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# What are the design issues of DES?

Data Encryption Standard (DES) is a block cipher algorithm that takes plain text in blocks of 64 bits and transform them to ciphertext utilizing keys of 48 bits. It is a symmetric key algorithm. It can define that the same key can be used for encrypting and decrypting data.

The design of DES was invented by IBM in 1994. There are some tests on DES have proved that it satisfies some of the required element as claimed. There are some design issued which are as follows −

**S-Boxes** − S-Boxes is a procedure that accepts the 48-bit input from the XOR operation containing the compressed key and expanded RPT and makes a 32-bit output using the substitution technique. The substitution is implemented by the eight substitution boxes (also known as the S-boxes).

There are several properties of S-Boxes are as follows −

The entries of each row are permutations of values among 0 and 15.

S-boxes are non-linear. In other terms, the output is not an affine transformation of the input.

If it can change an individual bit in the input, two or more bits will be transformed in the output.

If two inputs to an S-box differ only in two middle bits (bits 3 and 4), the output should differ in partly two bits. In other words, S(x) and S(x ⊕ 001100) must differ in partly two bits where x is the input and S(x) is the output.

If two inputs to an S-box differ in the first two bits (bits 1 and 2) and are the similar in the last two bits (5 and 6), the two outputs should be different. In other words, it is required to have the following relation S(x) ≠ S(x ⊕ 11bc00), in which b and c are arbitrary bits.

**D-Boxes** − A D-box is a permutation box having similar features as traditional transposition ciphers. There are the following properties of D-Boxes are as follows −

Each S-box input appears from the output of a different S-box (in the earlier round).

No input to a given S-box appears from the output from the similar box (in the earlier round).

The four outputs from each S-box go to 6 distinct S-boxes (in the later round).

No two output bits from an S-box go to the same S-box (in the next round).

For each S-box, the two output bits go to the first or last two bits of an S-box in the next round. The another two output bits show up the middle bits of an S-box in the later round.

**Number of Rounds** − DES uses sixteen rounds of Feistel ciphers. It has been demonstrated that after eight rounds, each ciphertext is a function of every plaintext bit and every key bit; the ciphertext is intensely a random function of plaintext and ciphertext. Thus, it looks like eight rounds should be adequate.

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