There exists many unicode space characters, each with a different width. This creates the means to encode plain text as unicode space characters. It enables you to embed space-encoded text in plain sight within a document.
You could watermark a document or code file, or transmit hidden information within the subject line of an email.
It can be combined with the encryption capabilities in platform to include encryption to further strengthen the security.
Not something that I have built. Just integrated with the OS platform.
Link to actual project : https://www.codeproject.com/Articles/1202195/Invisible-Ink