function sha256x2 (buffer) {
  var tmp = createHash('sha256').update(buffer).digest()
  return createHash('sha256').update(tmp).digest()
}
function sha256x2 (buffer) {
  var tmp = createHash('sha256').update(buffer).digest()
  return createHash('sha256').update(tmp).digest()
}
function sha256x2 ( buffer ) { var tmp = createHash ( 'sha256' ) . update ( buffer ) . digest ( ) return createHash ( 'sha256' ) . update ( tmp ) . digest ( ) }
SHA256(SHA256(buffer))
SHA256 ( SHA256 ( buffer ))