Bug: the precision of sortition is not enough

[algorand]

Bug: the precision of sortition is not enough

[algoradam]

Thank you very much for reporting this! Just wanted to let you know that we're looking into it.

[derbear]

Hi @Nanyan,

• We changed the sortition code so Select() should now return
  reasonable values when the VRF output is very high. In particular,
  floating-point precision issues should now never result in getting
  more votes than you would get if we had infinite precision. While this
  doesn't actually increase the precision, it does fix the issue with
  sybil-resistance. We've released a software update containing this
  fix. (See 0ffcdca)

• We're considering increasing the precision beyond 64-bit floats, but
  first we need to be sure that libboost's binomial cdf calculations are
  both sufficiently accurate and consistent across platforms if we
  increase the precision. We're also looking into other, simpler ways to
  compute the sortition function and retain sybil-resistance.

Thanks again for looking into this sortition issue and reporting this
bug! We really appreciate it. :)

• the Algorand security team