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Encrypt or decrypt text using the classic polyalphabetic substitution algorithm. Ideal for cryptography learning and testing.
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Overview
Understand what the tool solves, how it works, and the boundaries of its data.
The classic Vigenère cipher shifts each English letter according to a repeating key. For the A–Z alphabet, assign A=0 through Z=25. If a plaintext letter has value P and its aligned key letter has value K, encryption is C = (P + K) mod 26; decryption is P = (C − K) mod 26. The key starts again after its last letter.
Encrypt: C = (P + K) mod 26
Decrypt: P = (C − K) mod 26
With the key KEY, the message “Hello” becomes “Rijvs”: H shifts by K, e by E, and so on. The tool preserves letter case and leaves spaces, punctuation, digits, and non-English characters as entered. Those unchanged characters do not advance the key; only A–Z letters use a key position.
Choose Encrypt for plaintext or Decrypt for ciphertext when you already know the keyword. The key field accepts text, but the calculation uses only its A–Z letters, case-insensitively. For instance, “Key!” supplies the same shifts as “KEY.” If the key contains no English letters, there is no effective shift and the input is returned unchanged.
The output updates as you change the text, key, or mode. Copy the result when you are ready to reuse it. To reverse an encryption, use the same keyword and select Decrypt; a wrong key or a different handling convention will produce different text.
Guide
Follow the workflow and verify inputs and outputs with practical examples.
Select Encrypt for a message you are transforming, or Decrypt for ciphertext whose keyword you already know.
Type the key in the key field. Only English letters A through Z contribute; punctuation and other characters in the key are ignored.
Letters are shifted in sequence. Spaces, punctuation, digits, and characters outside A–Z remain unchanged and do not consume a key letter.
Check the output, then copy it. For a round-trip check, use the same key and switch from Encrypt to Decrypt.
Example: with KEY and Encrypt, “Hello World!” becomes “Rijvs Uyvjn!”; switching to Decrypt with the same key restores the original text.
Use cases
See how the tool fits into real work and everyday tasks.
Use a known keyword to compare letter-by-letter shifts with a hand-worked Vigenère table, then decrypt the result with the same key.
If a puzzle provides both the cipher text and its keyword, test the decryption and retain its punctuation and spacing while evaluating the recovered message.
Q&A
Find concise answers to common questions and confusing cases.
No. It applies a keyword you provide; it does not analyze ciphertext or recover a key.
The shifting alphabet is A–Z only. Characters outside English A–Z pass through unchanged and do not advance the repeating key.
Check that Decrypt is selected, the key is correct, and the original text used the same alphabet and convention. This tool’s key alignment advances on English letters only.
Notes
Review scope, result limitations, and important precautions before use.
The Vigenère cipher is a classical educational cipher and should not be used to protect sensitive information. This page does not provide cryptanalysis, key recovery, or modern encryption. The mathematical rule described here applies to the classic repeating A–Z keyword behavior.
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