Question

Was just wondering if anybody can look at the following code and explain to me why the length variable returns 0:

textToBeEncrypted = Encoding.ASCII.GetString(buffer);
txtEncryptedText = AESEncryption(textToBeEncrypted, key, true);
byte[] encText = Encoding.ASCII.GetBytes(txtEncryptedText);

NetworkStream stream = s.GetStream();
stream.Write(encText, 0, PACKET_SIZE);

s.ReceiveTimeout = Timeout;

int length = stream.Read(buffer, 0, PACKET_SIZE);
if (length == PACKET_SIZE)
{
    string decText = Encoding.ASCII.GetString(encText);
    txtDecryptedText = AESDecryption(decText, key, true);
    buffer = Encoding.ASCII.GetBytes(txtDecryptedText);
    retval = Decode();
}            

After I’ve encoded everything using AES, I’m writing out 1366 bytes of data in encText (PACKET_SIZE is 1036). I get no complaints regarding the Send; the data is sent out happily. When it tries to read it back in, however, length is always 0, meaning I don’t get to enter the decode statement bracket. Any ideas? (retval is a string, before anyone asks)

Answer 1

If the length is zero from this:

int length = stream.Read(buffer, 0, PACKET_SIZE);

it means the other machine has closed their outbound socket (your inbound socket), and no more data is ever going to be available.

You should also be very careful about this:

if (length == PACKET_SIZE)
{...}

There is absolutely no guarantee about what you read. What you should do here is buffer the data until you have an entire message (frame), and then process what you buffered. In particular, if the other end sends less than PACKET_SIZE bytes, your code is guaranteed to do nothing. Even if the other end sent exactly PACKET_SIZE bytes, it would be pretty unlikely to arrive in exactly a single chunk of PACKET_SIZE bytes.

For example:

int length;
MemoryStream ms = new MemoryStream();
while((length = stream.Read(buffer, 0, buffer.Length)) > 0) {
    ms.Write(buffer, 0, length); // append what we just recieved

    // now: could check `ms` to see if we have a "frame" here...
}
//...or you could just process the entire recieved data here