In room-size metal boxes, secure against electromagnetic leaks, the National Security Agency is racing to build a computer that could break nearly every kind of encryption used to protect banking, medical, business and government records around the world. According to documents provided by former NSA contractor Edward Snowden, the effort to build “a cryptologically useful quantum computer” — a machine exponentially faster than classical computers — is part of a $79.7 million research program titled, “Penetrating Hard Targets.” Much of the work is hosted under classified contracts at a laboratory in College Park. The development of a quantum computer has long been a goal of many in the scientific community, with revolutionary implications for fields like medicine as well as for the NSA’s code-breaking mission. With such technology, all forms of public key encryption would be broken, including those used on many secure Web sites as well as the type used to protect state secrets.