CryptGetKeyParam(hPubKey, KP_CERTIFICATE, sb, ref pcbData, 0);
var pCertContext = CertCreateCertificateContext(X509_ASN_ENCODING, sb, (int)pcbData);
X509Certificate2 x509 = new X509Certificate2();
[b]x509[/b].Import(sb);
var isCapiValid = x509.Verify(); //проверка просто так

            PdfReader reader = null;
            FileStream fs = null;
            PdfStamper st = null;
            string destFNameTmp = "";
            string fPath = "";

            fPath = "F:\\Etalon.pdf";
            destFNameTmp = "F:\\Etalon-sign.pdf";

            reader = new PdfReader(fPath);
            fs = new FileStream(destFNameTmp, FileMode.Create, FileAccess.Write);
            st = PdfStamper.CreateSignature(reader, fs, '\0', null, true);
           
            AcroFields af = reader.AcroFields;
            List<string> signFieldNames = new List<string>();
            signFieldNames = af.GetSignatureNames();

            PdfSignatureAppearance sap = st.SignatureAppearance;

            sap.CertificationLevel = PdfSignatureAppearance.CERTIFIED_FORM_FILLING_AND_ANNOTATIONS;

            X509CertificateParser parser = new X509CertificateParser();
            Org.BouncyCastle.X509.X509Certificate[] chain =
                    new Org.BouncyCastle.X509.X509Certificate[] { parser.ReadCertificate(x509.RawData) };

            sap.Certificate = chain[0];
            sap.SignDate = DateTime.Now;
            sap.Reason = "Reason!";
            sap.Location = "Location!";

            // Выбираем подходящий тип фильтра
            PdfName filterName = new PdfName("CryptoPro PDF");
            // Создаем подпись
            PdfSignature dic = new PdfSignature(filterName, PdfName.ADBE_PKCS7_DETACHED);
            dic.Date = new PdfDate(DateTime.Now);
            dic.Name = "Sign-test";
            dic.Reason = "Reason!";
            dic.Location = "Location!";
            sap.CryptoDictionary = dic;
            int intCSize = 4000;
            Dictionary<PdfName, int> hashtable = new Dictionary<PdfName, int>();
            hashtable[PdfName.CONTENTS] = intCSize * 2 + 2;

            sap.PreClose(hashtable);
            Stream s = sap.GetRangeStream();

            MemoryStream ms = new MemoryStream();
            int read = 0;
            byte[] buff = new byte[8192];
            while ((read = s.Read(buff, 0, 8192)) > 0)
            {
                ms.Write(buff, 0, read);
            }
            ContentInfo contentInfo = new ContentInfo(ms.ToArray());
            SignedCms signedCms = new SignedCms(contentInfo, true);
            CmsSigner cmsSigner = new CmsSigner([b]x509[/b]); [b]//Кажется тут я делаю совершенно не верно, так как x509 не содержит ссылку на закрытый ключ на токене[/b]
            byte[] pk = null;

            try
            {
                // вываливаюсь на этой строке с ошибкой:
                // { "Ключ не существует.\r\n"}

                signedCms.ComputeSignature(cmsSigner, false); //если не ввести container password - исключение
                pk = signedCms.Encode();
            }
            catch (Exception Ex)
            {
            }

            byte[] outc = new byte[intCSize];
            PdfDictionary dic2 = new PdfDictionary();
            Array.Copy(pk, 0, outc, 0, pk.Length);

            dic2.Put(PdfName.CONTENTS, new PdfString(outc).SetHexWriting(true));
            sap.Close(dic2);
            fs.Close();
            st.Close();
            reader.Close();

signedCms.ComputeSignature(cmsSigner, false); //если не ввести container password - исключение

Gost3410_2012_256CryptoServiceProvider //и из него получается ссылка на 
certGOST = cspGost.ContainerCertificate;

[b]x509[/b].Import(sb);