DES Implementation in Logisim Evolution

This README provides an overview of the Data Encryption Standard (DES) algorithm, its key schedule and encryption rounds.

1. Introduction

2. DES Structure Overview

3. Key Scheduling

   • PC-1 Permutation

   • C/D Shifts

   • PC-2 Permutation

4. Encryption Rounds (Feistel Network)

5. References

---

Introduction

The Data Encryption Standard (DES) is a symmetric-key algorithm for the encryption of digital data. Developed in the 1970s, DES became a widely adopted standard for secure communications. Although superseded by more modern algorithms (like AES), DES remains an excellent educational tool to learn about block ciphers and Feistel networks.

---

DES Structure Overview

• Block size: 64 bits

• Key size: 64 bits (56 bits used; 8 bits for parity)

• Rounds: 16 Feistel rounds

At a high level, DES encryption consists of:

1. Initial Permutation (IP)

2. 16 rounds of the Feistel function

3. Final Permutation (IP-1)

---

Key Scheduling

The DES key schedule generates 16 round keys (48 bits each) from the original 64-bit key.

PC-1 Permutation

• Input: 64-bit key (including parity bits)

• Output: 56-bit key (parity bits dropped, bits permuted)

C/D Shifts

• Split the 56-bit result into two 28-bit halves: C and D.

• Perform circular left shifts on each half according to the DES shift schedule:

Round	Shifts
1, 2, 9, 16	1 bit
All others	2 bits

PC-2 Permutation

• Input: 56-bit concatenation of shifted C/D

• Output: 48-bit round key

---

Logisim key schedule diagram

Encryption Rounds (Feistel Network)

Each round applies the following steps:

1. Expand the right half (32 → 48 bits)

2. XOR with the round key

3. Substitute via 8 S-boxes (48 → 32 bits)

4. Permutate (P-box)

5. Swap halves (except after final round)

---

References

• FIPS PUB 46-3: Data Encryption Standard

• Schneier, B. (1996). Applied Cryptography

• Data Encryption Standard (DES): Encryption by Christof Paar

• Data Encryption Standard (DES): Key Schedule and Decryption by Christof Paar

---