Tight Security Bounds for Triple Encryption

A blockcipher is said to be secure if there is no known attack faster than exhaustive key search. On the other hand, without utilizing any weakness of a blockcipher, one can recover its secret key simply by trying all possible keys over a small number of plaintext-ciphertext pairs. So the key length of a blockcipher can be viewed as the maximum level of security that the blockcipher is able to provide. However the key length providing a sufficient level of security might change over time.