return 0, errors.New("Cannot compute decrypted size of an unencrypted object")
	}
	if !o.isMultipart() {
		size, err := sio.DecryptedSize(uint64(o.Size))
		if err != nil {
			err = errObjectTampered // assign correct error type
		}
		return int64(size), err
	}

	var size int64
	for _, part := range o.Parts {
		partSize, err := sio.DecryptedSize(uint64(part.Size))
		if err != nil {