byte[] data = new byte[256];
            data[0] = 0xDE;
            data[1] = 0xAD;
            data[2] = 0xBE;
            data[3] = 0xEF;
            //... остальное 0

                  var msgHandle = Crypt32.CryptMsgOpenToEncode(CryptEncoding.PKCS_7_OR_X509_ASN_ENCODING,
                                                                    CryptMSGOpenFlags.CMSG_EMPTY_FLAG,
                                                                    CryptMSGType.CMSG_ENVELOPED,
                                                                    encodeDataHandle,
                                                                    contentTypePtr,
                                                                    IntPtr.Zero);
                    byte[] encodedContent;
                    // see RFC 2315 <#section-8>. Data content type RSA Data must be OCTET encoded string
                    // Пришлось ставить подпорку
                    if (Environment.OSVersion.Platform != PlatformID.Unix // Вот тут
                        && 
                        string.Compare(this.ContentInfo.ContentType, PartsOid.Data, StringComparison.OrdinalIgnoreCase) == 0)
                    {
                        byte[] content = this.ContentInfo.Content;
                        var dataBlob = new CRYPTOAPI_BLOB();

                        var handle = GCHandle.Alloc(content, GCHandleType.Pinned);
                        try
                        {
                            dataBlob.cbData = (uint)content.Length;
                            dataBlob.pbData = handle.AddrOfPinnedObject();

// кодируем посредством CryptEncodeObject
                            EncodeObject(EncodeObjectType.X509_OCTET_STRING,
                                                        dataBlob, 
                                                        out encodedContent);
                        }
                        finally
                        {
                            handle.Free();
                        }
                    }
                    else
                    {
                        encodedContent = this.ContentInfo.Content;
                    }

                    if (encodedContent.Length > 0)
                    {
                        Crypt32.CryptMsgUpdate(_cryptMsgHandle, encodedContent, (uint)encodedContent.Length, true);
                    }