Verification does not catch modification

[FastCatch]

We have been experimenting with itextsharp a lot lately and found that in certain cases it seems to fail to correctly verify modified PDF documents -- or (more likely) we do something the wrong way.

Attached, please find two files:

1. test_sign.pdf is a PDF file created and signed with itextsharp, verifies OK both in itextsharp and in Acrobat Reader;
2. test_modified.pdf is the same file modified by hand in a random byte.

test_modified.pdf fails verification in Acrobat Reader ("Signature is invalid", "Document has been altered or corrupted since it was signed") but our code below verifies it as OK.

 byte[] pdfcontent = File.ReadAllBytes(@"test_modified.pdf");

 PdfReader reader = new PdfReader(pdfcontent);

 AcroFields acrofields = reader.AcroFields;
 ArrayList namelist = acrofields.GetSignatureNames();
 bool IsModified = false;
 string[] names = (String[])namelist.ToArray(typeof(String));
 foreach ( string name in names ) {
         PdfPKCS7 pdfpkcs7 = acrofields.VerifySignature(name);
         if ( !pdfpkcs7.Verify() )
             IsModified = true;
 }

 Console.WriteLine(IsModified);

What is the right way to check message integrity? What do we do wrong?

Thx,

FC