Secure File Transfer System (Python + SSL/TLS)

This project implements a secure client-server file transfer application using Python's built-in socket and ssl libraries.

Unlike standard file transfers (like basic FTP) that send data in cleartext, this application establishes an encrypted SSL/TLS tunnel before transmitting any file data. It demonstrates the implementation of Server-Authenticated TLS, where the client verifies the server's identity using a digital certificate.

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Skills Demonstrated

• Network Programming: TCP Socket communication (binding, listening, accepting connections).
• Applied Cryptography: Implementing SSL/TLS contexts for data encryption in transit.
• Public Key Infrastructure (PKI): Handling X.509 certificates (.crt) and private keys (.key).
• Client-Server Architecture: Building a distinct server listener and client initiator.
• File I/O: Binary reading and writing of files.

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Project Structure

The repository contains the following essential files:

• server.py: The server script. It binds to port 8443, wraps the socket in SSL using the private key/cert, and listens for requests.
• client.py: The client script. It connects to the server, verifies the server's certificate, and requests the file.
• server.crt: The public SSL Certificate (used by the server to prove identity, and by the client to verify it).
• server.key: The server's Private Key (used to decrypt the handshake).
• textfile.txt: The sample file to be transferred.

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How to Run

Prerequisites: Python 3. (No external pip installs required).

Step 1: Setup

Ensure all files (server.py, client.py, keys, and textfile.txt) are in the same directory.

Step 2: Start the Server

Open a terminal in the project directory and run:

python server.py

Step 3: Start the Client

Open a separate terminal window in the same directory and run:

python client.py

Step 4: Verify Success

Client Terminal: You will see Downloaded file successfully. for file textfile.txt.

Server Terminal: You will see Connection from... and Server has sent the file....

Directory: A new file named downloaded_textfile.txt will appear in your folder.

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Security Details

Encryption: All data transferred is encrypted using TLS. If a packet sniffer (like Wireshark) intercepted the traffic, the file contents would be unreadable.

Authentication: The client code uses context.load_verify_locations('server.crt'). This ensures the client is connecting to the intended server and not a "Man-in-the-Middle" attacker.

Secure Context: The server creates an SSL context with ssl.create_default_context(ssl.Purpose.CLIENT_AUTH), ensuring modern security protocols are used for the handshake.

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Security Note

Educational Use Only: This repository includes the server.key (Private Key) for demonstration purposes so the code can be run immediately. In a real-world deployment, the private key must remain secret and never be uploaded to a version control system (GitHub).