🔐 Vigenère Cipher

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Keyword: {{ sanitizedKeyword }} (Length: {{ sanitizedKeyword.length }}) Enter letters only - numbers and symbols will be ignored
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Keyword: {{ sanitizedKeyword }}
Key Pattern: {{ getKeyPattern() }}
Shifts: {{ keyShifts.join(',') }}
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Total Letters: {{ analysis.total_letters }}
Unique Letters: {{ analysis.unique_letters }}
Key Length: {{ analysis.key_length }}
Key Strength: {{ analysis.key_strength }}
Most Frequent: {{ analysis.most_frequent }}
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Cipher Type: Polyalphabetic substitution
Security: Much stronger than monoalphabetic ciphers
Key Requirement: Shared secret keyword
Historical Use: Diplomatic and military communications
Current Setup: {{ cipherInfo }}
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ℹ️ About Vigenère Cipher

Vigenère cipher encoder and decoder transforms A-Z text with a repeating keyword, applying a different Caesar-style shift to each letter. It is a classic polyalphabetic substitution cipher: the same plaintext letter can encrypt to different ciphertext letters depending on the current key letter. For historical background and the tabula-recta method, see Britannica's Vigenère cipher overview and its broader cryptology discussion of Vigenère ciphers.

Common uses:

  • Encode or decode Vigenère cipher text for puzzle hunts, escape rooms, ARG clues, and CTF warm-ups.
  • Demonstrate how a keyword cipher improves on a single-shift Caesar cipher while still leaking patterns.
  • Compare polyalphabetic substitution with ROT13, ROT-N, Atbash, Baconian, and rail fence ciphers.
  • Teach repeated-key cryptanalysis concepts such as key length, Kasiski examination, and index of coincidence.
  • Obfuscate non-sensitive text when the goal is historical flavor or casual concealment, not real secrecy.

How the keyword shift works:

  • Letters are normalized as A=0, B=1, through Z=25.
  • Encoding uses C = (P + K) mod 26; decoding reverses it with P = (C - K) mod 26.
  • The keyword repeats across alphabetic characters. Spaces, digits, and punctuation can be preserved without consuming key positions.
  • A one-letter key degenerates into a Caesar cipher. Short or repeated keys create periodic patterns that are easier to attack.

Examples: With keyword KEY, HELLO becomes RIJVS because the key shifts are 10, 4, 24, 10, and 4. The classic keyword LEMON maps ATTACKATDAWN to LXFOPVEFRNHR. Decoding with the same keyword subtracts the same shifts to recover the original plaintext.

Limitations and strength note: Vigenère is useful for learning, but it is not modern encryption. Repeated-key ciphertext can reveal key length through repeated fragments and statistical tests. Michigan Tech's Vigenère tutorial covers Kasiski's method and the index of coincidence, two classic ways to analyze this cipher family. Do not use Vigenère for passwords, private messages, API keys, or production data; use modern authenticated encryption when confidentiality matters.

API Usage Example:

POST /api/tools/encoding-decoding/vigenere
Content-Type: application/json

{
    "text": "ATTACKATDAWN",
    "operation": "encode",
    "key": "LEMON",
    "preserve_case": true,
    "preserve_non_alpha": true
}
Ready