The infamous "DRM clause" of the GFDL reads:
You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute.
A criticism of this language is that it is too broad, because it applies to private copies made but not distributed. This means that a licensee is not allowed to save document copies "made" in a proprietary file format or using encryption.
In 2003, Richard Stallman said about the above sentence on the debian-legal mailing list:
This means that you cannot publish them under DRM systems to restrict the possessors of the copies. It isn't supposed to refer to use of encryption or file access control on your own copy. I will talk with our lawyer and see if that sentence needs to be clarified.
On the other hand, sending GFDL-licensed material over HTTPS/SSL/whatever is definitely not your "own copy"; you are merely obstructing unscrupulous individuals from reading the copy as it travels through the network, but it is obstruction nonetheless.
Edit: I just noticed that the GPL has a requirement that could potentially be interpreted similarly. Section 6, final paragraph:
Corresponding Source conveyed, and Installation Information provided, in accord with this section must be in a format that is publicly documented (and with an implementation available to the public in source code form), and must require no special password or key for unpacking, reading or copying.