var rndKey;
function randomnum(num) {
    var m = "";
    for (var i = 0; i < num; i++)
    { var n = Math.floor(Math.random() * 10); m += n; }
     return m;
 }
 function GetKey() {
     var key = randomnum(4);
     rndKey = key;
     document.getElementById("CheckImgKey").value = key;
     document.getElementById("CheckImg").src = "Login/ImgKey.aspx?key=" + key;
 }
 window.document.write("<input tabindex='3' name='UserKey' type='text' id='UserKey' class='input_text' size='5' style='width:67px;height:16px;' maxlength='4' /><input id='CheckImgKey' name='CheckImgKey' type='hidden' />");
 window.document.write(" <img id='CheckImg' align='absmiddle' src='images/ImgKey.jpg' style='width:50px;height:22px;cursor:pointer;' onclick='GetKey();' alt='点击换一个'>");
 GetKey();
 $(function() { 
 window.setTimeout("ResetKey()", 1000); });
 function ResetKey() {
     $("#CheckImgKey").attr("value", rndKey); 
 }