AES key generation in Android

Question

Currently I am working in generating a key for AES encryption/decryption. The key is based on a password an a random salt per user. My first idea was to made a SecretKeyFactory with the algorithm "PBKDF2WithHmacSHA1". The problem is that Android currently does not support it.

Doing some search I found the answer from erickson recommended the use of that algorithm for the same purpose (AES 256 bit encryption). My question is how diffent would be the encryption process if I use "PBEWITHSHA256AND256BITAES-CBC-BC" instead of having "PBKDF2WithHmacSHA1"? There are any other idea of how to generate a key for AES in android in a secure way and not not just hashing the password with the salt fro the key (I don't think that approach to follow).

Answer 1

Try this:

public static String encrypt(String seed, String cleartext) throws Exception {
               byte[] rawKey = getRawKey(seed.getBytes("UTF-16"));
               byte[] result = encrypt(rawKey, cleartext.getBytes("UTF-16"));
               return toHex(result);
       }
       
       public static String decrypt(String seed, String encrypted) throws Exception {
               byte[] rawKey = getRawKey(seed.getBytes("UTF-16"));
               byte[] enc = toByte(encrypted);
               byte[] result = decrypt(rawKey, enc);
               return new String(result);
              }

       private static byte[] getRawKey(byte[] seed) throws Exception {
               KeyGenerator kgen = KeyGenerator.getInstance("AES");
               SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
               sr.setSeed(seed);
           kgen.init(128, sr); // 192 and 256 bits may not be available
           SecretKey skey = kgen.generateKey();
           byte[] raw = skey.getEncoded();
           return raw;
       }

       
       private static byte[] encrypt(byte[] raw, byte[] clear) throws Exception {
           SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
               Cipher cipher = Cipher.getInstance("AES");
           cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
           byte[] encrypted = cipher.doFinal(clear);
               return encrypted;
       }

       private static byte[] decrypt(byte[] raw, byte[] encrypted) throws Exception {
           SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
               Cipher cipher = Cipher.getInstance("AES");
           cipher.init(Cipher.DECRYPT_MODE, skeySpec);
           byte[] decrypted = cipher.doFinal(encrypted);
               return decrypted;
       }

       public static String toHex(String txt) {
               return toHex(txt.getBytes());
       }
       public static String fromHex(String hex) {
               return new String(toByte(hex));
       }
       
       public static byte[] toByte(String hexString) {
               int len = hexString.length()/2;
               byte[] result = new byte[len];
               for (int i = 0; i < len; i++)
                       result[i] = Integer.valueOf(hexString.substring(2*i, 2*i+2), 16).byteValue();
               return result;
       }

       public static String toHex(byte[] buf) {
               if (buf == null)
                       return "";
               StringBuffer result = new StringBuffer(2*buf.length);
               for (int i = 0; i < buf.length; i++) {
                       appendHex(result, buf[i]);
               }
               return result.toString();
       }
       private final static String HEX = "0123456789ABCDEF";
       private static void appendHex(StringBuffer sb, byte b) {
               sb.append(HEX.charAt((b>>4)&0x0f)).append(HEX.charAt(b&0x0f));
       }
       

Answer 2

As the name says, "PBEWITHSHA256AND256BITAES-CBC-BC" will use SHA256 as HMAC, instead if using SHA1. Because this is a different algorithm, it will generate a different Key for the chosen password.

If you en/decrypt only on android, this should be fine.