Question

Someone asked me to decrypt with PHP a string encrypted with the following Java Class.

public class CryptoLibrary {

private Cipher encryptCipher;
private sun.misc.BASE64Encoder encoder = new sun.misc.BASE64Encoder();

public CryptoLibrary() throws SecurityException{

    java.security.Security.addProvider(new com.sun.crypto.provider.SunJCE());

    char[] pass = "NNSHHETJKKSNKH".toCharArray();
    byte[] salt = {
    (byte) 0xa3, (byte) 0x21, (byte) 0x24, (byte) 0x2c,
    (byte) 0xf2, (byte) 0xd2, (byte) 0x3e, (byte) 0x19 };

    init(pass, salt, iterations);

}

public void init(char[] pass, byte[] salt, int iterations)throws SecurityException{

        PBEParameterSpec ps = new javax.crypto.spec.PBEParameterSpec(salt, 20);
        SecretKeyFactory kf = SecretKeyFactory.getInstance("PBEWithMD5AndDES");
        SecretKey k = kf.generateSecret(new javax.crypto.spec.PBEKeySpec(pass));

        encryptCipher = Cipher.getInstance("PBEWithMD5AndDES/CBC/PKCS5Padding");
        encryptCipher.init(Cipher.ENCRYPT_MODE, k, ps);
    }
}

public synchronized String encrypt(String str)  throws SecurityException{
    if(str!=null){
        byte[] utf8 = str.getBytes("UTF8");
        byte[] enc = encryptCipher.doFinal(utf8);
        return encoder.encode(enc);
    }
    else {
        return null;
    }
}
}

I don’t know any Java so I need some help to understand this encryption.

1) what is the meaning of this line?
PBEParameterSpec ps = new javax.crypto.spec.PBEParameterSpec(salt,20);

2) what value should I use for the first parameter of
string mcrypt_encrypt ( string $cipher , string $key , string $data , string $mode [, string $iv ] )

3) When should I use MD5 in my php script?

Answer 1

1) It creates the parameters for Password Based Encryption, the salt, which is included in the hash calculations, and the number of iterations that the hash method is executed (on it’s own output). It is used to defeat rainbow table attacks, basically an attacker has to go through the same number of iterations to check if the password is correct, and he cannot use a precalculated table because the salt will be different for each password (so you cannot see if somebody has the same password as another user).

2) MCRYPT_DES, and you will need MCRYPT_MODE_CBC for the mode, and PKCS#5 padding of course.

3) Only when you are absolutely sure that its weaknesses are not exposed or when absolutely required for compatibility. Fortunately, it is relatively secure for key derivation functions. Download a pbkdf1 method for PHP and put it in there – if not already included.