The multiplication in prss_share_random cannot be a local
multiplication. Tomas writes the following in:

  http://article.gmane.org/gmane.comp.cryptography.viff.devel/241

[...] looking at this lead Mikkel and me to look at prss_share_random
in runtime.py, and there seems to be either a bug (information leak)
/or/ a possibility of optimisation when sharing a bit in Zp.

The problem is the following:

  result = self.open(Share(self, field, share*share),
  threshold=2*self.threshold)

Is the "*" in "share*share" a multiplication protocol or a
multiplication of actual values? If it is actual values, then we
*cannot* simply call it shares and open it, as the polynomial is not
uniformly random (this can also be done with PRSS and no communication).

If on the other hand it is an invoation of the multiplication
protocol, then it is secure but can be optimised with the PRSS version
mentioned above.