  Expansion and amplification of weak randomness with untrusted quantum devices
has recently become a very fruitful topic of research. Here we contribute with
a procedure for amplifying a single weak random source using tri-partite
GHZ-type entangled states. If the quality of the source reaches a fixed
threshold $R=\frac{1}{4}\log_{2}(10)$, perfect random bits can be produced.
This technique can be used to extract randomness from sources that can't be
extracted neither classically, nor by existing procedures developed for
Santha-Vazirani sources. Our protocol works with a single fault-free device
decomposable into three non-communicating parts, that is repeatedly reused
throughout the amplification process.
