Support encypting files individually by password

The last time this seems to have been discussed is three or more years ago. So I thought I might start a new thread.

I'm looking for a way to keep some highly sensitive information on my iPad. I'm thinking about files being encrypted by an individual password on the iPad from within Textastic. The file should be encrypted when it its closed (probably asks for an explicit "close the file now" button). The password must be entered to decrypt the file when it is opened in Textastic. Encrypted files would stay encrypted even when sent to other places (cloud, mail, dropbox, WebDav, etc.). 

Thanks
Peter