pdf-decrypt-lite 🔓

Ultra-lightweight PDF decryption library (~8KB!) with RC4 40/128-bit support

Built by PDFSmaller.com - Try our free online PDF tools with this decryption built-in!

🚀 Why pdf-decrypt-lite?

When building PDFSmaller.com, we needed lightweight PDF decryption that worked within edge constraints. Every existing solution pulled in all of pdf.js (2MB+) just to remove a password. There was literally no standalone PDF decryption package on npm.

So we built one.

This is the companion to our @pdfsmaller/pdf-encrypt-lite package — same crypto engine, reverse direction.

The Problem We Solved:

• ❌ pdf.js: 2MB+ just for decryption
• ❌ qpdf/poppler: Native binaries, not browser-compatible
• ❌ No npm package: pdf-decrypt, pdf-unlock — nothing existed!
• ✅ pdf-decrypt-lite: Only ~8KB! 🎉

✨ Features

• 🔓 Real PDF decryption - RC4 40/128-bit that actually works
• 📦 Tiny size - Only ~8KB total (MD5 + RC4 implementations)
• ⚡ Edge-ready - Works in Cloudflare Workers, Vercel Edge, Deno Deploy
• 🌐 Browser compatible - No Node.js dependencies
• 🔑 Dual password support - Works with both user and owner passwords
• 🔍 Encryption detection - Check if a PDF is encrypted before decrypting
• 📱 PDF Standard compliant - Implements Algorithms 2, 4-7 from PDF spec
• 🚀 Zero dependencies - Just needs pdf-lib as peer dependency
• 🤝 Companion package - Pairs perfectly with @pdfsmaller/pdf-encrypt-lite

📥 Installation

npm install @pdfsmaller/pdf-decrypt-lite pdf-lib

💻 Usage

import { decryptPDF, isEncrypted } from '@pdfsmaller/pdf-decrypt-lite';

// Decrypt a PDF
const decryptedPdfBytes = await decryptPDF(encryptedPdfBytes, 'password123');

// Check if encrypted first
const info = await isEncrypted(pdfBytes);
if (info.encrypted) {
  console.log(`Encrypted: V=${info.version}, R=${info.revision}, ${info.keyLength}-bit`);
  const decrypted = await decryptPDF(pdfBytes, password);
}

// Full example
async function unlockPDF() {
  // Load your encrypted PDF
  const encryptedBytes = await fetch('protected.pdf').then(res => res.arrayBuffer());

  // Decrypt it
  const decryptedBytes = await decryptPDF(
    new Uint8Array(encryptedBytes),
    'secret123'
  );

  // Save the unlocked PDF
  const blob = new Blob([decryptedBytes], { type: 'application/pdf' });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = 'unlocked.pdf';
  a.click();
}

🔐 Encrypt + Decrypt Pair

Use with our companion encryption package for a complete solution:

import { encryptPDF } from '@pdfsmaller/pdf-encrypt-lite';
import { decryptPDF } from '@pdfsmaller/pdf-decrypt-lite';

// Encrypt
const encrypted = await encryptPDF(pdfBytes, 'password');

// Later... Decrypt
const decrypted = await decryptPDF(encrypted, 'password');

🔥 Use Cases

Perfect for:

• Edge Functions (Cloudflare Workers, Vercel Edge, Netlify Edge)
• Browser applications (Like PDFSmaller.com)
• Serverless functions with size limits
• Client-side PDF unlocking without server uploads
• Automated document pipelines that need to process protected PDFs
• Lightweight Node.js applications

🎯 Real-World Example

See it in action at PDFSmaller.com/unlock-pdf - our free online tool uses this exact library to decrypt PDFs directly in your browser. No uploads, no server processing, just pure client-side decryption!

🏗️ How It Works

1. Parse encryption dict - Reads /Encrypt, /ID from PDF trailer via pdf-lib
2. Validate password - Tries as user password, then owner password
3. Derive encryption key - MD5-based key derivation per PDF spec
4. Decrypt all objects - RC4 decrypts every stream and string with per-object keys
5. Strip encryption - Removes /Encrypt entry, outputs clean PDF

Total crypto code: ~8KB 🤯

📊 Comparison

| Library | Size | Real Decryption | Edge Compatible | Browser | |---------|------|-----------------|-----------------|---------| | pdf-decrypt-lite | ~8KB ✅ | ✅ | ✅ | ✅ | | pdf.js | 2,000KB+ | ✅ | ❌ | ✅ | | qpdf | Native | ✅ | ❌ | ❌ | | pdf-lib alone | 0KB | ❌ | ✅ | ✅ |

⚠️ Supported Encryption

| Type | Version | Status | |------|---------|--------| | RC4 40-bit | V=1, R=2 | ✅ Supported | | RC4 128-bit | V=2, R=3 | ✅ Supported | | AES 128-bit | V=4, R=4 | 🔜 Planned (v2) | | AES 256-bit | V=5, R=6 | 🔜 Planned (v2) |

RC4 encryption covers the majority of password-protected PDFs in the wild. AES support is planned for v2 using the Web Crypto API.

🤝 Contributing

We welcome contributions! This library powers PDFSmaller.com, so we maintain high standards for security and compatibility.

📜 License

MIT License - Use it freely in your projects!

🙏 Credits

Built with ❤️ by PDFSmaller.com - Your free PDF toolkit

If this library helps you, check out our other free PDF tools:

• Unlock PDF - Uses this library!
• Protect PDF - Uses pdf-encrypt-lite!
• Compress PDF - Reduce PDF size by up to 90%
• Merge PDF - Combine multiple PDFs
• 20+ more tools - All free, all private

🚀 Quick Start for Cloudflare Workers

import { decryptPDF } from '@pdfsmaller/pdf-decrypt-lite';

export default {
  async fetch(request, env) {
    const formData = await request.formData();
    const file = formData.get('pdf');
    const password = formData.get('password');

    const pdfBytes = new Uint8Array(await file.arrayBuffer());
    const decrypted = await decryptPDF(pdfBytes, password);

    return new Response(decrypted, {
      headers: { 'Content-Type': 'application/pdf' }
    });
  }
}

📧 Support

• 🐛 Report issues
• 💡 Request features
• 🌐 Visit PDFSmaller.com
• 📧 Contact us

⭐ Star this repo if it helps you!

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