code: function swarmHashBlock (data, totalLength) {
    var hash = new Keccak(256)
    var tmp = Buffer.alloc(8)
    tmp.writeUIntLE(totalLength, 0, 6)
    hash.update(tmp)
    hash.update(data)
    return Buffer.from(hash.digest('bin'), 'binary')
  }
docs: NOTE: it only supports 48 bit lengths