How to Enable Post‑Quantum TLS (X25519+Kyber) on an Ubuntu 24.04 VPS with Nginx or Caddy (2025)

Introduction

As quantum computing advances, traditional cryptographic algorithms face potential vulnerabilities. Post-quantum cryptography (PQC) provides a proactive defense against future quantum attacks on TLS connections. The X25519+Kyber hybrid key exchange combines the proven security of classical elliptic curve cryptography with quantum-resistant algorithms.

In this tutorial, you’ll learn how to implement post-quantum TLS on your Ubuntu 24.04 VPS using either Nginx or Caddy. This configuration enhances your server’s cryptographic resilience while maintaining compatibility with modern browsers that support hybrid key exchange mechanisms.

Prerequisites

Before starting this tutorial, ensure you have:

• An Ubuntu 24.04 VPS with root access (minimum 2GB RAM, 2 vCPU cores)
• A registered domain name pointing to your server’s IP address
• Basic familiarity with command-line operations
• Either Nginx 1.25+ or Caddy 2.7+ installed
• Valid SSL/TLS certificates (Let’s Encrypt recommended)

Note: This tutorial assumes you have either Nginx or Caddy already configured with basic HTTPS. If you need guidance on web server setup, check our Caddy vs Nginx comparison guide.

Step-by-Step Tutorial

Step 1: Update System Packages

Start by updating your Ubuntu 24.04 system to ensure you have the latest security patches:

sudo apt update && sudo apt upgrade -y
sudo reboot

Step 2: Verify OpenSSL Version

Post-quantum TLS support requires OpenSSL 3.0+ with quantum-safe algorithms. Check your version:

openssl version -a
openssl list -kem-algorithms | grep -i kyber

Ubuntu 24.04 ships with OpenSSL 3.0.2+, which includes experimental post-quantum support.

Step 3: Configure Nginx for Post-Quantum TLS

If you’re using Nginx, modify your server configuration to enable hybrid key exchange:

server {
    listen 443 ssl http2;
    server_name your-domain.com;
    
    # SSL Certificate Configuration
    ssl_certificate /etc/letsencrypt/live/your-domain.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/your-domain.com/privkey.pem;
    
    # Post-Quantum TLS Configuration
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ecdh_curve X25519:prime256v1;
    
    # Enable post-quantum key exchange groups
    ssl_conf_command Groups "X25519Kyber768Draft00:X25519:prime256v1";
    
    # Cipher suites supporting post-quantum cryptography
    ssl_ciphers ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305;
    ssl_prefer_server_ciphers off;
    
    # Security headers
    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload" always;
    add_header X-Content-Type-Options nosniff;
    add_header X-Frame-Options DENY;
    
    location / {
        root /var/www/html;
        index index.html;
    }
}

Test the Nginx configuration and reload:

sudo nginx -t
sudo systemctl reload nginx

Step 4: Configure Caddy for Post-Quantum TLS

For Caddy users, create or modify your Caddyfile:

your-domain.com {
    # Enable post-quantum key exchange
    tls {
        curves x25519_kyber768 x25519 p256
        ciphers TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256
    }
    
    # Security headers
    header {
        Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
        X-Content-Type-Options "nosniff"
        X-Frame-Options "DENY"
    }
    
    root * /var/www/html
    file_server
}

Reload Caddy to apply the configuration:

sudo systemctl reload caddy

Step 5: Verify Post-Quantum TLS Implementation

Test your post-quantum TLS configuration using OpenSSL:

# Test TLS connection with post-quantum support
openssl s_client -connect your-domain.com:443 -groups X25519Kyber768Draft00 -brief

# Check supported curves
echo | openssl s_client -connect your-domain.com:443 2>&1 | grep -i "server temp key"

You can also use online tools like SSL Labs to verify your quantum-resistant cryptography implementation, though full PQC support in testing tools is still evolving.

Step 6: Monitor Browser Compatibility

Check browser support for your post-quantum implementation:

• Chrome 116+: Supports X25519Kyber768 (experimental)
• Firefox 120+: Limited post-quantum support
• Safari: No native support yet

Create a simple test page to verify connectivity across different browsers and monitor your server logs for any connection issues.

Best Practices

Performance Considerations

Post-quantum algorithms typically have larger key sizes and may impact performance:

• Monitor CPU utilization during TLS handshakes
• Consider enabling TLS session resumption to reduce overhead
• Use HTTP/2 or HTTP/3 to minimize connection establishment costs
• Implement proper caching strategies for static content

Security Recommendations

Enhance your post-quantum TLS implementation with additional security measures:

• Keep systems updated: Post-quantum cryptography standards are evolving
• Implement defense in depth: Consider deploying CrowdSec protection alongside PQC
• Regular security audits: Monitor for new developments in quantum-resistant algorithms
• Fallback compatibility: Maintain support for classical algorithms during the transition period

Monitoring and Maintenance

Set up monitoring for your post-quantum TLS deployment:

# Create a simple monitoring script
cat << 'EOF' > /usr/local/bin/pqc-monitor.sh
#!/bin/bash
LOG_FILE="/var/log/pqc-monitor.log"
echo "[$(date)] Testing post-quantum TLS..." >> $LOG_FILE
openssl s_client -connect localhost:443 -groups X25519Kyber768Draft00 -quiet < /dev/null
if [ $? -eq 0 ]; then
    echo "[$(date)] PQC TLS: OK" >> $LOG_FILE
else
    echo "[$(date)] PQC TLS: FAILED" >> $LOG_FILE
fi
EOF

chmod +x /usr/local/bin/pqc-monitor.sh

Conclusion

You’ve successfully implemented post-quantum TLS using X25519+Kyber hybrid key exchange on your Ubuntu 24.04 VPS. This configuration provides quantum-resistant protection while maintaining compatibility with current cryptographic standards.

Key takeaways from this implementation:

• Post-quantum cryptography offers proactive protection against future quantum threats
• Hybrid approaches like X25519+Kyber provide the best balance of security and compatibility
• Regular monitoring and updates are essential as PQC standards evolve
• Performance considerations must be balanced with security requirements

As cryptographic resilience becomes increasingly important, we recommend exploring Onidel VPS in Singapore, Sydney, or Amsterdam for hosting applications that require advanced security features. Our high-performance EPYC Milan processors and enterprise-grade infrastructure provide the computational power needed for quantum-resistant cryptography implementations.