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# What is the most security-critical component of DES round function?

There are various component of DES which are as follows −

**Key Transformation** − An initial 64-bit key is changed into a 56-bit key by discarding each 8^{th} bit of the initial key. Therefore for each round, a 56 bit key is applicable. From this 56-bit key, there are several 48-bit subkey is produced during every round utilizing a procedure is known as key transformation. The 56-bit key is breaked into two halves, each 28 bits.

Round | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Number of key bits shifted | 1 | 1 | 2 | 2 | 2 | 2 | 2 | 2 | 1 | 2 | 2 | 2 | 2 | 2 | 2 | 1 |

No of key bits shifted per round

In key transformation procedure, it includes permutation and selection of a 48-bit key from original 56-bit key it is known as compression permutation. In compression permutation technique, there are multiple subset of key bits is utilized in each round. This creates DES not simply to crack.

**Expansion Permutation** − After initial permutation, it had two 32-bit plaintext areas, known as Left Plain Text and Right Plain Text. During expansion permutation, the RPT is diffuse from 32 bits to 48 bits. It can enhancing the bit size from 32 to 48, the bits are permuted as well known as the expansion permutation.

As it can see firstly the key transformation process compresses the 56-bit key to 48- bits. Therefore, the expansion permutation process develop the 32-bit RPT to 48-bits.

**S-box Substitution** − S-box substitution is a procedure that accepts the 48-bit input from the XOR operation containing the compressed key and expanded RPT, and develops a 32-bit output utilizing the substitution technique.

The substitution is implemented by the eight substitution boxes (also known as the S-boxes). Each of the eight S-boxes has a 6-bit input and a 4-bit output. The 48-bit input block is divided into 8 sub-blocks (each including 6 bits), and each sub-blocks is provided to an S-box.

**P-box Permutation** − The output of the S-box includes 32 bits. These 32 bits are permuted using a P-box. This straightforward permutation structure contains simple permutation including replacement of each bit with another bit as defined in the Pbox table, without any expansion or compression. This is known as P-box Permutation.

**XOR and SWAP** − It can be implementing all these operations only on the 32-bit right half portion of the 64-bit original plaintext (i.e., on the RPT). The left half portion (i.e., LPT) was unaffected so far.

At this juncture, the left half portion of the initial 64-bit plain-text block (i.e., LPT) is XORed with the output created by P-box permutation. The result of this XOR operation develop into the new right half (i.e., RPT). The old right half (i.e., RPT) develop into the new left half in a procedure of swapping.

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