All Wufoo forms and reports are SSL-encrypted by default, on all plans—free and paid.
No matter how you share your form or report, you'll notice that the URL starts with https://, indicating that it's being served securely over SSL.
Learn more about Wufoo's security practices here: Our Security Measures
When there's an SSL certificate installed on a website, it tries to encrypt all the elements served on the page—including any files you reference in your themes (like logos or custom CSS) or within the body of your form (like images added with HTML).
If elements being referenced on the page aren't served over SSL, like the logo on your form, people trying to access your form may see a browser warning that says they're trying to access unsecured content. This doesn't indicate a failure of the SSL on your site, but it can still be jarring for your users and it can affect form completion rates.
We highly recommend hosting all of your external content over SSL to avoid these warnings.
You can also encrypt individual fields on your form for increased security.