There is no easy answer to this question since it depends on many distinct factors. Not only must the work and computational resources required by the cryptanalyst be reasonable, but the amount and type of data required for the attack to be successful must also be taken into account.

One classification distinguishes among cryptanalytic attacks according to the data they require in the following way: chosen plaintext or chosen ciphertext, known plaintext, and ciphertext-only. (This classification is not particular to secret-key ciphers and can be applied to cryptanalytic attacks on any cryptographic function.)