function captcha() {
	this.value = '';
}
captcha.prototype.draw = function(n) {
	var div = document.getElementById('captcha_div');
	var range = '0123456789';
	var img_path = 'modules/mod_captcha/img/';
	var ext = '.png';
	for (var i=0; i<n; i++) {
		var s = Math.floor(Math.random()*10);
		this.value += s;
		var img_name = range.charAt(s);
		img_name = SHA1(img_name).substr(0,27); //27 e' la lungezza della sottostringa del digest sha1 usata come filename per l'immagine
		img_fullpath = img_path + img_name + ext;
		var img = document.createElement('IMG');
		img.src = img_fullpath;
		div.appendChild(img);
	}
}