Secure files - is it really secure?


the documentation about secure files says:

These files are stored securely outside of your project’s repository and are not version controlled. It is safe to store sensitive information in these files.

Is there any information available about how the security (encryption?) is implemented internally? I think the statement it is safe depends on the actual context and having more details would help to evaluate this really great feature (at least for my own context).

Thanks in advance!