🔒 Document Verification System (Console-Based Blockchain Python Application)

📄 Project Overview

The Document Verification System is a beginner-friendly, console-based Python application that uses a simple blockchain to verify documents. When a user provides a file path, the app computes its SHA-256 hash and stores it as a block on the chain. Users can later verify if a document has been seen before by comparing its hash against the blockchain. The chain persists to a local JSON file for reuse across sessions.

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🎯 Objectives

• Provide a minimal, educational blockchain for document verification
• Compute and store document hashes securely (SHA-256)
• Verify whether a file was previously added to the chain
• Persist the blockchain to JSON for durability between runs
• Offer optional file extension restrictions via a simple menu
• Validate blockchain integrity (links and hashes) on demand

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🧱 System Architecture

🔹 Components

Component	Description
Block	Dataclass with index, timestamp, data (document hash), previous_hash, hash
Blockchain	Manages the chain: create genesis block, add blocks, validate integrity, save/load JSON
File Hashing	Streams file bytes to compute file SHA-256 efficiently
CLI Menu	Console-driven flows to add/verify documents, print/validate chain, configure extensions
Persistence	Read/write blockchain.json with a simple list of blocks

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🧪 Functional Modules

Module	Responsibilities
Add Document	Hash a file and append the hash as a new block (if not already present)
Verify Document	Check whether a file’s hash exists anywhere in the chain
Print Blockchain	Display readable details: index, timestamp, hashes
Validate Integrity	Ensure each block’s previous_hash links correctly and hashes match recomputation
Configure Extensions	Optionally allow only specific file extensions (e.g., .pdf,.docx,.png)
Persistence	Auto-save after mutations and on exit; manual save option

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🗂️ Data Format

Storage File: blockchain.json

Each block is stored as a JSON object:

[
  {
    "index": 0,
    "timestamp": "2025-09-15Z",
    "data": "GENESIS",
    "previous_hash": "0000000000000000000000000000000000000000000000000000000000000000",
    "hash": "<sha256>"
  },
  {
    "index": 1,
    "timestamp": "2025-09-15Z",
    "data": "<document_sha256>",
    "previous_hash": "<sha256_of_genesis>",
    "hash": "<sha256>"
  }
]

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🖥️ Sample Output (Console Snapshot)

============================================================
Document Verification System (Blockchain)
============================================================
1) Add document
2) Verify document
3) Print blockchain
4) Validate blockchain integrity
5) Configure allowed extensions
6) Save blockchain now
7) Exit
[All file extensions are allowed]
Tip: Use option 5 to restrict uploads (e.g., .pdf,.docx,.png).

Choose an option (1-7):

Examples of confirmations and errors:

Document added successfully.
Document already verified.
Document not found in the blockchain.
File not found. Please check the path and try again.

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🧑‍💻 How to Run

Prerequisites

• Python 3.9+ (works on Windows/macOS/Linux)

Setup (optional virtual environment)

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

Run the App

python app.py

Tip

• Restrict uploads by choosing option 5 and entering extensions like: .pdf,.docx,.png (leave empty to allow all).

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🧾 Example Workflow

1. Choose “Add document” and provide a file path (e.g., sample.txt).
2. The app computes SHA-256 and adds a block if not already present.
3. Choose “Verify document” later to confirm if the same file exists in the chain.
4. Use “Validate blockchain integrity” to check chain links and hashes.
5. Use “Save blockchain now” to persist immediately (auto-save also occurs on add and exit).

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🧪 Chain Integrity Rules

• Genesis block uses previous_hash of 64 zeros and index 0.
• Each subsequent block must point to the exact hash of the previous block.
• A block’s hash must equal the SHA-256 of its content (index, timestamp, data, previous_hash).

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🤝 Contributing

Pull requests are welcome! Feel free to fork the repository and submit improvements.

Contributions are welcome! Follow these steps:

1. Fork the project.
2. Create a feature branch:

   git checkout -b feature-name

3. Commit your changes:

   git commit -m "Add feature description"

4. Push to the branch:

   git push origin feature-name

5. Open a pull request.

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📄 License

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

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📧 Contact

For queries or suggestions:

• 📩 Email: spreveen123@gmail.com
• 🌐 LinkedIn: www.linkedin.com/in/preveen-s/

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🌟 Show Your Support

If you like this project, please consider giving it a ⭐ on GitHub!