DES & 3DES C++ Encryption Library

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Contents

• DES Algorithm Documentation & History
• Project Overview
• Features
• Requirements
• Operation Modes
• Key and IV Generation
• Usage
• Example
• Screenshots
• License

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DES Algorithm Documentation & History

DES Block Cipher Overview

The Data Encryption Standard (DES) is a symmetric-key block cipher that operates on 64-bit blocks of data using a 56-bit key (plus 8 parity bits). Developed by IBM in the 1970s and adopted as a U.S. federal standard in 1977, DES was the foundation of digital cryptography for decades. Although DES is now considered obsolete for secure communications, it remains a cornerstone for understanding block cipher design and symmetric encryption.

DES Encryption Pipeline

1. Initial Permutation (IP)

• The 64-bit plaintext block is subjected to a fixed permutation called the Initial Permutation (IP), designed for hardware efficiency.
• Example:
  If the input is plaintext[0..63], the output is a reordered sequence where each output bit is mapped from a specific input bit according to the IP table.

2. Feistel Network Structure

DES uses a Feistel network with 16 rounds.
Each round splits the block into left (L) and right (R) 32-bit halves:

L0 = IP_block[0..31]
R0 = IP_block[32..63]

For each round i (1 ≤ i ≤ 16):

Li = Ri-1
Ri = Li-1 XOR F(Ri-1, Ki)

Where:

• F is the round function (see below)
• Ki is the round subkey

3. Round Function (F-function)

The F-function is central to DES security, introducing both confusion and diffusion:

• Expansion (E-Box):
  The 32-bit right half is expanded to 48 bits by duplicating and permuting certain bits, according to the E-table:

  expanded_block[0..47] = ExpansionPermutation(Ri-1)
  

  This ensures each S-box input depends on multiple plaintext bits.

• XOR with Subkey:
  The expanded block is XORed with the 48-bit round subkey:

  xored_block = expanded_block XOR Ki
  

• Substitution (S-Boxes):
  The 48-bit block is divided into eight 6-bit chunks.
  Each chunk is mapped through a specific S-box, producing a 4-bit output.
  This yields a 32-bit output, dramatically increasing non-linearity.

  for i in 0..7:
      sbox_output[i] = Sbox[i](xored_block[i*6 : (i+1)*6])
  combined_sbox = concat(sbox_output[0..7]) // 32 bits
  

• Permutation (P-Box):
  The combined S-box output is permuted to further diffuse the bits:

  permuted = PPermutation(combined_sbox)
  

4. Swap and Repeat

• After each round, the left and right halves are swapped for the next round.
• After the 16th round, the halves are recombined as (R16, L16).

5. Final Permutation (IP-1)

• The combined block is passed through the inverse of the initial permutation (IP-1), producing the final ciphertext.

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DES Key Schedule

• The original 64-bit key is reduced to 56 bits by discarding every 8th bit (parity).
• The 56-bit key is split into two 28-bit halves.
• For each round:
  • Each half is rotated left by 1 or 2 bits (depending on the round).
  • 48 bits are selected using a fixed permutation (PC-2) to form the subkey for that round.

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Triple DES (3DES)

3DES (“Triple DES”) enhances DES security by applying the DES cipher three times in sequence with three different keys:

Ciphertext = DES_encrypt(K3, DES_decrypt(K2, DES_encrypt(K1, Plaintext)))

• 3 independent keys: 168-bit key strength
• Same block size and mode support as DES

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Example: DES Round Operations

Let’s walk through the operations for a single round:

Input Block:
plaintext_block[64] = [b0, b1, ..., b63]

1. Initial Permutation:
   IP_block[64] = InitialPermutation(plaintext_block)

2. Split into Halves:

   L0[32] = IP_block[0..31]
   R0[32] = IP_block[32..63]
   

3. Round 1:

   L1 = R0
   R1 = L0 XOR F(R0, K1)
   

4. F-function Steps:

   expanded_R0[48] = ExpansionPermutation(R0)
   xored = expanded_R0 XOR K1
   sbox_out[32] = SBoxSubstitution(xored)
   permuted = PPermutation(sbox_out)
   

5. Combine and Swap:
   After 16 rounds, final output is (R16, L16), which is then permuted using IP-1.

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DES Modes of Operation

This library supports all major block cipher modes, for both DES and TripleDES:

• ECB (Electronic Codebook)
• CBC (Cipher Block Chaining)
• CFB (Cipher Feedback)
• OFB (Output Feedback)
• CTR (Counter Mode)

Each mode provides different security properties and is suitable for various applications. All are implemented in des.hpp with a unified API.

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Key and IV Generation

For cryptographic safety and demonstration flexibility, this library includes secure key and IV generators:

• DESUtils::GenerateKey() — 8 bytes for DES
• DESUtils::GenerateIV() — 8 bytes for IV (for CBC/CFB/OFB/CTR)
• DESUtils::GenerateTripleKey() — 24 bytes for 3DES

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Project Overview

This project delivers a modern, header-only C++11 implementation of DES and 3DES algorithms, including all common block cipher modes and secure random key/IV generation.
It’s ideal for cryptography education, experimentation, and understanding the inner workings of classic symmetric ciphers.

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Features

• Full DES & 3DES support with unified API
• ECB, CBC, CFB, OFB, CTR modes for both DES and 3DES
• PKCS7 padding for all modes
• Clean, educational code in a single header (des.hpp)
• Simple, modern C++ usage

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Requirements

• C++11 or later
• Standard C++ compiler (g++, clang++, MSVC)

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Operation Modes

• ECB (Electronic Codebook)
• CBC (Cipher Block Chaining)
• CFB (Cipher Feedback)
• OFB (Output Feedback)
• CTR (Counter Mode)

All modes are available for both DES and TripleDES with identical API.

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Usage

1. Include the Header

#include "des.hpp"

2. Define Arguments

std::string message("something to encrypt with DES");
std::string key("12345678");

// additionally, define 2 more keys for 3DES...
std::string key2("12345678"); // note that these should be different and secure keys...
std::string key3("12345678");

// now define the IV for modes using an IV...
std::vector<bool> iv(64, 1); // example iv with all bits set, im using bool type because it's ok to represent bits...

3. Encrypt and Decrypt

DES Example (CBC Mode)

auto cbc_enc = DES::CBC::Encrypt(message, key, iv);
auto cbc_dec = DES::CBC::Decrypt(cbc_enc.toString(), key, iv);

TripleDES Example (CFB Mode)

auto tdes_enc = DES::CBC::Encrypt3DES(message, key, key2, key3);
auto tdes_dec = DES::CBC::Decrypt3DES(tdes_enc.toString(), key, key2, key3);

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Screenshots

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License

This project is licensed under the MIT License.

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