How to use the tink.proto.tink_pb2.Keyset.FromString function in tink

def _decrypt(encrypted_keyset: tink_pb2.EncryptedKeyset,
             master_key_aead: aead.Aead) -> tink_pb2.Keyset:
  """Decrypts an EncryptedKeyset and returns a Keyset."""
  try:
    keyset = tink_pb2.Keyset.FromString(
        master_key_aead.decrypt(encrypted_keyset.encrypted_keyset, b''))
    # Check emptiness here too, in case the encrypted keys unwrapped to nothing?
    _assert_enough_key_material(keyset)
    return keyset
  except message.DecodeError:
    raise tink_error.TinkError('invalid keyset, corrupted key material')

def _encrypt(keyset: tink_pb2.Keyset,
             master_key_primitive: aead.Aead) -> tink_pb2.EncryptedKeyset:
  """Encrypts a Keyset and returns an EncryptedKeyset."""
  encrypted_keyset = master_key_primitive.encrypt(keyset.SerializeToString(),
                                                  b'')
  # Check if we can decrypt, to detect errors
  try:
    keyset2 = tink_pb2.Keyset.FromString(
        master_key_primitive.decrypt(encrypted_keyset, b''))
    if keyset != keyset2:
      raise tink_error.TinkError('cannot encrypt keyset: %s != %s' %
                                 (keyset, keyset2))
  except message.DecodeError:
    raise tink_error.TinkError('invalid keyset, corrupted key material')
  return tink_pb2.EncryptedKeyset(
      encrypted_keyset=encrypted_keyset, keyset_info=_keyset_info(keyset))