BU is a great place to do a PhD in security. We have a very active and well funded research group. Our students publish at top conferences and our graduates are in high demand. Our group is only six years old and thus far, nine of our alumni have taken tenure-track faculty jobs (at UCSB, HKUST, Johns Hopkins,USC, UConn, Georgetown, George Mason, NC State, UMass Amherst) and several others are working at top research labs.
Boston is a great place to do a PhD -- the city is full of students, and there are good relationships between all the schools, and they are all walking distance from each other. This leads to a very active research community, arguably one of the best in the US. As a small example of the level of activity, see for example these local conferences, many of which regularly happen/were founded at BU: New England Network and Systems Day (NENS), New England Security Day (NESD), crypto day.
Admission to our group is very competitive. If you'd like to work with me, be make sure to follow these instructions and make sure to mention my name on your application.
Feel free to email me to ask about your prospects, but don't get discouraged if I don't answer. (I'm not great with email. There are many better things I could be doing, like research, or talking to my students!)
I look forward to seeing your application!