Raffler

Raffler is an algorithm to select a winner from a provably fair source of randomness.

The Algorithm

The algorithm is as follows:

• Prior to starting, create an ordered list of participants (L_0)
• Select some random string S_0 from an unbiased source of randomness
• For each prize n, (n = 1, 2, ..., N):
  • Calculate H_n = HMAC_SHA256(S_0, string(n))
  • Interpret H_n as a integer Z_n
  • The winner is Z_n mod length(L_(n-1))
  • Remove the winner from the list to get L_n

Some things to point out:

• There are no special requirements for the list. The order doesn't matter. You could use the names of the participants in some lexicographical order agreed upon beforehand, for example
• A good source of randomness could be the hash of the first blockchain block generated after the submission deadline