Build a Python PDF Encryptor in 20 Lines

A tiny, well-commented Python script that encrypts a PDF with a password in about 20 lines of code.

Overview

This repo shows how to build a self-contained PDF encryptor in Python that can lock any PDF file with a password. The focus is on clarity and minimalism: a single script, a single function, and just enough code to be understandable for beginners.

By the end, you’ll be able to run one command (or a short Python snippet) to take an existing PDF and produce a password-protected version.

This repository supports the PLEX tutorial:

Build a Python PDF Encryptor in 20 Lines
https://plexdata.online/post/build-a-python-pdf-encryptor-in-20-lines

You can also follow Dorian’s broader writing on Medium:
https://medium.com/@doriansotpyrc

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Features

• 🔐 Password-protect any PDF using a single function.
• 🧾 Minimal code (~20 lines), easy to read and adapt.
• 🧩 No complex frameworks – just Python and pypdf.
• 🧪 Includes a small test to verify that encryption runs successfully.
• 🧑‍💻 Works both as a CLI script and as an importable function.

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Project Structure

build-a-python-pdf-encryptor/
├── pdf_encryptor.py        # ~20-line core script to encrypt a PDF with a password
├── examples/
│   └── demo_encrypt.py     # Example usage importing the encrypt function
├── tests/
│   └── test_pdf_encryptor.py  # Minimal test to verify encrypted output is created
├── requirements.txt        # External dependency pin (pypdf)
└── README.md               # Project documentation and usage guide

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How It Works

At a high level:

1. Read the input PDF using pypdf.PdfReader.
2. Copy all pages into a new pypdf.PdfWriter object.
3. Apply encryption with a user-supplied password.
4. Write the encrypted PDF to a new file.

Conceptual flow:

[input.pdf] --read--> [PdfReader] --copy pages--> [PdfWriter] --encrypt(password)--> [encrypted.pdf]

The core logic lives in a single function encrypt_pdf inside pdf_encryptor.py. The same file also exposes a small CLI so you can run it directly from the command line.

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Installation

1. Clone the repo

git clone git@github-dorian:dorian-sotpyrc/build-a-python-pdf-encryptor.git
cd build-a-python-pdf-encryptor

2. (Optional) Create and activate a virtual environment

python -m venv .venv
# Windows
.venv\\Scripts\\activate
# macOS / Linux
source .venv/bin/activate

3. Install dependencies

pip install -r requirements.txt

This will install pypdf, the only external dependency.

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Usage

1. Command-line usage

You can run the encryptor directly as a script:

python pdf_encryptor.py input.pdf output_encrypted.pdf mypassword

• input.pdf – path to the existing PDF you want to protect.
• output_encrypted.pdf – path where the encrypted PDF will be written.
• mypassword – the password required to open the encrypted PDF.

Example:

python pdf_encryptor.py examples/sample.pdf examples/sample_encrypted.pdf secret123

If the command finishes without errors, you should see sample_encrypted.pdf in the examples/ directory. Open it with a PDF viewer and you’ll be prompted for the password secret123.

2. Import and use from Python

You can also import the function in your own scripts:

from pdf_encryptor import encrypt_pdf

encrypt_pdf(
    input_path="examples/sample.pdf",
    output_path="examples/sample_encrypted.pdf",
    password="secret123",
)

A complete example is provided in examples/demo_encrypt.py.

Run it with:

python examples/demo_encrypt.py

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Code Walkthrough (20-line core)

The heart of the project is the encrypt_pdf function in pdf_encryptor.py. It:

1. Opens the input PDF with PdfReader.
2. Creates a PdfWriter and copies each page.
3. Calls writer.encrypt(password).
4. Writes the result to output_path.

The script also includes a small if __name__ == "__main__": block that parses three command-line arguments and calls encrypt_pdf.

The code is heavily commented so you can understand each step even if you’re new to Python or PDF libraries.

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Running Tests

This repo includes a minimal test to verify that encryption runs and produces an output file.

1. Install pytest (if you don’t already have it):

pip install pytest

2. Run tests from the repo root:

pytest

The test in tests/test_pdf_encryptor.py will:

• Create a tiny one-page PDF in memory.
• Save it to a temporary file.
• Call encrypt_pdf to produce an encrypted version.
• Assert that the encrypted file exists and is readable.

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PLEX Article & Further Reading

This repo is explained in more detail in the PLEX article:
https://plexdata.online/post/build-a-python-pdf-encryptor-in-20-lines

You can also follow Dorian’s broader writing on Medium:
https://medium.com/@doriansotpyrc

There you’ll find more tutorials on Python, security, and practical automation.

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Roadmap / Extensions

If you want to extend this project, here are some ideas:

• Batch encryption: encrypt all PDFs in a folder with the same password.
• Password prompt: hide the password input using getpass instead of passing it on the command line.
• GUI wrapper: build a tiny Tkinter or PyQt interface for non-technical users.
• Decryption helper: add a companion function to decrypt PDFs (where you know the password).
• Metadata handling: preserve or modify PDF metadata (title, author, etc.) during encryption.

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License

This project is licensed under the MIT License. You are free to use, modify, and distribute it, with attribution.