How do modern cryptographers defend against brute-force attacks?

How do modern cryptographers defend against brute-force attacks?

• Use an algorithm that requires the attacker to have both ciphertext and plaintext to conduct a successful attack.
• Use a keyspace large enough that it takes too much money and too much time to conduct a successful attack.
• Use frequency analysis to ensure that the most popular letters used in the language are not used in the cipher message.
• Use statistical analysis to eliminate the most common encryption keys.
  

  Answers Explanation & Hints: In a brute-force attack, an attacker tries every possible key with the decryption algorithm knowing that eventually one of them will work. To defend against the brute-force attacks, modern cryptographers have as an objective to have a keyspace (a set of all possible keys) large enough so that it takes too much money and too much time to accomplish a brute-force attack. A security policy requiring passwords to be changed in a predefined interval further defend against the brute-force attacks. The idea is that passwords will have been changed before an attacker exhausts the keyspace.