c# Расшифровываем файл, зашифрованный ключом из сертификата x509 / x509 Certificate Decrypt file

public static MemoryStream DecryptFile(string dumpFilePath, RSACryptoServiceProvider rsaKey)
        {
            MemoryStream outFs = new MemoryStream();
            using (AesManaged aesManaged = new AesManaged())
            {
                aesManaged.KeySize = 256;
                aesManaged.BlockSize = 128;
                aesManaged.Mode = CipherMode.CBC;
                byte[] LenK = new byte[4];
                byte[] LenIV = new byte[4];
                using (FileStream inFs = new FileStream(dumpFilePath, FileMode.Open))
                {
                    inFs.Seek(0, SeekOrigin.Begin);
                    inFs.Seek(0, SeekOrigin.Begin);
                    inFs.Read(LenK, 0, 3);
                    inFs.Seek(4, SeekOrigin.Begin);
                    inFs.Read(LenIV, 0, 3);
                    int lenK = BitConverter.ToInt32(LenK, 0);
                    int lenIV = BitConverter.ToInt32(LenIV, 0);
                    int startC = lenK + lenIV + 8;
                    int lenC = (int)inFs.Length - startC;
                    byte[] KeyEncrypted = new byte[lenK];
                    byte[] IV = new byte[lenIV];
                    inFs.Seek(8, SeekOrigin.Begin);
                    inFs.Read(KeyEncrypted, 0, lenK);
                    inFs.Seek(8 + lenK, SeekOrigin.Begin);
                    inFs.Read(IV, 0, lenIV);
                    byte[] KeyDecrypted = rsaKey.Decrypt(KeyEncrypted, false);
                    using (ICryptoTransform transform = aesManaged.CreateDecryptor(KeyDecrypted, IV))
                    {
                        int count = 0;
                        int offset = 0;
                        int blockSizeBytes = aesManaged.BlockSize / 8;
                        byte[] data = new byte[blockSizeBytes];
                        inFs.Seek(startC, SeekOrigin.Begin);
                        using (CryptoStream outStreamDecrypted = new CryptoStream(outFs, transform, CryptoStreamMode.Write))
                        {
                            do
                            {
                                count = inFs.Read(data, 0, blockSizeBytes);
                                offset += count;
                                outStreamDecrypted.Write(data, 0, count);
                            }
                            while (count > 0);

                            outStreamDecrypted.FlushFinalBlock();
                        }
                    }

                }

            }
            return outFs;

        }