java字节流加密图片，具体操作是单个字节异或一个数字，为什么加密后的图片无法打开

FileInputStream fis = null;
FileOutputStream fos = null;
try {
    fis = new FileInputStream("haha.jpg");
    fos = new FileOutputStream("hahasecret.jpg");

    byte[] buffer = new byte[20];
    int len;
    while ((len = fis.read(buffer)) != -1) {
        for (int i = 0; i < len; i++) {
            buffer[i] = (byte) (buffer[i] ^ 1);
        }

        fos.write(buffer, 0, len);
    }
} catch (IOException e) {
    e.printStackTrace();
} finally {
    if (fos != null) {
        try {
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
    if (fis != null) {
        try {
            fis.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

不应该是可以打开但是里面的内容是乱的吗