How to edit the appearance of an existing digital signature?

[Luiz Kobayashi <luiz.kobayashi (at) incor.usp.br>]

Hello all,

first of all, many thanks again to Paulo, who helped me a lot in my 
previous question.

My next question is: How to edit the appearance of an existing digital 
signature? Is this possible?

One thing that I want to do is to sign a pdf and display the digital 
signature in base64 in the n2 layer.

The way I thought to do was:
1) Sign the pdf as shown in the tutorial
2) Read the signed pdf file and retrieve the signature bytes (thanks to 
Paulo who showed me how to do it!)
3) Get the n2 layer of the signature and change it.

But when I try to do

PdfReader reader2 = new PdfReader(baos.toByteArray()); // baos is the 
byte array where the signed pdf is stored
PdfStamper stp2 = new PdfStamper(reader2, fout, '\0', true);
PdfSignatureAppearance sap2 = stp2.getSignatureAppearance();

I get a java.lang.NullPointerException whenever I try doing anything 
with sap2, like sap2.getLayer(2) or sap2.setLayer2Text().

So, I was wondering what would be the right way to do, if there is one.

Any help would be greatly appreciated.

Thanks in advance,

Luiz Kobayashi

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