The simplest means to “brute-force” a key using a sample of both the encrypted message and the original plaintext. A known-plaintext attack is a cryptographic attack in which an individual has the plaintext and its encrypted version (ciphertext), thereby allowing him or her to use both to reveal further secret information—such as the secret key. Encrypted archived ZIP files are said to be prone to known-plaintext attacks because using software available on the Internet, crackers are able to determine the key needed to decrypt the archived files.
GNU_FDL. Known-Plaintext Attack. [Online, 2004.] GNU Free Documentation License Website: http://www.wordiq.com/definition/Known-plaintext_attack.