Is it possible to write a chrome extension to encrypt downloaded files? I also need to insert a header to encrypted file which makes file larger than original one.
Yes, it is possible. If you wan't to use one that already exists use this https://chrome.google.com/webstore/detail/fluffypuffin-file-encrypt/klbknjbjhpikjddjfbmlmmpnhngonljp?hl=en-GB