Cryptanalysis
The study of breaking ciphers without the key — cryptography's less polite twin.
The oldest surviving technique is frequency analysis, described by the Arab scholar al-Kindi in the 9th century. The field has shadowed ciphers ever since: Kasiski and Friedman took apart Vigenère, Polish and British mathematicians dismantled Enigma, and in the 1990s differential and linear cryptanalysis gave researchers real leverage against modern block ciphers such as DES.
Attacks are classified by what the attacker gets to see: ciphertext only, known plaintext or chosen plaintext. A cipher counts as broken the moment someone finds a route faster than brute force, even if it still takes 2120 operations. Cryptographers call that an academic break; everyone else should read it as a warning.