Advanced Vigenère Key Length Analysis
📌 Next Step: Use the key length analysis above to begin frequency analysis. Select a key length below to begin frequency analysis.
English Letter Frequency vs Your Decrypted Text
Compare to validate if decryption is correct. Good decryptions match English patterns.
The tool scans your ciphertext for repeated patterns (trigrams or longer). In Vigenère encryption, the same plaintext encrypted with the same portion of the key produces identical ciphertext.
For each repeated sequence, we measure the distance (in characters) between occurrences. These distances are always multiples of the key length.
We find all factors of each distance. The key length will be a common factor across many distances. Factors that appear frequently are strong candidates.
We test each probable key length by calculating IC values. Correct key lengths produce IC values close to 0.065 (English plaintext), while incorrect lengths stay near 0.038 (random text).
Once the key length is determined, split the ciphertext into groups (one for each key position). Each group is a simple Caesar cipher that can be solved with frequency analysis.
The Kasiski examination was published by Friedrich Kasiski in 1863, though Charles Babbage discovered it independently around 1854. This method broke the supposedly "unbreakable" Vigenère cipher that had been in use for centuries.
Before Kasiski, the Vigenère cipher was considered secure because it resisted simple frequency analysis. Kasiski's insight was recognizing that repeated sequences in ciphertext revealed the key length, reducing the problem to multiple simple substitution ciphers.
The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to encrypt text. Each letter of the keyword determines a Caesar shift for the corresponding plaintext letter. The keyword repeats to match the plaintext length.
Very accurate for texts over 200 characters with a reasonable key length (5-15 letters). Short texts or very long keys may produce ambiguous results. The IC analysis helps validate findings.
This usually means: (1) the text is too short, (2) the key is very long, or (3) it's not a Vigenère cipher. Try with longer ciphertext or consider other cryptanalysis methods.
No. The Vigenère cipher is a classical cipher from the 1500s. Modern encryption (AES, RSA) is mathematically secure against all known attacks. This tool is for educational purposes and historical cipher analysis only.
Kasiski examination uses repeated sequences to find the key length. The Index of Coincidence (IC) validates those findings by testing whether splitting the text at that length produces monoalphabetic-looking distributions. They complement each other.