Vigenère Cipher
Encode or decode with the Vigenère polyalphabetic cipher. Uses a keyword to vary the shift per letter. 100% client-side.
🔑 Key analysis
📜 Historical context
The Vigenère cipher was published in 1586 by Blaise de Vigenère, a French diplomat. It was considered "le chiffre indéchiffrable" (the unbreakable cipher) for over three centuries — a title it earned because the polyalphabetic nature defeated the frequency analysis that broke simple substitution ciphers.
It was finally broken in the 19th century: Charles Babbage around 1854 (unpublished), and Friedrich Kasiski in 1863 (published). Their approach — finding repeated sequences to determine the key length, then breaking each Caesar shift separately — is the basis of modern cryptanalysis of this cipher.
About Vigenère Cipher
The Vigenère cipher is a polyalphabetic substitution cipher invented by Blaise de Vigenère in the 16th century. It was considered "le chiffre indéchiffrable" (the indecipherable cipher) for over 300 years because — unlike the Caesar cipher — it uses a different Caesar shift for each letter of the message, determined by a repeating keyword. For example, with key "KEY", the first letter shifts by K (10), the second by E (4), the third by Y (24), then repeats. This defeats simple frequency analysis: the same plaintext letter encrypts differently depending on its position. The Vigenère cipher was eventually broken by Charles Babbage and Friedrich Kasiski in the 19th century through Kasiski examination and Friedman analysis. This tool encodes or decodes with any keyword.
How It Works
- Enter your message (plaintext for encode, ciphertext for decode).
- Enter a keyword (letters only — the key shift for each position).
- Choose Encode or Decode.
- See the output plus a per-letter breakdown with the shifts used.
✨ Key Features
- ✓Encode with any keyword
- ✓Decode with the same keyword
- ✓Preserves letter case
- ✓Non-letter characters skipped (spaces, punctuation, digits)
- ✓Per-letter breakdown showing the shift applied
- ✓Key analysis — length, unique letters, positions
- ✓Tabula recta (Vigenère table) preview
- ✓Live conversion
- ✓Copy result
- ✓100% client-side — no data sent to any server
🎯 Common Use Cases
- →Learning polyalphabetic ciphers
- →Decoding Vigenère puzzles and CTF challenges
- →Encrypting toy messages for fun
- →Teaching cryptography history
- →Understanding how modern ciphers evolved
- →Creating puzzle content
- →Practicing classical cryptanalysis
- →Comparing Vigenère with Caesar and modern ciphers
💡 Advanced Tips & Pro Insights
- ▸The Vigenère cipher is a polyalphabetic cipher — different letters use different shifts, defeating simple frequency analysis. This is what made it unbreakable for centuries.
- ▸The keyword repeats cyclically. For key "KEY" and message "ATTACK", the shifts are: A+K, T+E, T+Y, A+K, C+E, K+Y.
- ▸Non-letters (spaces, punctuation, digits) are typically left unchanged and do NOT consume a key letter. Some implementations skip this rule — this tool uses the standard convention.
- ▸Kasiski examination: repeated ciphertext sequences usually come from repeated plaintext sequences. The distance between them is often a multiple of the key length, revealing the key size.
- ▸Friedman test (Index of Coincidence): statistical analysis that estimates the key length by measuring letter frequency distribution across the text.
- ▸Once the key length is known, the cipher reduces to N separate Caesar ciphers — each can be cracked by frequency analysis.