Text Encryption & Decryption Script

I need a straightforward, well-documented script that can both encrypt and decrypt plain-text data for a small-scale cyber-security exercise.
My priority is speed—I’d like to start as soon as possible and wrap up quickly—so lean, readable code is essential.

Scope and expectations
• Build a reliable function or CLI tool that takes a text string (or .txt file) and returns either the encrypted or decrypted version, depending on a flag or argument.
• Encryption algorithm is flexible—AES-256, ChaCha20, or another modern, widely accepted standard is fine—as long as you explain the choice and provide the corresponding key/IV handling.
• Include clear instructions on how to generate, store, and rotate keys securely.
• Deliver the complete source code (Python preferred, but I’m open to Go, Rust, or C if it keeps setup simple), plus a short README with usage examples and any library requirements.

Success criteria
• Running a single command reliably encrypts then decrypts the sample text back to its original form.
• No hard-coded secrets; keys must be loaded from environment variables or a config file.
• Code passes a quick static-analysis check and basic unit test covering both paths.

If you’re confident you can turn this around fast, let’s get started today.