Simple (but good) Message Encryption?

Is XOR Encryption with the One-Time pad an option for you?
If the key you use is random, this encryption will be uncrack-able.
And the encrypted message isn't longer then the not-encrypted message.

Here's some pseudo-code:

var key = <random data>
var message;
function encryptMessage()
{
  message ^= key;
}

function decryptMessage()
{
   message ^= key;
}