Encryption / Decryption Problem

[Hari Dhanakoti]

Hi,

I had been working on the encryption and decryption process..

I repeatedly facing the problem of
javax.crypto.BadPaddingException: Given final block not properly padded

This problem arises whenever i try to decrypt. In the java API documentation here its specified as

Note: under doFinal method its specified as

if any exception is thrown, this cipher object may need to be reset before it can be used

again. I am not sure how to over come this problem and dont know how to reset the cipher object again.

http://java.sun.com/j2se/1.4.2/docs/api/javax/crypto/Cipher.html#doFinal

Thanks
Hari

[K Aditi]

dont know how to reset the cipher object again

Did you try using init method again?
By the way, it would be helpful if you provide your code snippet for diagnosis.

[Hari Dhanakoti]

Encryption

private static String encry(String rand) throws Exception{
KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
 
// Generate the secret key specs.
SecretKey skey = kgen.generateKey();
byte[] raw = skey.getEncoded();
 
SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
 
        // Instantiate the cipher
Cipher ecipher = Cipher.getInstance("AES");
 
ecipher.init(Cipher.ENCRYPT_MODE, skeySpec);
 
byte[] encrypted = ecipher.doFinal(rand.toString().getBytes());
 

return asHex(encrypted);
}

Decryption

private static void decry(String randomVal) throws Exception{
 
KeyGenerator dkgen = KeyGenerator.getInstance("AES");
dkgen.init(128);
 
// Generate the secret key specs.
SecretKey dskey = dkgen.generateKey();
byte[] draw = dskey.getEncoded();
 
Cipher dcipher = Cipher.getInstance("AES");
 
SecretKeySpec dskeySpec = setKey(draw);
 
dcipher.init(Cipher.DECRYPT_MODE, dskeySpec);
 
byte[] decrypted = hex2byte(randomVal);
 
String tempString = new String(decrypted);
System.out.println("tempString is "+tempString);
 
byte[] original = dcipher.doFinal(decrypted);
String originalString = new String(original);
 
System.out.println("originalString is "+originalString);
}
    

If both encryption and decryption operation performed in the main() block it works fine. If we tempt to invoke on different methods it creates such a problem.

[K Aditi]

ecipher.init(Cipher.ENCRYPT_MODE, skeySpec);

dcipher.init(Cipher.DECRYPT_MODE, dskeySpec);

I think you are using two different keys for encryption and decryption.
AES is a symmetric cryptography algorithm, which means that encryption and decryption processes use a single key which has to be shared beforehand between both the parties.

[Ulf Dittmer]

Are you certain that hex2byte precisely reverses the effect of asHex?