How does the key schedule in DES generate the subkeys used in each round of encryption and decryption?
The Data Encryption Standard (DES) is a symmetric block cipher cryptosystem that operates on 64-bit blocks of data. The key schedule in DES is responsible for generating the subkeys used in each round of encryption and decryption. The key schedule takes the original 64-bit key and produces 16 round subkeys, each consisting of 48 bits.
The key schedule begins with a process called key permutation, where the 64-bit key is permuted according to a fixed table known as PC-1. This permutation rearranges the bits of the key to produce a 56-bit intermediate key. The purpose of this permutation is to remove parity bits and to mix the key bits to increase their diffusion throughout the subsequent key schedule process.
After the key permutation, the 56-bit intermediate key is split into two 28-bit halves, known as C0 and D0. Each half is then subjected to a series of left circular shifts. In each round, the number of shifts is determined by a predefined schedule known as the key rotation schedule. The key rotation schedule specifies the number of shifts for each of the 16 rounds.
After the left circular shifts, the two halves, C0 and D0, are combined to form a 56-bit key, which is then subjected to another permutation known as PC-2. This permutation maps the 56-bit key onto a 48-bit round subkey. The PC-2 permutation is similar to the PC-1 permutation, but it produces a different mapping.
The process of generating the round subkeys is repeated for each round of encryption and decryption. The only difference is that the key rotation schedule is applied in reverse order for decryption, starting with the last round and working back to the first round.
To illustrate the key schedule process, let's consider an example. Suppose we have an original 64-bit key:
0110111001101010010101100111101100101000011100100110111001110
After the key permutation (PC-1), we obtain the following 56-bit intermediate key:
11110000110011001010101011110101010101100110011110001111
Next, we split the intermediate key into two 28-bit halves, C0 and D0:
C0: 1111000011001100101010101111
D0: 0101010101100110011110001111
Applying the key rotation schedule, we perform left circular shifts on C0 and D0 to obtain C1 and D1:
C1: 1110000110011001010101011111
D1: 1010101011001100111100011110
After combining C1 and D1, we obtain a 56-bit key:
111000011001100101010101111110101010110011001100111100011110
Finally, applying the PC-2 permutation, we obtain the first round subkey:
110011000000110010101010111111101010101100110011
This process is repeated for each round, with the key rotation schedule determining the number of left circular shifts for each round. The resulting subkeys are used in the encryption and decryption rounds of DES.
The key schedule in DES generates the subkeys used in each round of encryption and decryption through a process of key permutation, left circular shifts, and another permutation. The key rotation schedule determines the number of shifts applied to the key halves. The resulting subkeys provide the necessary variation and diffusion of the key throughout the encryption and decryption process.
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