title: Java itext4.x digital signature
body: <p>I am using itext4 and I know this method of implementation  has been removed in new versions but I am able to generate pdf but its damaged.
  Appearance method setcrypto is used to sign.
  I added sign field while creating document.
  Later open same pdf and apply sign  logic as mentioned in itext in action 2nd edition page 397.</p>
  <p>First stamper.close give null pointer exception and next if i comment that line and generate pdf ,the pdf wont open or its damaged</p>
  <p>Please advise I am using itext 4.2.1 jar</p>