Question

I’m making an app that encrypts some files. I want to use gnu’s cryptix library. It says it is no longer developed since 2005, but I guess it has everything I need… should I use something else?

And I have a question about encrypting a single file. Right now I do it with a loop like this:

for(int i=0; i+block_size < bdata.length; i += block_size)
    cipher.encryptBlock(bdata, i, cdata, i);

So my question is how to encrypt the last block that may not have the same size as the block_size. I was thinking maybe a should add some extra data to the last block, but than I don’t know how to decrypt that…

Answer 1

I would strongly suggest using AES encryption and it too comes with the JAVA SDK. Have a look at: Using AES with Java Technology which will give you some great example. To read up more on AES see: Advanced Encryption Standard – Wikipedia.

Never use your own encryption scheme or an older form of an encryption scheme. AES has been tried and tested by people with far greater knowledge in that field then us, so you know it will work. Where as with your own or an old encryption scheme we might miss a fatal loop hole that will leave our data open to attacks.

See this question here to see the difference in the encryption schemes: Comparison of DES, Triple DES, AES, blowfish encryption for data

Addendum:

AES in java will work flawlessly for 192 and 256bit keys but you will have to install the newer JCE Policy Files. See here and here. You should also place the files in your JDK or else it wont work when executed from your IDE.

Note: Make sure you download the correct JCE policy files, depending on your Java version i.e 1.4, 1.5 1.6 or 7.

However if you use 128bit keys no need to install the newer JCE files.

Here is a template of some secure AES usage in java it use CBC/AES/PKCS5Padding and a random IV using RandomSecure.

Note you need both the key and IV for decrypting:

import java.io.UnsupportedEncodingException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import javax.crypto.*;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

/**
 * This program generates a AES key, retrieves its raw bytes, and then
 * reinstantiates a AES key from the key bytes. The reinstantiated key is used
 * to initialize a AES cipher for encryption and decryption.
 */
public class AES {

    /**
     * Encrypt a sample message using AES in CBC mode with a random IV genrated
     * using SecyreRandom.
     *
     */
    public static void main(String[] args) {
        try {
            String message = "This string contains a secret message.";
            System.out.println("Plaintext: " + message + "\n");

            // generate a key
            KeyGenerator keygen = KeyGenerator.getInstance("AES");
            keygen.init(128);  // To use 256 bit keys, you need the "unlimited strength" encryption policy files from Sun.
            byte[] key = keygen.generateKey().getEncoded();
            SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

            // build the initialization vector (randomly).
            SecureRandom random = new SecureRandom();
            byte iv[] = new byte[16];//generate random 16 byte IV AES is always 16bytes
            random.nextBytes(iv);
            IvParameterSpec ivspec = new IvParameterSpec(iv);

            // initialize the cipher for encrypt mode
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            cipher.init(Cipher.ENCRYPT_MODE, skeySpec, ivspec);

            System.out.println("Key: " + new String(key, "utf-8") + " This is important when decrypting");
            System.out.println("IV: " + new String(iv, "utf-8") + " This is important when decrypting");
            System.out.println();

            // encrypt the message
            byte[] encrypted = cipher.doFinal(message.getBytes());
            System.out.println("Ciphertext: " + asHex(encrypted) + "\n");

            // reinitialize the cipher for decryption
            cipher.init(Cipher.DECRYPT_MODE, skeySpec, ivspec);

            // decrypt the message
            byte[] decrypted = cipher.doFinal(encrypted);
            System.out.println("Plaintext: " + new String(decrypted) + "\n");
        } catch (IllegalBlockSizeException | BadPaddingException | UnsupportedEncodingException | InvalidKeyException | InvalidAlgorithmParameterException | NoSuchPaddingException | NoSuchAlgorithmException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * Turns array of bytes into string
     *
     * @param buf   Array of bytes to convert to hex string
     * @return  Generated hex string
     */
    public static String asHex(byte buf[]) {
        StringBuilder strbuf = new StringBuilder(buf.length * 2);
        int i;
        for (i = 0; i < buf.length; i++) {
            if (((int) buf[i] & 0xff) < 0x10) {
                strbuf.append("0");
            }
            strbuf.append(Long.toString((int) buf[i] & 0xff, 16));
        }
        return strbuf.toString();
    }
}