Encode and decode messages using the Vigenere polyalphabetic cipher with frequency analysis and key-length detection.
The Vigenere cipher is a polyalphabetic substitution. Each plaintext letter is shifted by a letter of the key, and the key repeats across the whole message, so the same plaintext letter encodes differently depending on its position. This tool encodes and decodes with the key you supply, and it can also attack ciphertext when the key is unknown: it estimates the key length with Kasiski's examination and the index of coincidence, then recovers the key by frequency analysis. The surprise for most users is that the cipher offers no real security. It is a historical and educational cipher, fully breakable by the analysis this page runs.
A concrete input and expected output from the current implementation.
Input
plaintext 'meetme', key 'key'
Expected output
WICDQC
Each plaintext letter is shifted by the corresponding repeating key letter (K=10, E=4, Y=24): m+10=W, e+4=I, e+24 wraps past Z to C, t+10=D, m+4=Q, e+24=C. This follows the standard convention where A is a shift of 0; implementations that give A a shift of 1 return different output.
Number the alphabet A=0 through Z=25. Each plaintext letter is added to its key letter's value, wrapping around the alphabet; key letters repeat, K-E-Y-K-E-Y..., across the whole message. Decryption subtracts the key letter instead of adding. Because the shift changes at every position, repeated plaintext letters produce different ciphertext letters, which is what makes the cipher look stronger than it is.
When no key is given, the tool estimates key length. Kasiski's examination finds repeated letter sequences in the ciphertext; distances between repeats are usually multiples of the key length, and the greatest common divisor gives a candidate. The index of coincidence confirms: English text scores about 0.0667, random text about 0.0385. Each key-length column forms a Caesar cipher, and the shift is recovered by matching letter frequencies to English.
Usually one of three things: the suggested key length or key letters are wrong, the key you typed does not match the one used to encrypt, or the tool's letter convention differs from the encoder's (A shifting 0 or 1, spaces counted or skipped). Decrypt a short phrase you know first, like 'meetme' with key 'key', to see which convention this tool uses.
No. With a repeating key it is fully breakable: the analysis here recovers the key and plaintext from ciphertext alone, a weakness Kasiski exposed in 1863 and Friedman formalized with the index of coincidence. Only a one-time pad - a truly random key never reused - resists this, and that is a different cipher with different requirements.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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