key, err := k.GenerateKey(context.Background(), "", ctx)
	if err != nil {
		return nil, err
	}
	stream, err := algorithm.Stream(key.Plaintext)
	if err != nil {
		return nil, err
	}
	nonce := make([]byte, stream.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return nil, err
	}

	const (
		MaxMetadataSize = 1 << 20 // max. size of the metadata
		Version         = 1
	)
	var (
		header [5]byte