 26 - Subtraction-based variant, self-reciprocal
Autokey Cipher: Uses plaintext as part of the key after initial keyword, eliminating key repetition
Trithemius Cipher: Progressive key with incrementing shifts (0, 1, 2, 3...) C = (P + K) mod 26 - Traditional addition-based encryption
Beaufort Cipher: C = (K - P) mod 26 - Subtraction-based variant, self-reciprocal
Autokey Cipher: Uses plaintext as part of the key after initial keyword, eliminating key repetition
Trithemius Cipher: Progressive key with incrementing shifts (0, 1, 2, 3...)

📚 Practical Tips

For Best Results: Use keys at least 1/3 the message length, avoid dictionary words as keys, random keys provide maximum security, never reuse keys for different messages. For absolute security, use one-time pad (key length = message length).
Common Mistakes: Using short or predictable keys, reusing the same key multiple times, using names or dates as keys. Use keys at least 1/3 the message length, avoid dictionary words as keys, random keys provide maximum security, never reuse keys for different messages. For absolute security, use one-time pad (key length = message length).
Common Mistakes: Using short or predictable keys, reusing the same key multiple times, using names or dates as keys.

Frequently Asked Questions About Vigenère Cipher

What is a Vigenère cipher and how does it differ from Caesar cipher?

The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to create multiple different Caesar cipher shifts, unlike the Caesar cipher which uses only one fixed shift for all letters. This makes Vigenère much more secure as it disguises frequency patterns. For example, the letter 'E' might be encrypted as 'X' in one position but 'M' in another position, depending on the keyword letter at that position.

How do I choose a strong key for Vigenère cipher?

A strong Vigenère key should be: (1) Long - ideally at least one-third the length of your message, (2) Random - avoid dictionary words, names, or predictable patterns, (3) Non-repetitive - keys without repeated letters are stronger, (4) Memorable yet unpredictable - if you need to remember it. For maximum security, use a truly random key that is as long as your message (this creates a one-time pad, which is theoretically unbreakable).

Can Vigenère cipher be cracked without knowing the key?

Yes, Vigenère cipher can be broken using several cryptanalytic methods: (1) Kasiski Examination - finding repeated sequences in ciphertext to determine key length, (2) Friedman Test - using statistical analysis (Index of Coincidence) to estimate key length, (3) Frequency Analysis - once key length is known, treating it as multiple Caesar ciphers and analyzing each position separately. Modern computers can crack Vigenère ciphers in seconds if the message is long enough and the key follows patterns.