Biham–Kocher attack
A known-plaintext attack that breaks the legacy password encryption of ZIP archives with about a dozen known bytes.
Eli Biham and Paul Kocher published it in 1994. PKZIP's stream cipher, now called ZipCrypto, keeps its state in three 32-bit words updated with CRC-32 and a linear congruential step. Given roughly twelve or thirteen bytes of known plaintext, the entire internal state can be recovered. The password itself is never needed: the state is enough to decrypt every file in the archive encrypted under the same password.
Known plaintext is rarely hard to come by. File headers have fixed bytes, and archives often contain a file available unencrypted elsewhere. The tools pkcrack and later bkcrack turn the attack into a matter of minutes. That ZipCrypto remains the default in several archivers says more about backward compatibility than about cryptography.