itext signing pdf - pdf

Question

I'm currently developing an ERP application which uses iText for creating and signing PDF files. The idea is that the app can generate a PDF file with a bill, then use PdfStamper or any other class to sign it with a digital signature. Here's my code:
CREATING AND EDITING THE BILL
File f1 = null;
f1 = new File("myFilePath");
f1.delete();
if ((f1 != null) && (f1.createNewFile()))
{
//Here I call the procedure that creates the bill
Document document = new Document();
PdfWriter writer = PdfWriter.getInstance(document, new FileOutputStream(f1));
PdfFormField field = PdfFormField.createSignature(writer);
PdfSigner pdfSigner = new PdfSigner();
pdfSigner.setFileIn(f1.getAbsolutePath());
pdfSigner.setFileKey("myKeyPath.pfx");
pdfSigner.setFileKeyPassword("myPassword");
pdfSigner.setFileOut("myOutputPath");
if (pdfGenerator.factura(writer, pdfSigner, document))
{
//Here I show the File to the user
}
}
else
{
//Here I show an error
}
PROCEDURE "factura"
public boolean factura (PdfWriter writer, PdfSigner signer, Document document) throws NullPointerException
{
try
{
//Here I set a PdfPageEvent before attaching it to the PdfWriter
writer.setPageEvent(myPdfPageEvent);
document.open();
//Here I manipulate the Document to generate the bill
signer.signPdf();
document.close();
return true;
}
//catch 4 or 5 different types of exceptions and return false if needed
}
CLASS PdfSigner
public class PdfSigner
{
private String fileKey = null;
private String fileKeyPassword = null;
private String fileIn = null;
private String fileOut = null;
public PdfSigner() {}
public boolean signPdf() throws IOException, DocumentException, Exception
{
if (fileKey == null || fileKeyPassword == null || fileIn == null || fileOut == null) return false;
try
{
KeyStore ks = KeyStore.getInstance("pkcs12");
ks.load(new FileInputStream(fileKey), fileKeyPassword.toCharArray());
String alias = (String) ks.aliases().nextElement();
PrivateKey key = (PrivateKey) ks.getKey(alias, fileKeyPassword.toCharArray());
Certificate[] chain = ks.getCertificateChain(alias);
//BOOOOOM!
PdfReader pdfReader = new PdfReader((new File(fileIn)).getAbsolutePath());
FileOutputStream outputFile = new FileOutputStream(fileOut);
PdfStamper pdfStamper = PdfStamper.createSignature(pdfReader, outputFile, '?');
PdfSignatureAppearance sap = pdfStamper.getSignatureAppearance();
sap.setCrypto(key, chain, null, PdfSignatureAppearance.SELF_SIGNED);
sap.setVisibleSignature(new Rectangle(10, 10, 50, 30), 1, "sign_rbl");
pdfStamper.close();
return true;
}
catch (Exception key)
{
throw new Exception(key);
}
}
//getters and setters
}
Well, this is wrong, but I don't know where does it fail. If I try to run it, it usually throw an exception in the line that I've marked with a "BOOOOM!", before setting the PdfReader. But if I try to sign it outside the procedure "factura", after closing the document, the exception is usually thrown almost in the last line, when I close the PdfStamper. In both cases, the cause is always the same: "PDF header signature not found."
Anyone has an idea of what's happening? I am 99% sure the paths I'm giving to the program are right, and the digital signature and password are also right...
Thanks
PS: I swear I've tried to find a solution among the multiple answers in this page, but none of them proved to be of any use to me