if metadata == nil {
		return map[string]string{MetaMultipart: ""}
	}
	metadata[MetaMultipart] = ""
	return metadata
}

// IsETagSealed returns true if the etag seems to be encrypted.

// 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"))

func TestIsETagSealed(t *testing.T) {
	for i, test := range isETagSealedTests {
		etag, err := hex.DecodeString(test.ETag)
		if err != nil {
			t.Errorf("Test %d: failed to decode etag: %s", i, err)
		}
		if sealed := IsETagSealed(etag); sealed != test.IsSealed {
			t.Errorf("Test %d: got %v - want %v", i, sealed, test.IsSealed)
		}
	}
}

var removeInternalEntriesTests = []struct {
	Metadata, Expected map[string]string
}{