Ciphertext-only attack

In cryptography, a ciphertext-only attack (COA) or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. 
The attack is completely successful if the corresponding plaintexts can be deduced (extracted) or, even better, the key. The ability to obtain any amount of information from the underlying ciphertext is considered a success. 
Every modern cipher attempts to provide protection against ciphertext-only attacks. The process of selecting a new cipher design standard usually takes many years and includes exhaustive testing of large quantities of ciphertext for any statistical different approach from random noise. On the other hand, poor cipher depends on home-grown proprietary algorithms that have not been subjected to extensive testing and inspection; for this reason, many computer-age encryption systems are still subject to ciphertext-only attack but still in use. Examples include: