Generated signed PDF crashes Adobe Reader but no other PDF reader

Question

We have code that modifies a PDF and then digitally signs the modified PDF. We use the LGPL version of the iTextSharp library (4.1.6) for digitally signing the PDFs.

public static Stream DigitallyCertifyPdfStream(Stream uncertifiedFileStream, CertificationBundle certificationBundle)
{
    using (var memoryStream = new MemoryStream())
    {
        var pdfReader = new PdfReader(uncertifiedFileStream);
        var signatureStamper = PdfStamper.CreateSignature(pdfReader, memoryStream, '\0', null);
        signatureStamper.SetEncryption(null, Encoding.UTF8.GetBytes(certificationBundle.Password), PdfWriter.ALLOW_PRINTING | PdfWriter.ALLOW_MODIFY_ANNOTATIONS, PdfWriter.STANDARD_ENCRYPTION_128);

        var signatureAppearance = signatureStamper.SignatureAppearance;
        signatureAppearance.Reason = "Approval of design";
        signatureAppearance.Location = "";

        var privateKey = certificationBundle.PrivateKey;
        var signingCertificates = new[] { certificationBundle.Certificate };
        signatureAppearance.SetCrypto(privateKey, signingCertificates, null, PdfSignatureAppearance.WINCER_SIGNED);

        pdfReader.Close();
        signatureStamper.Close();

        return new MemoryStream(memoryStream.ToArray());
    }
}

Here is a sample PDF which exhibits the issue. The PDF will open initially but then freeze and not be navigable. Whether or not you have our certificates to verify this signature installed, the issue seems to occur.

This issue does not seem to be happening consistently, and the problem only exists in Adobe Reader. Browser PDF viewers and Foxit Reader (which does signature verification) handle it just fine. Sometimes an error box will come up after a while that says something like "There was an error opening the stream."

Additionally of interest, on PDFs that have gone through this same digital signature process, we have observed the following in the Appearance Integrity Report

At the moment we are unsure if these are related to the problem. I mention them because they may be relevant.

The question, then, is why does this digitally signed PDF crash Adobe Reader and how can we remedy it?

Answer 1

Your PDF contains a broken image:

16 0 obj
<</Type/XObject/BitsPerComponent 8/Interpolate true/Width 736/ColorSpace/DeviceRGB/Filter/DCTDecode/Length 0/Height 1242/Subtype/Image>>stream

endstream
endobj 

This Image XObject claims to contain an RGB bitmap image (736x1242, 24bit) and at the same time is empty (Length 0). PDF viewers may fail if encountering such missing data (though it is impressive how Adobe Reader locks up for some time...).

Please check whether that stream already is broken in your source PDF.

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One issue by the way:

    pdfReader.Close();
    signatureStamper.Close();

You close the reader before closing the stamper. As the stamper may need to access the reader in the process of closing, this is a bad idea. Simply switch the order of the Close calls.

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By the way, your code produces a adbe.pkcs7.sha1 signature. This is a bad idea security-wise as this mechanism uses SHA1 for a first document hash no matter which security algorithms you use in you signature otherwise, and SHA1 is generally not considered safe anymore.