Validity unknown symbol is not being displayed on PDF

Question

I am using the below c# code (IText7 library) to set the appearance object to show the digital signature on my pdf.
    string destPdf = "D:\UnsignedPDF.pdf";
    string srcPdf = "D:\SignedPDF.pdf";

PdfReader reader = new PdfReader(srcPdf);
    PdfSigner signer = new PdfSigner(reader,
    new FileStream(destPdf, FileMode.Create),
    new StampingProperties());

PdfSignatureAppearance appearance = signer.GetSignatureAppearance();
    appearance.SetReason("My reson to sign...")
        .SetLocation("India")
        .SetPageRect(new Rectangle(36, 648, 200, 100))
        .SetPageNumber(1);
    signer.SetFieldName("MyFieldName");

A digital signature is printed on my pdf but not as what I expected, it should show a validity unknown symbol or signature valid symbol but it is not getting printed. I am getting it printed like below image.

but my expectation is to print it like below images:

mkl · Answer

A digital signature is printed on my pdf but not as what I expected, it should show a validity unknown symbol or signature valid symbol but it is not getting printed.

No, it shouldn't.
What you describe is a behavior that has been deprecated in 2003 when Adobe Acrobat 6 has been published. In particular that behavior has never been specified in the ISO PDF specification ISO 32000; on the contrary it has been forbidden in the update ISO 32000-2. For references read this answer.

With respect to iText:
iText(Sharp) 5.x, having inherited its signing API implementation base from iText 2.x/4.x, still offered a backward switch (PdfSignatureAppearance.Acro6Layers) which set to false allowed the creation of signature appearances supporting those in-document validation status marks.
iText 7.x, being a product started in the 2010s, got rid of this burden.
If you happen to work in a context which technologically got stuck in the early 2000s, and if you indeed are required to enable such (nowadays invalid) behavior, you may consider re-integrating the code for generating those pre-Acrobat6 layers in the current iText 7 sign module. If you compare the PdfSignatureAppearance.GetAppearance() source iText 5 and iText 7, you find quickly where with Acro6Layers == false additional layers where added.

Validity unknown symbol is not being displayed on PDF

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