Drop Once

🔒 Drop Once -- Secure One-Time File Sharing

Long-Form Project Summary for KNWS Showcase

Drop Once is a secure, self-hosted file sharing platform where uploaded files can be downloaded exactly once before being automatically and permanently deleted from the server. It provides a simple workflow: upload a file, receive a shareable URL containing a 64-character cryptographic token, and send that link to your recipient. The moment they download the file, it is erased -- no copies remain, no second chances.

Built with Python/Flask and backed by MySQL, the application includes user authentication (email/password and Google OAuth), CSRF protection, secure filename sanitisation, and per-user storage isolation. It is designed for scenarios where sensitive documents need to be transferred with confidence that they will not persist on an intermediate server.

URL: https://droponce.knws.co.uk GitHub: github.com/datagram1/droponce

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🚀 Project Goals

1. Provide truly ephemeral file sharing -- files exist on the server only until the first download.
2. Generate cryptographically secure share links using 64-character random tokens that are practically unguessable.
3. Enforce strict security -- CSRF protection, secure filename handling, and user isolation.
4. Support multiple authentication methods -- traditional email/password registration and Google OAuth for convenience.
5. Limit file types and sizes to prevent abuse -- 100MB maximum with a whitelist of 10 allowed extensions.
6. Organise storage per user -- each authenticated user has their own isolated upload directory.
7. Deploy cleanly on KNWS infrastructure with minimal dependencies.

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🧠 Core Features

1. One-Time Download Mechanism

The core functionality:

• User uploads a file through the web interface
• Server generates a unique 64-character cryptographic token using Python's secrets module
• A shareable URL is constructed: https://droponce.knws.co.uk/download/<token>
• The file metadata (original filename, storage path, token, uploader, timestamp) is stored in MySQL
• When a recipient visits the URL:
  • The file is streamed to them as a download
  • The physical file is immediately deleted from the server filesystem
  • The database record is marked as consumed
• Any subsequent visit to the same URL returns a "file no longer available" message

There is no recovery mechanism by design. Once downloaded, the file is gone.

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2. Cryptographic Token Generation

Security of the sharing mechanism:

• Tokens are generated using secrets.token_urlsafe(48), producing 64-character base64url strings
• The token space is vast enough (approximately 2^288 possibilities) to make brute-force enumeration infeasible
• Tokens are stored hashed in the database -- even a database breach does not reveal valid download URLs
• No sequential or predictable patterns in token generation

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3. User Authentication

Dual authentication support:

• Email/Password -- standard registration and login with password hashing (Werkzeug security)
• Google OAuth 2.0 -- sign in with Google for frictionless access
• Session management via Flask-Login
• Remember-me functionality with secure cookies
• Password reset capability

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4. File Security & Validation

Multiple layers of protection:

• Maximum file size: 100MB enforced at both client and server level
• Allowed extensions: 10 whitelisted file types to prevent executable uploads
• Filename sanitisation: Werkzeug's secure_filename() strips dangerous characters and path traversal attempts
• Per-user directories: Each user's uploads are stored in an isolated directory named by their user ID
• CSRF protection: Flask-WTF CSRF tokens on all forms
• No directory listing: Upload directories are not browsable

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5. User Dashboard

Authenticated users see:

• List of their active (not yet downloaded) shared files
• Upload date and file size for each
• Shareable link with one-click copy
• Ability to manually delete a file before it is downloaded
• Download status indicator (pending / consumed)

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6. Clean, Responsive UI

The frontend is built with Bootstrap 5:

• Simple drag-and-drop or click-to-upload interface
• Progress bar during upload
• Instant link display after successful upload
• Copy-to-clipboard button for the share URL
• Mobile-friendly responsive layout
• Clear visual feedback for file status and errors

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🛠 Technical Architecture

Backend

• Language: Python 3.x
• Framework: Flask
• ORM: SQLAlchemy with MySQL backend
• Authentication: Flask-Login + Flask-OAuthlib (Google OAuth)
• Forms: Flask-WTF with CSRF protection
• File Storage: Server filesystem with per-user directory isolation
• Token Generation: Python secrets module
• File Handling: Werkzeug secure_filename, size validation, extension whitelist

Database Schema

• Users -- id, email, password hash, OAuth provider, created timestamp
• Files -- id, user_id, original_filename, storage_path, token_hash, file_size, uploaded_at, downloaded_at, status

Frontend

• CSS Framework: Bootstrap 5
• Templates: Jinja2
• JavaScript: Vanilla JS for upload progress, clipboard API, drag-and-drop

Infrastructure

• Hosted on KNWS web server (192.168.10.10)
• MySQL on the dedicated database server
• Apache with WSGI for Flask deployment
• Virtual host configured (pending activation)

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🔧 Pipeline & Workflow

1. User signs up or logs in via email or Google OAuth.
2. User uploads a file through the web interface.
3. Server validates file type, size, and filename.
4. Server stores the file in the user's isolated directory and generates a cryptographic token.
5. User copies the shareable URL and sends it to the recipient.
6. Recipient clicks the link -- file downloads and is immediately deleted from the server.
7. Link becomes invalid -- any future access returns a gone message.

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📈 Outcome & Impact

Drop Once demonstrates:

• Security-first design -- cryptographic tokens, CSRF protection, filename sanitisation, and automatic file deletion
• OAuth integration -- Google sign-in alongside traditional authentication
• Flask full-stack development -- backend API, database ORM, template rendering, and file handling
• Privacy engineering -- files are truly ephemeral with no server-side retention
• Practical utility -- a genuinely useful tool for sharing sensitive documents, credentials, or one-time access files
• Clean architecture -- well-separated concerns with SQLAlchemy models, Flask blueprints, and template inheritance

The project addresses a real need for secure file transfer where the sender wants assurance that shared content does not persist beyond its intended recipient.

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