Exhaustive key search, or brute-force search, is the basic technique of trying every possible key in turn until the correct key is identified. To identify the correct key it may be necessary to possess a plaintext and its corresponding ciphertext, or if the plaintext has some recognizable characteristic, ciphertext alone might suffice. Exhaustive key search can be mounted on any cipher and sometimes a weakness in the key schedule of the cipher can help improve the efficiency of an exhaustive key search attack.
Advances in technology and computing performance will always make exhaustive key search an increasingly practical attack against keys of a fixed length. When DES was designed, it was generally considered secure against exhaustive key search without a vast financial investment in hardware. Over the years, this line of attack will become increasingly attractive to a potential adversary.