The passphrase+salt get hashed. This hashed-passphrase is what decrypts the file. You can store the hashed-passphrase in local storage. This means you can auto

The passphrase+salt get hashed. This hashed-passphrase is what decrypts the file. You can store the hashed-passphrase in local storage. This means you can auto-decrypt again. You don't need the original passphrase. If bad guys get your computer, they can read the file. However, they cannot trivially work out what your original passphrase was, so it protects you a little if you reused your passphrase elsewhere.