WinFax has public key

The new WinFax 4.0 appears to have public-key encryption. It can, according to the manual, send a binary file as a fax, with encryption and a digital signature on it. Has anyone tried this out? How secure is it? The manual says nothing about how it works, but the encryption is the type used in Microsoft At Work. It allows you to create a key pair and export the public key. --- Mike