🔐 Client–Server Diffie–Hellman Key Exchange using WebAssembly and Native C (WASM + Next.js)

A Web-based Diffie–Hellman Key Exchange System demonstrating how native C code can be executed on both:

• Client → via WebAssembly (WASM)
• Server → via a natively compiled C binary

to compute modular exponentiation securely and efficiently.

This project shows end-to-end integration of:
C → WebAssembly → JavaScript → Next.js Frontend → Next.js API → Native C
to establish a shared secret key between a client and a server.

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🧭 Project Overview

This project was developed to demonstrate how Diffie–Hellman Key Exchange (DHKE) can be implemented in a modern web environment:

• Client-side exponentiation (g^a mod p) happens using C compiled to WASM

• Server-side exponentiation (g^b mod p and x^b mod p) happens using native C executed from Next.js

The system performs the complete DH key exchange:

1. Client randomly generates private key a

2. Computes x = gᵃ mod p using WASM

3. Sends <g, p, x> to server

4. Server generates private key b

5. Computes:

   • y = gᵇ mod p
   • K = xᵇ mod p

6. Returns <K, y> to the client

7. Client displays <a, y, K> as the shared secret key exchange result

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🧰 Technologies Used

Category	Technology	Purpose
Core Logic	C (modexp implementation)	Modular exponentiation used in DHKE
Client Engine	WebAssembly (WASM)	Execute modexp C code in browser
WASM Compiler	Emscripten	Compile C → WebAssembly + JS loader
Frontend	Next.js (React)	UI and client computations
Backend	Next.js API Routes	Execute native C program using child_process
Runtime	Native GCC C binary	Fast server-side modular exponentiation
Styling	TailwindCSS	Responsive UI

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🌐 Key Features

🔐 Real Diffie–Hellman Key Exchange between client & server

🔐 Real Diffie–Hellman Key Exchange between client and server

⚙️ Client uses WASM to compute gᵃ mod p

⚙️ Server uses native C to compute gᵇ mod p and xᵇ mod p

🔄 JSON-based communication between client and server

📡 Server API executes native C using execFile

📊 UI displays:

• Shared secret key K
• Server public key y
• Client private key a

🎨 Clean UI for entering p and g, and generating values

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🎯 Objective

To learn how native C programs can be compiled to WebAssembly and used inside a Next.js frontend, while also executing native C on the backend to perform:

• Fast modular exponentiation

• Full Diffie–Hellman key exchange

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📚 Learning Outcomes

✔️ How to compile C → WebAssembly using Emscripten

✔️ How to expose C functions to JavaScript using Emscripten bindings

✔️ How to load and use WASM modules in Next.js

✔️ How to execute native C programs from a Next.js API route

✔️ How to pass data between WASM memory and JS

✔️ How Diffie–Hellman Key Exchange works internally

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🛠️ Steps Implemented

Step 1: Implement Modular Exponentiation in C

• A complete Modexp implementation result = (g^a) mod p was written manually (no libraries).
• The file is located at: src/modexp.c

Step 2: Compile C ➝ WebAssembly using Emscripten

Commands used:

emcc modexp.c -O3 \
   -s WASM=1 \
   -s MODULARIZE=1 \
   -s 'EXPORT_NAME="ModExpModule"' \
   -s EXPORTED_FUNCTIONS='["_modexp"]' \
   -s EXPORTED_RUNTIME_METHODS='["ccall","cwrap"]' \
   -o ../public/modexp.js

This generates:

• modexp.wasm
• modexp.js (JS glue code to load WASM)

Placed inside:

public/
│── modexp.js
└── modexp.wasm

Step 3: Compile Native C Binary for Server

• gcc modexp.c -o modexp

src/
│── modexp.c
└── modexp

Step 4: Create Next.js Frontend

Frontend includes:

• Input for prime number p
• Input for generator g
• Button to compute x = gᵃ mod p via WASM
• Sending <g, p, x> to backend API
• Display server response: K, y, and client private key a

Located in:

pages/index.js
hooks/useModExp.js
styles/global.css

Step 5: Create Next.js Backend(API)

Server API endpoint executes native C using Node.js:

pages/api/useModExpApi.js

Backend performs:

• Random generation of b
• y = gᵇ mod p (native C)
• K = xᵇ mod p (native C)
• Returns <K, y> to the client

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⚙️ Installation & Running Locally

🧩 Prerequisites

Tool	Description	Version
Node.js	JavaScript runtime	>= 20.x
Emscripten	C → WASM compiler toolchain	Latest
GCC	Native C compiler	Latest

🔧 Install Emscripten

git clone https://github.com/emscripten-core/emsdk.git
cd emsdk
./emsdk install latest
./emsdk activate latest
source ./emsdk_env.sh

1️⃣ Clone the repository

git clone https://github.com/Fuzzy-programmer/diffie-hellman-wasm.git
cd DiffieHellmanExApp

2️⃣ Install dependencies

npm install

3️⃣ Build WASM + Start Next.js

npm run build
npm start

4️⃣ Open browser

👉 http://localhost:3000

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🗂️ Project Structure

DiffieHellmanExApp/
├── hooks
│   └── useModExp.js
├── package.json
├── package-lock.json
├── pages
│   ├── api
│   │   └── useModExpApi.js
│   ├── _app.js
│   └── index.js
├── postcss.config.js
├── public
│   ├── modexp.js
│   └── modexp.wasm
├── src
│   ├── modexp
│   └── modexp.c
├── styles
│   └── global.css
└── tailwind.config.js

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🖼️ Screenshots

📸 Home Page UI

📸 Generate & Exchange Keys

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📜 License

This project is licensed under the MIT License.
See the LICENSE file for details.

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