Data encryption standard, or DES, is the most widely used block cipher. In 1973, the US National Bureau of Standards, or NBS, which later became the National Institute of Standards and Technology, NIST, issued the request for proposal for a national cipher standard. As a result, IBM's design got accepted and got adopted as the data encryption standard in 1977. Let's revisit the Feistel Cipher and build on that to describe DES. As discussed previously, Feistel Cipher supports a product cipher and incorporates both substitution and permutation. Feistel Cipher is a structure framework for blocked ciphers and DES is the specific implementation based on the structure. As discussed earlier, here are some of the main design parameters for a Feistel Cipher. DES parameters are shown here, the block length is 64 bits. So the left half and the right half of the data is 32 bits each. The key length is 56 bits, the number of rounds is 16. As for the key length of 56 bits, the precursor of DES is the Lucifer algorithm and actually had 128 bits key, which is longer than 56 bits and can have better security. However, IBM, IBM, the inventor of Lucifer and DES, wanted a more marketable algorithm that could fit on a single chip, and the result was 56 bits key long for DES. We will discuss more about the subkey generation algorithm and the round function design later in this module. Now that we've established some of the parameter values, let's build on the Feistel Cipher to describe DES. The figure is the same figure that we use when describing Feistel Cipher. Let's focus on the encryption process, because the decryption process is essentially the same, but with different inputs. As discussed previously, DES has 16 rounds, so that n is equal to 16. The Feistel Cipher had this in as a design parameter variable. DES adds additional permutation steps at the beginning and at the end. These permutations are appropriately called Initial Permutation and Final Permutation. The block length is 64 bits and this length of 64 bits travels across the rounds. Because it is based on Feistel Cipher, the block processes the data in halves after splitting it to the left half and the right half. The subkeys for each round are 48 bits, these will be different bits for each round. We previously mentioned that DES uses a 56 bit key, we will later describe how the subkey generation works. And how it takes 56 bit long key and outputs 16 times 48 bits long subkeys. Also, from the figure, we see that the round function, F, within each round takes the right half of the block data, which is 32 bits as an input. The round function, F, then outputs 32 bits, which afterward gets exhorted with the left half of the block data.