RSA decryption, BadPaddingException: Data must start with zero

[Andreas Pax Lück]

hi friends!

i have got a problem with RSA en-/decryption. following simple example uses the public key from the keypair for encryption, and the private key for decryption. sometimes it works and sometimes there's a badpaddingexcpetion trown. i searched the internet about this problem but didn't find any working solution. perhaps, this is bug? i hope you can help me. here comes the code:

 
    public static void main(String[] args) throws NoSuchAlgorithmException
    {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(512);
        KeyPair rsaKeyPair = kpg.genKeyPair();
        String txt = "This is a secret message.";
        System.out.println("Original clear message: " + txt);
        
        // encrypt
        Cipher cipher;
        try
        {
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            cipher.init(Cipher.ENCRYPT_MODE, rsaKeyPair.getPublic());
            txt = new String(cipher.doFinal(txt.getBytes()));
        }
        catch (Throwable e)
        {
            e.printStackTrace();
            return;
        }
        System.out.println("Encrypted message: " + txt);
        
        // decrypt
        try
        {
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            cipher.init(Cipher.DECRYPT_MODE, rsaKeyPair.getPrivate());
            txt = new String(cipher.doFinal(txt.getBytes()));
        }
        catch (Throwable e)
        {
            e.printStackTrace();
            return;
        }
        System.out.println("Decrypted message: " + txt);
    }
 

[Andreas Pax Lück]

alright, guys.

i tried to use a byte array for the message (not string) and now it works. this helps me. but do you have any idea why the string conversion destroys the encryption?

   
    public static void main(String[] args) throws NoSuchAlgorithmException
    {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(512);
        KeyPair rsaKeyPair = kpg.genKeyPair();
        byte[] txt = "This is a secret message.".getBytes();
        System.out.println("Original clear message: " + new String(txt));
        
        // encrypt
        Cipher cipher;
        try
        {
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            cipher.init(Cipher.ENCRYPT_MODE, rsaKeyPair.getPublic());
            txt = cipher.doFinal(txt);
        }
        catch (Throwable e)
        {
            e.printStackTrace();
            return;
        }
        System.out.println("Encrypted message: " + new String(txt));
        
        // decrypt
        try
        {
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            cipher.init(Cipher.DECRYPT_MODE, rsaKeyPair.getPrivate());
            txt = cipher.doFinal(txt);
        }
        catch (Throwable e)
        {
            e.printStackTrace();
            return;
        }
        System.out.println("Decrypted message: " + new String(txt));
    }  

[Ulf Dittmer]

Encrypted stuff is binary data, not character data, so it can't be meaningfully converted to a String. If for some reason you need an ASCII representation of something that's encrypted (maybe for storing it in a text-only database field), then you should run it through an encoding like base-64.

[Andreas Pax Lück]

ok, thanks ulf.