Zombie_Killer, разбирайтесь.
<input type="password" id="input" placeholder="Пароль: open"/>
<div id="box" style="display:none">
Very important infortation.
</div>
<script>document.getElementById('input').addEventListener('keyup',function(){
if(this.value=='open')
document.getElementById('box').style.display='block';
});</script>