# What are the structure of DES in information security?

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Data Encryption Standard (DES) is a block cipher algorithm that creates plain text in blocks of 64 bits and transform them to ciphertext using keys of 48 bits. It is a symmetric key algorithm, which defines that the similar key can be used for encrypting and decrypting information.

The encryption phase is create of two permutations (P-boxes), which it can define initial and final permutations, and 16 Feistel rounds. Each round need a particular 48- bit round key created from the cipher key in accordance with a definite algorithm.

The elements of DES cipher at the encryption site is as follows −

Initial and Final Permutations − Each permutations creates a 64-bit input and permutes them in accordance with a definite rule. It can have shown only some input ports and the corresponding output ports. These permutations are keyless straight permutations that are the opposite of each other.

For instance, in the initial permutation, the 58th bit in the input develop into the first bit in the output. Likewise, in the final permutation, the first bit in the input develop into the 58th bit in the output.

In other words, if the rounds among these two permutations do not continue, and the 58th bit entering the initial permutation is the equal as the 58th bit leaving the final permutation.

Rounds − DES uses 16 rounds. Each round of DES is a Feistel cipher. The round creates LI−1and RI−1from previous round and makes LI and RI, which go to the next round. It can consider that each round has two cipher elements (such as mixer and swapper).

Each of these elements is invertible. The swapper is invertible. It can swaps the left half of the text with the right half. The mixer is invertible due to an XOR operation. All non-invertible elements are collected within the function f(RI−1, KI).

Cipher and Reverse Cipher − By using mixers and swappers, it can create the cipher and reverse cipher, each having 16 rounds. The cipher can be used at the encryption site and the reverse cipher can be used at the decryption site. The whole concept is to create the cipher and the reverse cipher algorithms similar.

In the first method, it can manage this objective, one method is to create the last round (round 16) distinct from the others and it has only a mixer and no swapper. Although the rounds are not horizontal, the elements (mixer or swapper) are horizontal.

In the first approach, round 16 is different from other rounds and there is no swapper in this round. This is required to create the final mixer in the cipher and the first mixer in the opposite cipher regulate. It is used to create all 16 rounds the similar by containing one swapper to the 16th round and insert a more swapper after that (two swappers eliminate the result of each other).