Encryptron

What I built

I built a Flask app with account registration, encryption and decryption flows, and an interface for managing file permissions. Users select a file, enter a passphrase, and download the result with an .enc extension.

The frontend uses HTML, CSS, JavaScript, and Jinja templates. The backend handles sessions, compression, file transformations, and SQLite records. It is an educational prototype with a custom cipher.

The upload flow

I split the interface into three steps, with JavaScript controlling which fields and instructions are visible.

1. Choose a file

   The selected filename replaces “Browse Files.” Reset clears the selection, and Next moves to the passphrase step.

2. Enter a passphrase

   A password field collects the passphrase. Back returns to the file step; an empty passphrase prevents moving forward.

3. Submit and download

   The final screen names the file being processed. On submission, the selected file and passphrase are copied into a multipart form and sent to Flask.

Behind the form

Returning the file

The processing route wraps the result in BytesIO and returns it with Flask’s send_file. The file contents stay in memory for this request; SQLite stores the associated records.

return send_file(
    io.BytesIO(encrypted_data),
    mimetype='application/octet-stream',
    as_attachment=True,
    download_name=encrypted_filename
)

From the encryption route in app.py.

Decryption and sharing

Before decryption, the app looks up the uploaded filename and checks whether the signed-in user owns the record or has a matching permission entry. A missing record or failed permission check stops the request before processing.

The Manage Access page lists the owner’s files and the usernames with access. Its forms can grant access, revoke a permission, or remove a file’s database record. Granting access does not send the file itself.

Memory and storage

In-memory processing keeps this workflow straightforward, but the file size directly affects memory usage. A larger deployment would need explicit limits and a different strategy for large files.

SQLite makes local setup simple. Moving to multiple application instances would require revisiting database access and deployment choices.

What I’d carry forward

Building the full flow made the different responsibilities clearer: form validation, file processing, account authentication, and file permissions each need their own checks.

For the next backend project, I’d define access rules and failure cases earlier, and keep file-processing logic separate from route handlers so it can be tested independently.