# With the help of block diagram explain DES?

• Issued in 1977 by the National Bureau of Standards (now NIST) as Federal Information Processing Standard (FIPS) 46
• Was the most widely used encryption scheme until the introduction of the Advanced Encryption Standard (AES) in 2001
• Algorithm itself is referred to as the Data Encryption Algorithm (DEA)
• Data are encrypted in 64-bit blocks using a 56-bit key
• The algorithm transforms 64-bit input in a series of steps into a 64-bit output
• The same steps, with the same key, are used to reverse the encryption • there are two inputs to the encryption function: the plaintext to be encrypted and the key. In this case, the plaintext must be 64 bits in length and the key is 56 bits in length.

The processing of the plaintext proceeds in three phases

• First, the 64-bit plaintext passes through an initial permutation (IP) that rearranges the bits.
• This is followed by a phase consisting of sixteen rounds of the same function, which involves both permutation and substitution functions. The output of the last (sixteenth) round consists of 64 bits that are a function of the input plaintext and the key. The left and right halves of the output are swapped to produce the preoutput.
• Finally, the preoutput is passed through a final permutation [IP -1 ] that is the inverse of the initial permutation function, to produce the 64-bit ciphertext.
• With the exception of the initial and final permutations, DES has the exact structure of a Feistel cipher.
• The right-hand portion of Figure shows the way in which the 56-bit key is used. Initially, the key is passed through a permutation function. Then, for each of the sixteen rounds, a subkey(Ki ) is produced by the combination of a left circular shift and a permutation. The permutation function is the same for each round, but a different subkey is produced because of the repeated shifts of the key bits.
• As in Feistel cipher, decryption uses the same algorithm as encryption, except that the application of the subkeys is reversed. Additionally, the initial and final permutations are reversed. 