What is the term for an attack that tries every possible key?

• A. Brute-force attack
• B. Collision attack
• C. Birthday attack
• D. Chosen-plaintext attack

Answer: (A)

Brute-force attack is a straightforward key-search approach: the attacker systematically tries every possible key until the correct one is found that decrypts the ciphertext or produces the expected result. With an n-bit key, there are 2^n possible keys, so on average about half the keyspace must be tested before success, though the worst case is the full space. The practicality of this method depends on key length and computational power; longer keys make the search space grow exponentially and quickly become infeasible with current technology, which is why modern systems use 128-bit or 256-bit keys. For context, other listed attacks don’t aim to discover the key through exhaustive guessing—they target other weaknesses, such as hash collisions or plaintext-ciphertext relationships, rather than a complete key search.