// DerivePartKey derives an unique 256 bit key from an ObjectKey and the part index.
func (key ObjectKey) DerivePartKey(id uint32) (partKey [32]byte) {
	var bin [4]byte
	binary.LittleEndian.PutUint32(bin[:], id)

	mac := hmac.New(sha256.New, key[:])
	mac.Write(bin[:])
	mac.Sum(partKey[:0])
	return partKey
}

// SealETag seals the etag using the object key.
// It does not encrypt empty ETags because such ETags indicate

		return 0, nil
	}
	o.debugln("searching for ", search)
	var tmp metacacheBlock
	json := jsoniter.ConfigCompatibleWithStandardLibrary
	i := 0
	for {
		partKey := fmt.Sprintf("%s-metacache-part-%d", ReservedMetadataPrefixLower, i)
		v, ok := fi.Metadata[partKey]
		if !ok {
			o.debugln("no match in metadata, waiting")
			return -1, io.ErrUnexpectedEOF
		}
		err := json.Unmarshal([]byte(v), &tmp)
		if !ok {