It’s the argument that just won’t go away. For decades, governments have been lobbying for encryption “backdoors” : ways of accessing people’s private messages should law enforcement agencies want a peek. It will help them catch terrorists, they insist. But computer scientists have always pushed back. Weakening encryption damages everyone’s security , they say.

Now, Sacha Servan-Schreiber at the Massachusetts Institute of Technology and Archer Wheeler at Brown University in Rhode Island have proposed a potential compromise: backdoors with steps that would make them costly for law enforcement agencies to …