Cryptography Questions Long
The Data Encryption Standard (DES) is a symmetric key algorithm that was developed in the 1970s by IBM and later adopted by the National Institute of Standards and Technology (NIST) as a federal standard for encryption. DES operates on blocks of data, typically 64 bits in length, and uses a 56-bit key for encryption and decryption.
The working principle of DES involves several steps:
1. Key Generation: The 56-bit key is generated by taking an input key of 64 bits and applying a parity drop, which discards the least significant bit of each byte. This results in a key with 56 effective bits, while the remaining 8 bits are used for error detection.
2. Initial Permutation (IP): The input plaintext block is subjected to an initial permutation, which rearranges the bits according to a predefined permutation table. This step is performed to provide diffusion and confusion in the subsequent rounds.
3. Feistel Structure: DES employs a Feistel structure, which divides the plaintext block into two halves, left and right. The right half is expanded to 48 bits using an expansion permutation table, and then XORed with a round key derived from the main key.
4. Substitution (S-Box): The XORed result is then divided into eight 6-bit blocks, which are substituted using eight S-boxes. Each S-box takes a 6-bit input and produces a 4-bit output based on a predefined substitution table. This substitution step introduces non-linearity and further confuses the relationship between the plaintext and the ciphertext.
5. Permutation (P-Box): After the substitution step, the 32-bit output from the S-boxes is subjected to a permutation using a fixed permutation table known as the P-box. This permutation provides additional diffusion and confusion.
6. Rounds: The above steps (Feistel structure, S-box substitution, and P-box permutation) are repeated for a total of 16 rounds, with each round using a different round key derived from the main key. The round keys are generated by applying a key schedule algorithm that involves shifting and permuting the bits of the main key.
7. Final Permutation (FP): After the 16 rounds, the left and right halves of the output are swapped, and the resulting block is subjected to a final permutation, which is the inverse of the initial permutation. This final permutation ensures that the decryption process is the reverse of the encryption process.
The working principle of DES relies on the combination of these steps to provide both confusion and diffusion, making it resistant to various cryptographic attacks. However, due to advances in computing power, DES is now considered relatively weak and has been replaced by more secure algorithms such as the Advanced Encryption Standard (AES).