I think that its okay that you don't have side projects. I just graduated with my undergrad and was quite close to my professors. I'd say, if you can still go to office hours, go to them prepared and with good questions. Let them know that you did your research and noticed that they were into cryptology, and tell them that that's something you'd be interested in learning more about - ask them if they have books or papers that they would recommend. After a couple of office hours, or when you're comfortable, ask them if they would be willing to grab coffee. This will create more of a personal professional relationship rather than one just related to class. If you do not feel comfortable, continue to talk to them about the papers you've read and so on. Ask them how you can learn more, or if they'd be willing to help you code some stuff. Show them that you're truly interested.

I don't think you have to be at the top of your class to have a good relationship with your professors (I definitely wasn't!)