Diffie-Hellman Key Exchange Protocol using the pyDHE Python Library

by | Jun 15, 2021 | Cryptography And Python, Encryption, Featured

import math
import socket
import sys

import pyDHE

HOST = "127.0.0.1"
PORT = 12345

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()

    while True:
        connection, address = s.accept()

        with connection:
            print("Connected by ", address)

            alice = pyDHE.new(14)
            bob_pub_key_bytes = connection.recv(2048)
            bob_pub_key = int.from_bytes(bob_pub_key_bytes, sys.byteorder, signed=False)
            shared_key = alice.update(bob_pub_key)
            alice_pub_key = alice.getPublicKey()
            alice_pub_key_bytes = alice_pub_key.to_bytes(math.ceil(alice_pub_key.bit_length()/8), sys.byteorder, signed=False)
            connection.sendall(alice_pub_key_bytes)
            print(shared_key)

Now, as per the IETF RFC 3526, there are a set of values for g and p. And, each set of values is given an id. In the group id 14, g is 2 and p is a 2048-bit prime number that has a specific value. Please refer to the IETF RFC for details.

Please note that the server and the client can exchange bytes. So, we need to convert the bytes into an integer and use it for calculation.

The client can compute the same shared secret in the following way:

import math
import socket
import sys

import pyDHE

HOST = "127.0.0.1"
PORT = 12345

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))

    bob = pyDHE.new(14)
    bob_pub_key = bob.getPublicKey()
    bob_pub_key_bytes = bob_pub_key.to_bytes(math.ceil(bob_pub_key.bit_length()/8), sys.byteorder, signed=False)
    s.sendall(bob_pub_key_bytes)
    alice_pub_key_bytes = s.recv(2048)
    alice_pub_key = int.from_bytes(alice_pub_key_bytes, sys.byteorder, signed=False)
    shared_key = bob.update(alice_pub_key)
    print(shared_key)

Here also, the client is receiving shared parameters from the server and using them to calculate the same shared secret. Interested readers, who want to know more about how to implement the Diffie-Hellman Key Exchange algorithm using Python without using any specific library, please refer to the following article: How to implement the Diffie-Hellman Key Exchange algorithm using Python?

I hope this helps. However, readers who want to know more about how different cryptographic algorithms work and how they are used in various secure network protocols can refer to the book “Cryptography And Public Key Infrastructure.”

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Amrita Mitra

Author

Ms. Amrita Mitra is an author, who has authored the books “Cryptography And Public Key Infrastructure“, “Web Application Vulnerabilities And Prevention“, “A Guide To Cyber Security” and “Phishing: Detection, Analysis And Prevention“. She is also the founder of Asigosec Technologies, the company that owns The Security Buddy.

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