What is a brute-force search and what is its cryptographic relevance?
In a nutshell: If f(x) = y and you know y and can compute f, you can find x by trying every possible x. That’s brute-force search. Example: Say a cryptanalyst has found a plaintext and a corresponding ciphertext, but doesn’t know the key. He can simply try encrypting the plaintext using each possible key, until the ciphertext matches—or decrypting the ciphertext to match the plaintext, whichever is faster. Every well-designed cryptosystem has such a large key space that this brute-force search is impractical. Advances in technology sometimes change what is considered practical. For example, DES, which has been in use for over 30 years now, has 2^56, or about 10^17, possible keys. A computation with this many operations was certainly unlikely for most users in the mid-70’s, though some believe that the key length was shosen because NSA could break DES by brute force using its compute farms. The situation is very different today given the dramatic decrease in cost per processor operati
