asp.net - “无法解码 PKCS7SignedData 对象” - ASP.NET iTextSharp

Question

我正在尝试验证是否已使用 iTextSharp 从服务器端对 pdf 进行数字签名，但收到以下错误：代码上的“无法解码 PKCS7SignedData 对象”

PdfPKCS7 pk = af.VerifySignature(name);

这是功能：

public string verifyPdfSignature(string pdfFile)
    {
        var parser = new X509CertificateParser();

        PdfReader reader = new PdfReader(pdfFile);
        AcroFields af = reader.AcroFields;
        var names = af.GetSignatureNames();

        if (names.Count == 0)
        {
            return "No Signature present in pdf file.";
        }
        foreach (string name in names)
        {
            if (!af.SignatureCoversWholeDocument(name))
            {
                return (string.Format("The signature: {0} does not cover the whole document.", name));
            }
            PdfPKCS7 pk = af.VerifySignature(name);  //<--- ERROR HERE
            var cal = pk.SignDate;
            var pkc = pk.Certificates;

            if (!pk.Verify())
            {
                return "The signature could not be verified.";
            }
            if (!pk.VerifyTimestampImprint())
            {
                return "The signature timestamp could not be verified.";
            }
        }
        return "nothing";
    }