  We introduce a protocol through which a pair of quantum mechanical devices
may be used to generate n bits of true randomness from a seed of O(log n)
uniform bits. The bits generated are certifiably random based only on a simple
statistical test that can be performed by the user, and on the assumption that
the devices obey the no-signaling principle. No other assumptions are placed on
the devices' inner workings. A modified protocol uses a seed of O(log^3 n)
uniformly random bits to generate $n$ bits of true randomness even conditioned
on the state of a quantum adversary who may have had prior access to the
devices, and may be entangled with them.
