if out, err := sio.DecryptBuffer(key[:0], sealedKey.Key[:], unsealConfig); len(out) != 32 || err != nil {
		return ErrSecretKeyMismatch
	}
	return nil
}

// 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[:])

// multi-part PUT request. It derives an unique encryption key from
// the partID and the object key.
func EncryptMultiPart(r io.Reader, partID int, key ObjectKey) io.Reader {
	partKey := key.DerivePartKey(uint32(partID))
	return EncryptSinglePart(r, ObjectKey(partKey))
}

// DecryptSinglePart decrypts an io.Writer which must an object
// uploaded with the single-part PUT API. The offset and length

	for i, test := range derivePartKeyTest {
		expectedPartKey, err := hex.DecodeString(test.PartKey)
		if err != nil {
			t.Fatalf("Test %d failed to decode expected part-key: %v", i, err)
		}
		partKey := key.DerivePartKey(test.PartID)
		if !bytes.Equal(partKey[:], expectedPartKey) {
			t.Errorf("Test %d derives wrong part-key: got '%s' want: '%s'", i, hex.EncodeToString(partKey[:]), test.PartKey)
		}
	}
}