FRANKFURT AN DER ODER, Germany—Before 25-year-old Abhishek Budhiraja leaves his dorm to go to campus, he pats down his pockets. “Phone, keys, wallet, headphones,” he says, running through his checklist. Recently, that list has grown longer: “Student ID, passport, residence permit.” Today, like most other days, Budhiraja anticipates needing to prove his identity. FRANKFURT AN DER ODER, Germany—Before 25-year-old Abhishek Budhiraja leaves his dorm to go to campus, he pats down his pockets.