C#: how divide TCP input into multiple parts?

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C#

TcpClient

I want to divide the integrated input I get from a TCP connection(in bytes) into several different byte arrays.I have a server and client. server that encrypts a message by 3DES and encrypts the key and IV by RSA.
Then it concatenates them together like this (plaintext + ciphertext + key + IV) and sends it to the client.
The client must read from bytes 0 to 20 and save it as a plaintext, then read from bytes 21 to 129 and save it as the key and so on. Any Ideas on how to implement it in C#? I'd appreciate your help.

void ReadFromTCPClientStream(IAsyncResult iar)
        {
            TcpClient tcpClient;
            int nCountReadBytes = 0;
            try
            {
                tcpc = (TcpClient)iar.AsyncState;
                nCountReadBytes = tcpc.GetStream().EndRead(iar);

                if(nCountReadBytes == 0)
                {
                    printStatus("Client disconnected");
                    return;
                }

                readBuff = new byte[512];
                tcpc.GetStream().BeginRead(readBuff, 0, readBuff.Length,           ReadFromTCPClientStream, tcpClient);
                
            }
            catch (Exception exc)
            {
                MessageBox.Show(exc.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

i want to split readbuff to multiple byte arrays as i said.

What I have tried:

I think with the foreach it can read from one byte to another byte and then store it.But I don't know exactly how?