How attack become realistic

Question

At what point does an attack become realistic?

Answer 1

There is no simple answer to this question as it based on numerous distinct factors. Not only must the work and computational resources needed by the cryptanalyst be reasonable, however the amount and kind of data needed for the attack to be successful should as well be taken into account.

One categorization differentiates among cryptanalytic attacks according to the data they need in the given way: chosen plaintext or chosen cipher text, plaintext and cipher text-only. (This categorization is not specific to secret-key ciphers and can be exerted to cryptanalytic attacks on any cryptographic function.)