DES Design and the Lucifer Cipher

In 1971, Horst Feistel at IBM Research developed a cipher called Lucifer as part of a banking security project for Lloyd's of London. Lucifer was one of the first modern block ciphers, using a 128-bit key and 128-bit blocks.

Lucifer demonstrated that a strong block cipher could be built using the Feistel structure: alternating rounds of substitution and permutation applied to half-blocks. It was the direct ancestor of DES.

DES applies 16 rounds of the Feistel structure to 64-bit blocks. In each round, the 64-bit block is split into 32-bit halves. The right half is expanded to 48 bits, XORed with a 48-bit subkey, passed through S-boxes, permuted, and XORed with the left half.

The halves are then swapped and the process repeats. After 16 rounds, a final permutation produces the 64-bit ciphertext. Decryption runs the same structure with subkeys in reverse order.