Question

I am working on an android application, and I need to use encryption for one aspect of it. I am really indifferent to which algorithm I use (AES, DES, RSA, etc…). I am aware that Java has a crypto package, but I am not familiar with it at all. Can someone post an example on how to do an encrypt/decrypt function?

Answer 1

The java AES library has a flaw in it that allows, under the right circumstances, a listener to decrypt the packets sent. See Padding Oracle Exploit Tool vs Apache MyFaces.

That being said check out this SO question Java 256bit AES Encryption.

Bouncy Castle AES EXAMPLE stolen from: http://www.java2s.com/Code/Java/Security/EncryptionanddecryptionwithAESECBPKCS7Padding.htm

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class MainClass {
  public static void main(String[] args) throws Exception {
    Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());    
    byte[] input = "www.java2s.com".getBytes();
    byte[] keyBytes = new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 
                 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 
                 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 
                 0x15, 0x16, 0x17 };

    SecretKeySpec key = new SecretKeySpec(keyBytes, "AES");

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding", "BC");

    System.out.println(new String(input));

    // encryption pass
    cipher.init(Cipher.ENCRYPT_MODE, key);

    byte[] cipherText = new byte[cipher.getOutputSize(input.length)];
    int ctLength = cipher.update(input, 0, input.length, cipherText, 0);
    ctLength += cipher.doFinal(cipherText, ctLength);
    System.out.println(new String(cipherText));
    System.out.println(ctLength);

    // decryption pass
    cipher.init(Cipher.DECRYPT_MODE, key);
    byte[] plainText = new byte[cipher.getOutputSize(ctLength)];
    int ptLength = cipher.update(cipherText, 0, ctLength, plainText, 0);
    ptLength += cipher.doFinal(plainText, ptLength);
    System.out.println(new String(plainText));
    System.out.println(ptLength);
  }
}