The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST). DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm.

Here is a step-by-step guide to a single round of the DES algorithm:

1. **Initial Permutation:** The first step of the DES algorithm is to perform an initial permutation on the plaintext block. This permutation is simply a rearrangement of the bits, and it does not depend on the key. The output of this step is referred to as the "pre-round block."

2. **Splitting:** The pre-round block is split into two halves, each of 32 bits. These halves are referred to as the "left half" and the "right half."

3. **Expansion:** The right half of the block is expanded from 32 bits to 48 bits using an expansion table. This is done to match the size of the sub-key that will be used in this round.

4. **Key Mixing:** The expanded right half is combined with the round key using the XOR operation. The round key is a 48-bit key derived from the original encryption key using a key schedule.

5. **Substitution:** The result of the key mixing is then passed through 8 substitution boxes, or S-boxes. Each S-box takes 6 bits as input and produces 4 bits as output. The S-boxes provide the primary security of DES, as they are non-linear and their output is not a simple function of the input.

6. **Permutation:** The output of the S-boxes is then permuted according to a fixed permutation table.

7. **Combining:** The result of the permutation is then combined with the left half of the pre-round block using the XOR operation. The output of this step is the new right half of the block.

8. **Swapping:** The original right half of the pre-round block becomes the new left half, and the output of the combining step becomes the new right half. This completes one round of the DES algorithm.

After 16 rounds, a final permutation is performed on the block, and the result is the ciphertext block. The final permutation is the inverse of the initial permutation.

Question

The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST). DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm.

Here is a step-by-step guide to a single round of the DES algorithm:

1. **Initial Permutation:** The first step of the DES algorithm is to perform an initial permutation on the plaintext block. This permutation is simply a rearrangement of the bits, and it does not depend on the key. The output of this step is referred to as the "pre-round block."

2. **Splitting:** The pre-round block is split into two halves, each of 32 bits. These halves are referred to as the "left half" and the "right half."

3. **Expansion:** The right half of the block is expanded from 32 bits to 48 bits using an expansion table. This is done to match the size of the sub-key that will be used in this round.

4. **Key Mixing:** The expanded right half is combined with the round key using the XOR operation. The round key is a 48-bit key derived from the original encryption key using a key schedule.

5. **Substitution:** The result of the key mixing is then passed through 8 substitution boxes, or S-boxes. Each S-box takes 6 bits as input and produces 4 bits as output. The S-boxes provide the primary security of DES, as they are non-linear and their output is not a simple function of the input.

6. **Permutation:** The output of the S-boxes is then permuted according to a fixed permutation table.

7. **Combining:** The result of the permutation is then combined with the left half of the pre-round block using the XOR operation. The output of this step is the new right half of the block.

8. **Swapping:** The original right half of the pre-round block becomes the new left half, and the output of the combining step becomes the new right half. This completes one round of the DES algorithm.

After 16 rounds, a final permutation is performed on the block, and the result is the ciphertext block. The final permutation is the inverse of the initial permutation.

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