TextMode="Password"

One may detect enter key like following (however it is usually not a
proper way, if one has to detect enter key like this, then there has
probably been some mistake in document/object model).

<asp:TextBox runat="server" id="txtSampleTextBox"
onkeypress="KeyPressHandler(event)" />

//in javascript
   function KeyPressHandler(e)
   {
       var keynum;
       if(window.event) // IE
       {
           keynum = e.keyCode;
       }
       else if(e.which) // Netscape/Firefox/Opera
       {
           keynum = e.which;
       }
       if(keynum == 13)
       {
           // Ok this is 'Enter' key, do here whatever required for
'Enter' (perhaps call OnEnter() !! )
       }
   }

You can hide the password by setting the TextMode property of the
password text box to Password.