Vigenere cipher solver with automatic key recovery

Vigenere applies a different Caesar shift to each position, cycling through a keyword. That defeats plain frequency analysis, because E no longer maps to a single letter. It does not defeat frequency analysis applied per key position, and that two-step insight is the entire attack.

Step one: find the key length

The index of coincidence measures how likely two randomly chosen letters of a text are to be identical. English sits near 0.067; uniformly random text sits near 0.038. Slice the ciphertext into every Nth character for N = 1, 2, 3, ... and compute the IoC of each slice. When N equals the key length, every slice is a plain Caesar shift of English and the IoC jumps back toward 0.067. That spike is your period.

Step two: solve each column

Once the period is known, character positions 0, 4, 8, ... all share one shift. That slice is a Caesar cipher, so chi-squared gives its shift directly, and the winning shift for each column is the corresponding letter of the key. Concatenate the columns and you have recovered the keyword without ever guessing it.

Both steps degrade with short ciphertext. Below roughly 100 characters the IoC is noisy and the per-column statistics have too few samples. When automatic recovery fails on a short text, fall back to a crib: if you know the plaintext contains picoCTF, XOR-style crib dragging recovers key fragments directly.