Редактировать цифровые подписи в PNG через Java

Signature signature = new Signature("sampleSigned.pdf");
try 
{
    ImageSearchOptions options = new ImageSearchOptions();

    // search for image signatures in document
    List signatures = signature.search(ImageSignature.class,options);
    if (signatures.size() > 0)
    {
        ImageSignature imageSignature = signatures.get(0);
        imageSignature.setLeft(100);
        imageSignature.setTop(100);
        boolean result = signature.update("sampleSigned-output.png",imageSignature);
        if (result)
        {
            System.out.print("Image signature at location " + imageSignature.getLeft() + "x" + imageSignature.getTop() + " and Size " + imageSignature.getSize() + " was updated in the document [" + fileName + ".");
        }
        else
        {
            System.out.print("Signature was not updated in the document! Signature at location " + imageSignature.getLeft() + "x" + imageSignature.getTop() + " and Size " + imageSignature.getSize() + " was not found!");
        }
    }
} catch (Exception e) {
    throw new GroupDocsSignatureException(e.getMessage());
}