Does there exist a system wherein the encryption key/algorithm can be made indecipherable despite making arbitrarily many plaintext and ciphertext pairs available to the cracker? (Does this qualify as KPA attack security? ) If so, how strong is such a system, what kind of key and algorithms should be used?
What minimum size should the keyspace be to ensure recovering key from the known plaintext-ciphertext pairs is not efficient? Are there any known attacks which can recover key faster than a brute-force attack?