You want a simple way to authenticate the clients connecting to servers in a distributedsystem, but don’t need the complexity of something like SSL.
Simple but effective authentication can be performed by implementing a connectionhandshake using the hmac module. Here is sample code:
import hmacimport 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)
The general idea is that upon connection, the server presents the client with a messageof random bytes (returned by os.urandom(), in this case). The client and server bothcompute a cryptographic hash of the random data using hmac and a secret key knownonly to both ends. The client sends its computed digest back to the server, where it iscompared and used to decide whether or not to accept or reject the connection.Comparison of resulting digests should be performed using the hmac.compare_digest() function. This function has been written in a way that avoids timing-analysis-based attacks and should be used instead of a normal comparison operator (==).To use these functions, you would incorporate them into existing networking or mes‐saging code. For example, with sockets, the server code might look something like this:
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)...
A common use of hmac authentication is in internal messaging systems and interprocesscommunication. For example, if you are writing a system that involves multiple pro‐cesses communicating across a cluster of machines, you can use this approach to makesure that only allowed processes are allowed to connect to one another. In fact, HMAC-based authentication is used internally by the multiprocessing library when it sets upcommunication with subprocesses.It’s important to stress that authenticating a connection is not the same as encryption.Subsequent communication on an authenticated connection is sent in the clear, andwould be visible to anyone inclined to sniff the traffic (although the secret key knownto both sides is never transmitted).The authentication algorithm used by hmac is based on cryptographic hashing functions,such as MD5 and SHA-1, and is described in detail in IETF RFC 2104.