Hide SSN input and show only last 4 digits

I have a classic ASP application.

There is a form that accepts SSN as input.

I want to show the last 4 digits only and still capture the full SSN value because I will use that for further processing.

I have used a hidden field to hold the value entered in the input text field and displaying the value in the SSN field as *****1111 for example.

It is working fine until here.

But, when a user hits backspace to correct number, the code acting crazy when the user holds down the backspace continuously to clear the field.

Please let me know if you have any better code or alternatives or solutions.
The following is my code

I have kept the hidden field as a regular text field for testing purpose.

<html>
<head>
<script type="text/javascript">
function hideinput(obj){
var v = obj.value;
var l = v.length;
var val='';
var evtobj=window.event? event : obj;
var unicode=evtobj.keyCode? evtobj.keyCode : evtobj.charCode;
var h = document.getElementById('htext');
var hv = h.value;
if (unicode == 8)
{
    if (l>=0)
        hv=hv.substr(0,hv.length-1)
}
else
{
hv+=v.charAt(l-1);
}
h.value=hv;
for(var i=0;i<l;i++)
{
if(i<5)
val+='*';
else
val+=v.charAt(i);
}
obj.value=val;
return true;
}
</script>
</head>
<body>
<form>
<input type="text" onkeyup="hideinput(this)"/>
<input type="text" id="htext"/>
</form>
</body>
</html>

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spraocsAsked:
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Paul MacDonaldDirector, Information SystemsCommented:
You could break the field up into three inputs for the three sections of the SSN.  Make the first two "password" style, so they're masked.  Leave the last one as a normal input control.
sammySeltzerCommented:
Why bother with javascrip?

Why not simply use this:

<input type="text" <%=="***-**-" + Right([YourSSN],4)%>">

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leakim971PluritechnicianCommented:
Following @paulmacd instruction : http://jsfiddle.net/RpRKM/
HTML code :
<input class="ssn" id="ssn1" name="ssn1" type="password" size="1" maxlength="7" onkeyup="if(this.value.length==7) document.getElementById('ssn2').focus();" /><input class="ssn" id="ssn2" name="ssn2" type="text" size="1" maxlength="4" />

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a bit of CSS :
¿#ssn1 {
    border: none;
    border-top: orange 1px solid;
    padding-left: 4px;
    padding-bottom: 4px;
    border-left: orange 1px solid;
    padding-top: 4px;
    border-bottom: orange 1px solid;
}
#ssn2 {
    border: none;
    border-right: orange 1px solid;
    padding-right: 4px;
    border-top: orange 1px solid;
    padding-bottom: 4px;
    padding-top: 4px;
    border-bottom: orange 1px solid;
}

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spraocsAuthor Commented:
Thanks to @Paulmacd for the initial lead to the solution. I was going to try that and then I saw the code and the link from @leakim971, which made it very easy to figure out and customize the solution completely.
Thanks a lot!
leakim971PluritechnicianCommented:
You're welcome
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