Contrary to popular belief, in Cryptography, breaking a cipher is not the process of finding plaintext relating to a ciphertext. Breaking a cipher (Cryptanalysis) is the process of finding the Key used to encrypt a data. Once a Key is found, many instances of data can be decrypted. That's why in modern ciphers, even if you know the plaintext (or even select it), you should not be able to find the Key.