Encode a String into a BigInteger then decode back to String

I found an answer that almost solves my problem: https://stackoverflow.com/a/5717191/1065546

This answer demonstrates how to encode a BigInteger into a String then back into a BigInteger using Base64 encodings which uses Apache commons-codec.

Is there a way of encoding technique/method for a String to a BigInteger then back to a String? if so would someone please explain how to use it?

      String s = "hello world";
      System.out.println(s);

      BigInteger encoded = new BigInteger( SOME ENCODING.(s));
      System.out.println(encoded);

      String decoded = new String(SOME DECODING.(encoded));
      System.out.println(decoded);

Print:

      hello world
      830750578058989483904581244
      hello world

(The output is just an example and hello world doesn't have to decode to that BigInteger)

EDIT

More specific:

I am writing a RSA algorithm and I need to convert a message into a BigInteger so that I can then encrypt the message with the public key (send message) and then decrypt the message with the private key and then convert the number back into a String.

I would like a method of conversion that could produce the smallest BigInteger as I was planning on using binary until I realised how ridiculouslybig the number would be.

Solution

I don't understand why you want to go through complicated methods, BigInteger already is compatible with String :

// test string
String text = "Hello world!";
System.out.println("Test string = " + text);

// convert to big integer
BigInteger bigInt = new BigInteger(text.getBytes());
System.out.println(bigInt.toString());

// convert back
String textBack = new String(bigInt.toByteArray());
System.out.println("And back = " + textBack);

** Edit **

But why do you need BigInteger while you can work directly with the bytes, like DNA said?