Only letters A-Z are used. Longer keys provide stronger encryption.
About the Vigenère Cipher
How It Works
The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to shift each letter differently. Unlike the Caesar cipher which uses a single shift, Vigenère cycles through multiple shifts based on the key.
For example, with key "KEY": the 1st letter shifts by 10 (K), 2nd by 4 (E), 3rd by 24 (Y), then the pattern repeats.
Cryptanalysis Methods
- Kasiski Examination: Find repeated sequences to determine key length
- Index of Coincidence: Statistical measure that reveals key length
- Frequency Analysis: Once key length is known, analyze each column as a Caesar cipher
Historical Significance
Invented by Blaise de Vigenère in the 16th century, this cipher was considered unbreakable for 300 years and was nicknamed "le chiffre indéchiffrable" (the indecipherable cipher). It was finally broken by Charles Babbage and Friedrich Kasiski in the 19th century.
Security Note
The Vigenère cipher is not secure for modern use. It can be broken with sufficient ciphertext using the methods shown in this tool. For real encryption, use modern algorithms like AES.
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Frequently Asked Questions
Common questions about the Vigenère Cipher with Cryptanalysis
The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to shift letters. Unlike the Caesar cipher which uses a single shift, each letter in the keyword determines a different shift for the corresponding plaintext letter. This makes it much harder to break than simple substitution ciphers.