| Attack Type | Required Input | Success Rate | Complexity | |-------------|----------------|--------------|-------------| | Known-plaintext | ≥1 full message + matching ciphertext | >95% | Low (O(n)) | | Ciphertext-only | ≥2 messages with same initial key | ~70% (short texts) | Medium (O(n²)) | | Key-stream reuse | ≥2 ciphertexts, unknown plaintext | ~85% (English text) | Medium |

The Autokey cipher, a polyalphabetic substitution cipher, represents a significant evolutionary step in classical cryptography. It was designed to overcome the primary weakness of the standard Vigenère cipher: the periodic repetition of the key. While the Autokey successfully mitigates simple frequency analysis based on key length, it introduces a structural flaw that renders it vulnerable to a specific, elegant form of cryptanalysis: the .

Extract key stream by subtracting plaintext from ciphertext. First L bytes = initial key.

For the first $N$ characters: $$C_i = (P_i + K_i) \pmod26$$

Antares Auto-Key is a popular plugin that analyzes audio in real-time to identify its musical key and BPM. It is designed to work seamlessly with , automatically syncing key information across multiple instances to speed up the vocal tuning process. The Risks of "Crack Only" Downloads