Hash function for http://class.coursera.org/crypto

gistfile1.rb

class String
  def last(n)
    self[-n,n]
  end
end

require "digest"

def hash_message(message)
  sha256 = Digest::SHA2.new(256)
  sha256hash = sha256.digest message

  binstring = ""

  sha256hash.bytes.each do |byte|
    binval = byte.to_s(2)
    if binval.length < 8 
      binval = '0' * (8 - binval.length) + binval
    end
    binstring << binval
  end
  
  return binstring.last(50).to_i(2)
end