Form That Submits Content Securely

Form That Submits Content Securely

I have built a WP site for a client. One of the pages has a form which my
client's customers will use to submit material to my client, and one of
the required fields is a file upload.
My client requires that the material submitted via that form reach my
client securely. I see no WP contact form plugin that achieves this. (The
ones that call themselves "secure", such as fast secure contact form, use
that term in a different sense: protection against people submitting
malicious content via the form.)
Here is the approach I'm now thinking of:
I use a standard plugin to create the form (I'm using contact form 7).
The contents of the form submission are not emailed to my client. She just
receives an email notification that the form was submitted.
I've installed the plugin contact form DB, so that my client can access
the contents of the submission from the WP admin.
But there are still at least two issues that need to be resolved:
I need to make contact form 7 submit the content securely.
My client needs to be able to view the submitted content in the WP admin
securely.
Regarding issue # 1, it it's too much of a hassle to figure out how to
make contact form 7 or any of the other WP contact form plugins submit the
content securely, I could code the form myself.
I see a plugin called Dagon Design that calls itself secure -- but the
example they give is a form with a non-secure url, and the action of the
form also is non-secure, leading me to believe that this plugin too is not
using "secure" in the sense that I need.
Any assistance on how to implement this, or any suggestions for an
alternative approach, would be welcome.