import hmac
import os

def client_authenticate(connection, secret_key):
  '''
  Authenticate client to a remote service.
  connection represents a network connection.
  secret_key is a key known only to both client/server.
  '''
  message = connection.recv(32)
  hash = hmac.new(secret_key, message)
  digest = hash.digest()
  connection.send(digest)

def server_authenticate(connection, secret_key):
  '''
  Request client authentication.
  '''
  message = os.urandom(32)
  connection.send(message)
  hash = hmac.new(secret_key, message)
  digest = hash.digest()
  response = connection.recv(len(digest))
  return hmac.compare_digest(digest,response)

from socket import socket, AF_INET, SOCK_STREAM

secret_key = b'peekaboo'
def echo_handler(client_sock):
  if not server_authenticate(client_sock, secret_key):
    client_sock.close()
    return
  while True:

    msg = client_sock.recv(8192)
    if not msg:
      break
    client_sock.sendall(msg)

def echo_server(address):
  s = socket(AF_INET, SOCK_STREAM)
  s.bind(address)
  s.listen(5)
  while True:
    c,a = s.accept()
    echo_handler(c)

echo_server(('', 18000))

Within a client, you would do this:

from socket import socket, AF_INET, SOCK_STREAM

secret_key = b'peekaboo'

s = socket(AF_INET, SOCK_STREAM)
s.connect(('localhost', 18000))
client_authenticate(s, secret_key)
s.send(b'Hello World')
resp = s.recv(1024)