What the tool does, how to run it, and what to expect from the result.
The Vigenère cipher shifts each letter by the matching letter of a repeating keyword: with the key LEMON, the first letter shifts by 11 (L), the next by 4 (E), and so on. "ATTACK AT DAWN" with LEMON becomes "LXFOPV EF RNHR". It resisted attack for three centuries and was called "le chiffre indéchiffrable" until Kasiski published a general method to break it in 1863.
Short, plain-language definitions for the formats and settings above.
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Anyone who needs this done quickly, privately and free.
Using the vigenère cipher without installing anything or signing up.
Short answers for the questions people usually have before trying a utility like this.
Write the keyword repeatedly under the message and shift each letter by its key letter (A = 0, B = 1 ... Z = 25). Decoding shifts back by the same amounts.
Not here: the key advances only on letters, the usual convention, so spaces and punctuation pass through without shifting the key.
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Encode or decode as you type. Case, spaces and punctuation are kept.