Experiment 3 - DES Decryption
Theory
The DES decryption process involves reversing the steps of the encryption process. Let's break down the key components and steps involved:
Initial Permutation (IP) The process begins with the 'Initial Permutation,' where the bits of the ciphertext are rearranged. This step is the reverse of the 'Final Permutation' in encryption, serving to set the stage for the subsequent rounds.
Key Schedule The key schedule is precomputed and involves generating the 16 subkeys required for each round of decryption. These subkeys are derived from the original encryption key using the same key generation algorithm used in encryption.
16 Rounds of Decryption Each round of decryption involves the following steps:
a. Expansion Permutation The 'Expansion' permutation is applied to the right half of the data to expand it, similar to the encryption process. This expanded half is then XORed with the subkey for the current round.
b. S-Box Substitution The result is divided into 6-bit blocks, each of which is substituted using the S-boxes. The S-box substitution introduces non-linearity into the process, enhancing security.
c. Permutation After substitution, a 'Permutation' is applied to rearrange the bits. This step is the inverse of the 'Straight Permutation' used in encryption.
d. XOR with Left Half The result of the permutation is XORed with the left half of the data.
e. Swap Halves The left and right halves are swapped, preparing for the next round.
- Final Permutation After completing the 16 rounds, a 'Final Permutation' is applied to the combined data (left and right halves). This permutation is the inverse of the 'Initial Permutation' and produces the final decrypted output.
Conclusion The DES decryption process, like encryption, involves a series of carefully orchestrated steps. Understanding the intricacies of the 'Expansion,' 'S-Box' substitution, and permutation operations is crucial for grasping the inner workings of DES decryption.
It's worth noting that while DES was widely used, its 56-bit key length is now considered insecure against modern computing capabilities. Advanced encryption standards like AES with longer key lengths are recommended for secure communication.