Understanding DES Encryption and Decryption

What is DES?

Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST). It was once the standard encryption algorithm for securing electronic data, though it has since been superseded by more secure algorithms like AES.

The DES Algorithm

DES operates on 64-bit blocks of data, using a 56-bit key (although the key is usually expressed as 64 bits with parity bits). The algorithm can be expressed mathematically as:

\[C = F_K(M) = IP^{-1}(f_{K_{16}}(...f_{K_2}(f_{K_1}(IP(M)))))\]

Where:

• \(C\) is the ciphertext
• \(M\) is the plaintext message
• \(F_K\) is the DES function with key \(K\)
• \(IP\) is the initial permutation
• \(IP^{-1}\) is the final permutation (inverse of IP)
• \(f_{K_i}\) is the round function with subkey \(K_i\)

Calculation Steps

1. Key Schedule: Generate 16 48-bit subkeys from the original 56-bit key.
2. Initial Permutation: Rearrange the bits of the input block.
3. 16 Rounds of Feistel Network:
   • Split the block into left and right halves
   • Apply the round function to the right half and XOR with the left half
   • Swap the halves
4. Final Permutation: Inverse of the initial permutation.

Example

Let's consider encrypting the message "Hello" with the key "secret":

1. Convert "Hello" to binary: 0100100001100101011011000110110001101111
2. Pad to 64 bits: 0100100001100101011011000110110001101111000000000000000000000000
3. Apply Initial Permutation
4. Perform 16 rounds of the Feistel network
5. Apply Final Permutation
6. Result (in hex): 7AA38A029E73A0E7

Visual Representation

This diagram illustrates the high-level process of DES encryption, showing how the plaintext is transformed through the various stages of the algorithm, influenced by the key, to produce the final ciphertext.