In the Data Encryption Standard (DES) algorithm, 16 subkeys are generated through a process known as key scheduling. Here is a brief explanation of how the 16 subkeys are generated:
1. Initial Key Permutation (PC-1): The original 64-bit encryption key is permuted using a fixed permutation table called PC-1. This permutation rearranges the bits of the key to generate a 56-bit key.
2. Key Halving: The 56-bit key is then split into two halves, each containing 28 bits.
3. Key Rotation: Each half of the key is rotated left by a certain number of bits, which varies for each round of key generation. The rotation schedule is predefined and unique for each round.
4. Compression Permutation (PC-2): After the rotation, a 48-bit subkey is generated by selecting 24 bits from each half of the key and permuting them using another fixed permutation table called PC-2.
5. Repeat: Steps 3 and 4 are repeated for a total of 16 rounds, generating 16 different 48-bit subkeys.
These 16 subkeys are then used in the DES algorithm for the 16 rounds of encryption and decryption. Each subkey is used in a specific round to perform a permutation and substitution process on the data being encrypted or decrypted.
It's important to note that the DES algorithm is now considered relatively weak and has been replaced by more secure encryption algorithms such as AES (Advanced Encryption Standard).