Theory

• The Data Encryption Standard (DES) is a symmetric key block cipher developed by IBM and adopted by the U.S. National Institute of Standards and Technology (NIST) in 1977. It works on 64-bit blocks of plaintext and uses a 48-bit effective key derived from DES key generation process.
• The DES follows a Feistel network structure consisting of 16 rounds, where each round applies substitution and permutation operations to perform confusion and diffusion.
• The encryption process begins with an Initial Permutation (IP), followed by splitting the block into left and right halves. In each round, the right half is expanded, XORed with a round-specific subkey generated from the main key, and passed through S-boxes, which provide nonlinearity. The output is then permuted and XORed with the left half to form the new right half. After 16 rounds, the halves are swapped and passed through a Final Permutation (FP) to produce the ciphertext.