You could install an SSL certificate on the DNN site and secure the page containing the iFrame.  However, your users would probably still get a mixed content message.  I don't really see a way around that short of Windows authentication into both apps.

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Shane Miller

There really are two separate parts to this issue:  installing the SSL certificate and configuring it onto IIS; and then configuring it in DNN.

Are you familiar with the procedure for installing a certificate and then configuring SSL onto IIS?  If not let me know and I'll reply with some resources.  Otherwise, the next question is how to configure DNN for SSL.  I suggest using any of several third-party modules to do this.  You can search at SnowCovered and find several good ones.  My favorite choice, however, doesn't actually appear at Snow if you search for SSL modules.  I use the SSL module bundled with Portal Store.  I guess you can say it's a free bonus with Portal Store (which is available at SnowCovered, or directly from the vendor at www.dnn-modules.com)  It's simple but it works well, and for hosts it allows the use of wildcard certs (a capability the Portal Store people built into the module at my request; they are very responsive). 

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Shane Miller