probanjee/File-Encryption-Tool ? reverse-engineered prompt

Build me a cross platform file encryption app in C called FileVault.

I want a secure tool that can encrypt and decrypt files from both a command line and a simple desktop app. The core should use strong authenticated encryption, derive keys from a password, and handle large files without loading everything into memory. It should save encrypted files in a custom vault format, show progress while working, and refuse to decrypt anything that has been tampered with.

Please include basic password prompts, file info, safe file handling, and a way to keep old XOR encrypted files readable so users can migrate them. The desktop version should feel modern, support drag and drop, and have light and dark themes. Make it work on Windows, Linux, and macOS, and keep the code modular so the encryption engine, file format, utilities, and UI are cleanly separated.

If you need current docs for the crypto or GUI pieces, look them up online first.