7-Zip opens almost every archive you will be sent and can create 7z and ZIP files encrypted with AES-256. It is free, by Igor Pavlov, and the Windows program is the one everyone means. It is light and it does not bundle a browser.
The window looks old because it is a compression tool. On Linux, use the official console build or a maintained port, and do not grab a random “7zip” package from a blog. Encryption in ZIP is a mess of old and new modes. If you need the password to matter, use the 7z format and a long passphrase. 7-Zip does not sync, back up, or hide the fact that you sent the file.
Use it as the archiver on Windows. 7-zip.org
When you encrypt with 7-Zip, choose the 7z format and a passphrase you did not use anywhere else. Send the passphrase in a different channel from the file. ZIP encryption has old modes that are weak. If the dialog looks like 1998, slow down and read which method is selected. Double-clicking an encrypted archive on a shared computer leaves a temporary copy. Close it and clear temp files if the file mattered.
