def checksum(entropy)
      b = Bitcoin.sha256([entropy].pack('B*')).unpack('B*').first
      b.slice(0, (entropy.length/32))
    end
def checksum(entropy)
      b = Bitcoin.sha256([entropy].pack('B*')).unpack('B*').first
      b.slice(0, (entropy.length/32))
    end
calculate entropy checksum
 @param [String] entropy an entropy with bit string format
 @return [String] an entropy checksum with bit string format