lib/secretsharing/shamir.rb

   1 require 'openssl'
   2
   3 module SecretSharing
   4         class Shamir
   5                 attr_reader :n, :k, :secret, :secret_bitlength, :shares
   6
   7                 DEFAULT_SECRET_BITLENGTH = 128
   8
   9                 def initialize(n, k=n)
  10                         if k > n then
  11                                 raise ArgumentError, 'k must be smaller or equal than n'
  12                         end
  13                         if k < 2 then
  14                                 raise ArgumentError, 'k must be greater or equal to two'
  15                         end
  16                         @n = n
  17                         @k = k
  18                         @secret = nil
  19                         @shares = []
  20                 end
  21
  22                 def secret_set?
  23                         ! @secret.nil?
  24                 end
  25
  26                 def create_random_secret(bitlength = DEFAULT_SECRET_BITLENGTH)
  27                         raise RuntimeError, 'secret already set' if secret_set?
  28                         @secret = get_random_number(bitlength)
  29                         @secret_bitlength = bitlength
  30                 end
  31
  32                 private
  33                 def get_random_number(bitlength, highest_bit_one = true)
  34                         byte_length = (bitlength / 8.0).ceil
  35                         rand_hex = OpenSSL::Random.random_bytes(byte_length).each_byte.to_a.map { |a| "%02x" % a }.join('')
  36                         rand = OpenSSL::BN.new(rand_hex, 16)
  37                         begin
  38                                 rand.mask_bits!(bitlength)
  39                         rescue OpenSSL::BNError
  40                                 # never mind if there was an error, this just means
  41                                 # rand was already smaller than 2^bitlength - 1
  42                         end
  43                         if highest_bit_one then
  44                                 rand.set_bit!(bitlength)
  45                         end
  46                         rand
  47                 end
  48         end
  49 end