Implementing-cryptography-using-python

212 Chapter 7 ■ Message Integrity
Sending Secure Messages Over IP Networks
Sockets are used to send messages across various networks that can be logical,
local, or external. Sockets were introduced back in 1971 with the birth of ARPANET
and then later became the Berkeley sockets, which is an API in the Berkeley
Software Distribution OS that was released in 1983. Network programming
started to boom in the 1990s as the use of the internet increased; the use of
client-server applications of many types became widespread. The socket API
in Python provides an interface to the Berkley sockets.
There are two types of sockets that you can experiment with. The first is a
Transmission Control Protocol (TCP). TCP is used to produce reliability and has
an in-order data delivery system. The second type of socket is the User Datagram
Protocol (UDP). These sockets lack reliability and their data can be out
of order; in fact, there is no guarantee that your data will reach the destination.
Depending on the type of data you are sending, one type of socket will have
advantages over the other. In the next two code samples provided, you will use
Python to create a server listening for UDP communications and create a client
server that will communicate with it.
Create a Server Socket
The following Python code will allow you to create a server that is listening
on port 13000. Once this code executes, it will launch a command window that
states Waiting to receive message. . . . See Figure 7.3.
Figure 7.3: Output of Server.py