Question

I have a big problem, I developed an app which uses encryption for send message to another device.

Here cipher class, I “steal” that but I don’t remember where:

public class AesCipher {
    public static String encrypt(String seed, String cleartext) throws Exception {
        byte[] rawKey = getRawKey(seed.getBytes());
        byte[] result = encrypt(rawKey, cleartext.getBytes());
        return toHex(result);
    }

    public static String decrypt(String seed, String encrypted) throws Exception {
        byte[] rawKey = getRawKey(seed.getBytes());
        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(256,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 {
        try{
            SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
            Cipher cipher = Cipher.getInstance("AES");
            cipher.init(Cipher.DECRYPT_MODE, skeySpec);
            byte[] decrypted = cipher.doFinal(encrypted);
            return decrypted;
        }
        catch (Exception e) {return encrypted;}
    }

    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));
    }
}

so, if i send a message to same device, my app work fine, but if i use different device versions (andorid2.3 vs android4.0), receiver cannot decrypt message..

searching around I found that the problem is SecureRandom which cannot garantees compatibility over different implementation. how I can resolve it?

Sorry for my english…

Answer 1

Yes, I’ve tried to get that terrible example removed, but to no avail.

You should try and to retrieve the bytes from a 2.3 device and use the resulting raw[] byte array to create a SecretKeySpec. You can use this SecretKeySpec directly as key to decrypt anything that you’ve encrypted on a 2.3 device.

Unfortunately if you’ve encrypted anything using a 4 device and “thrown away” the raw[] then your only option is to break "AES/ECB/PKCS5Padding" (the default when you use "AES"). You may be able to get a tiny bit of information from ECB mode as this is insecure as well, but otherwise it would come down to breaking AES ciphertext security – and nobody in this world is – as far as we know – able to do that.

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If you start off with a password yourself when creating code then you should first call a Password Based Key Derivation Function to create a key from a password. Java has PBKDF2 build in.