// fail.
func DecryptETag(key crypto.ObjectKey, object ObjectInfo) (string, error) {
	if n := strings.Count(object.ETag, "-"); n > 0 {
		if n != 1 {
			return "", errObjectTampered
		}
		i := strings.IndexByte(object.ETag, '-')
		if len(object.ETag[:i]) != 32 {
			return "", errObjectTampered
		}
		if _, err := hex.DecodeString(object.ETag[:32]); err != nil {
			return "", errObjectTampered
		}