java - CipherInputStream and CipherOutputStream are not generating files

Question

I have the following code. However the files b.xlsx and c.xlsx are of 0 bytes. Why is CipherOuputSteam not working?

public static void main(String[] args) throws Exception {

    KeyPair keys = KeyPairGenerator.getInstance("RSA").generateKeyPair();
    Cipher cipher = Cipher.getInstance("RSA");

    cipher.init(Cipher.ENCRYPT_MODE, keys.getPublic());

    FileInputStream fis;
    FileOutputStream fos;
    CipherOutputStream  cos;

    fis = new FileInputStream("C:/temp/a.xlsx");
    fos = new FileOutputStream("C:/temp/b.xlsx");

    cos = new CipherOutputStream (fos, cipher);

    byte[] block = new byte[8];
    int i;
    while ((i = fis.read(block)) != -1) {
        cos.write(block, 0, i);
    }
    cos.close();
    fos.close();



    cipher.init(Cipher.DECRYPT_MODE, keys.getPrivate());
    CipherInputStream cis1, cis2;
    fis = new FileInputStream("c:/temp/b.xlsx");
    CipherInputStream cis = new CipherInputStream(fis, cipher);
    fos = new FileOutputStream("c:/temp/c.xlsx");

    while ((i = cis.read(block)) != -1) {
        fos.write(block, 0, i);
    }
    fos.close();
    fis.close();
    cis.close();
}

Answer 1

The problem lies in your usage - which is incorrect and in the implementation of CipherOutputStream which masks a very important exception - IllegalBlockSizeException.

The problem is that you cannot use an RSA key to encrypt data which is longer than the size of the key (which is 128 bytes in your example). you should use a symmetric encryption algorithm for large blocks of data - e.g. AES.

If you want to use asymmetric keys for a reason (safe transmition of data for example) - you can find a good example on this SO answer.