Hi Baki!
I'm afraid I can't comment much on the international student side of things, though I imagine that most schools have some method of recalculating/translating your current work into the American system. (For literal translations of your transcripts and such, they'll usually want you to provide a certified copy, and they'll specify whether or not you need to muck about with changing the grades to "American.")
What area of Security Studies are you interested in? It seems like many schools lean towards a specific type of security studies - Security Policy, or International Security, etc. You'll want to make sure that the school's coursework is related to your area of interest. Also, you may discover that defining your focus actually gives you new options outside of what is strictly termed as "security studies" - for example, I'm primarily interested in terrorism in the FSU, so I applied to area studies programs, as well.