Text revelation effect
var decrypted = document.getElementById("decoded");
var encrypted = document.getElementById("encoded");

function startdecrypt() {
    // Original text, split into an array and reversed (for faster pop())
    var originalText = decrypted.textContent.split('').reverse();
    var decryptedText = "";
    var i = 0;

    decrypted.textContent = "";

    var shuffleInterval = setInterval(function(){

      // Generate random strings. You can modify the generator function range
      // (Math.random()*(to-from+1)+from);
      var shuffledText = '';
      var j = originalText.length;
      while(j--) shuffledText += String.fromCharCode((Math.random()*94+33) | 0);
      // You can also use this generator to use only the remaining letters
      // while(j--) shuffledText += originalText[(Math.random()*j) | 0];

      // On every 10 cycles, remove a character from the original text to the decoded text
      if(i++ % 10 === 0) decryptedText += originalText.pop();

      // Display
      decrypted.textContent = decryptedText;
      encrypted.textContent = shuffledText;

      // Stop when done
      if(!shuffledText.length) clearInterval(shuffleInterval);

    // 50ms looks more dramatic
    },50);
}

if (window.addEventListener) {
    window.addEventListener('load', startdecrypt, false); //W3C
} else {
    window.attachEvent('onload', startdecrypt); //IE
}


Changes


Everything in one function
Simplified the code
Using setInterval rather than setTimeout. This is more efficient since you don't create timers everytime
More drama by adding random string generator, instead of just your original set of letters. This adds more mystery to the next letters.
In addition to the code, a monospace font would be more dramatic. It avoids the to-fro movement of the decoding text.