The first rule of NSA club...

According to the law, you can't use the NSA log -- or even mention the letters "NSA" or use the name "National Security Agency":

   Sec. 15. (a) No person may, except with the written permission
   of the Director of the National Security Agency, knowingly use the
   words 'National Security Agency', the initials 'NSA', the seal of
   the National Security Agency, or any colorable imitation of such
   words, initials, or seal in connection with any merchandise,
   impersonation, solicitation, or commercial activity in a manner
   reasonably calculated to convey the impression that such use is
   approved, endorsed, or authorized by the National Security Agency.

Well, you might argue, clearly that doesn't apply, but it is precisely this law that was used to justify censorship of a post critical of the NSA by cryptography professor Mathew Green's post. His university threatened Green with legal action unless he removed the NSA logo -- based on their interpretation of this law.