// UnsealETag unseals the etag using the provided object key.
// It does not try to decrypt the ETag if len(etag) == 16
// because such ETags indicate that the S3 client hasn't sent
// an ETag = MD5(object) and the backend has picked an ETag value.
func (key ObjectKey) UnsealETag(etag []byte) ([]byte, error) {
	if !IsETagSealed(etag) {
		return etag, nil
	}
	mac := hmac.New(sha256.New, key[:])
	mac.Write([]byte("SSE-etag"))