After breaking AES, in this Tutorial we would break its precursor: DES (Data Encryption Standard). It was redeemed on October 2000 by AES.

In this example the key length is 8 Byte which equates 64 bit. It must be pointed out, that the algorithm only consider the First-Round-Key. That means 48 relevant bits (8 S-Boxes x 6 Bits) but the original Key has 56 significant bits. The remaining bits to 64 can be neglect because there are just parity bits.

If the 48 bits were found by the CPA there are just 8 bits left which are important for the original key. This 8 bits means 2^8=256 prospects so you can brute-force them for example. To reconstruct the original key you have to consider that the CPA-encrypted key with 48 bits with 2x28Bit blocks based on a permutation which equates the actual 56 bit key.