如何仅加密/解密文件的前几个字节(其余的应未加密)

所以，我已经实现了C#加密/解密算法，它在加密/解密任何文件时工作得很好。我现在的问题是，我如何才能只加密（然后解密）文件的前几个（兆）字节。

例如：我有一个 2GB 的文件，我只想加密这个文件的 3MB。

我已经尝试了一些事情，但没有像我想要的那样工作。我尝试计算读取的字节数，如果读取的字节数超过限制（3MB），则停止加密并继续将正常（未加密的数据）写入文件。但是解密后，出现了"填充"异常，等等。另一个例子：我（使用这种方法）"成功"加密了 20kb 的 50kb .txt文件，但解密后，.txt文件的最后一行仍然有一些"奇怪"字符 - 所以这是行不通的（如果我想要 exaple 加密图像 - 解密后仍然"损坏"）。

对于加密，我使用此功能：

public static bool Encrypt(string inputFilePath, string outputfilePath, string EncryptionKey)
{
try
{
    using (FileStream fsInput = new FileStream(inputFilePath, FileMode.Open))
    {
        using (FileStream fsOutput = new FileStream(outputfilePath, FileMode.Create))
        {
            fsInput.Position = 0;                       
            using (Aes encryptor = Aes.Create())
            {
                Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
                encryptor.Key = pdb.GetBytes(32);
                encryptor.IV = pdb.GetBytes(16);
                using (CryptoStream cs = new CryptoStream(fsOutput, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    int bufferLen = 4096;
                    byte[] buffer = new byte[bufferLen];
                    int bytesRead;
                    int byteWriteCounter = 0;                           
                    do
                    {
                        bytesRead = fsInput.Read(buffer, 0, bufferLen);
                        //my method
                        if(byteWriteCounter <= 20000){ //if readed bytes <= 20kb
                            cs.Write(buffer, 0, bytesRead); // then encrypt 
                        }else{ // if bytes are over 20kb
                            fsOutput.Write(buffer, 0, bytesRead); //write normal (unecrypted)
                        }                           
                        byteWriteCounter += bytesRead;

                    } while (bytesRead != 0);
                    return true;
                }
            }
        }
    }
}
catch (SystemException se)
{
    Console.WriteLine(se);
    return false;
}
}

对于解密（我使用）是类似的功能/方法：

public static bool Decrypt(string inputFilePath, string outputfilePath, string EncryptionKey)
{
try
{
    using (FileStream fsInput = new FileStream(inputFilePath, FileMode.Open))
    {

        using (Aes encryptor = Aes.Create())
        {
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
            encryptor.Key = pdb.GetBytes(32);
            encryptor.IV = pdb.GetBytes(16);
            using (FileStream fsOutput = new FileStream(outputfilePath, FileMode.Create))
            {
                using (CryptoStream cs = new CryptoStream(fsOutput, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    int bufferLen = 4096;
                    byte[] buffer = new byte[bufferLen];
                    int bytesRead;
                    int byteWriteCounter = 0;
                    do
                    {
                        bytesRead = fsInput.Read(buffer, 0, bufferLen);
                        //my method
                        if(byteWriteCounter <= 20000){ //if readed bytes <= 20kb
                            cs.Write(buffer, 0, bytesRead); // then decrypt 
                        }else{ // if bytes are over 20kb
                            fsOutput.Write(buffer, 0, bytesRead); //write normal data
                        }
                        byteWriteCounter += bytesRead;
                    } while (bytesRead != 0);
                }
            }
        }                   
    }
    return true;
}
catch (SystemException s)
{
    Console.WriteLine(s);
    return false;
}
}