Vigenère uses a keyword to apply different shifts to each letter, making it a polyalphabetic cipher. This resists simple frequency analysis because the same plaintext letter can encrypt to different ciphertext letters depending on position.

Autokey mode uses the plaintext itself to extend the keyword. After the keyword is exhausted, the plaintext characters are used as the key for subsequent positions. This prevents the repeating pattern that makes standard Vigenère vulnerable to Kasiski examination.

IC measures how likely two randomly selected letters are identical. For English text, IC ≈ 0.0667. For random text, IC ≈ 0.0385. A Vigenère-encrypted text with a short keyword will have IC closer to 0.0385, while a value above 0.045 suggests the text may be monoalphabetic or the keyword is very long.

Yes. The analysis uses the browser's native Intl API for language-aware text segmentation. Word and character counting works correctly for most languages including Chinese, Japanese, Arabic, and accented Latin scripts.

Documents up to approximately 100,000 words are supported. Analysis is incremental, meaning results update instantly even for large texts without blocking the browser.