Diffie-Hellman key exchange

diffiehellman.py

#!/bin/python
# @itseco
# https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
import sys

alice_private_key = int(sys.argv[1]) # should be large enough to be secure
bob_private_key = int(sys.argv[2]) # should be large enough to be secure
modulus_prime = 17 # should be large enough to be secure (2048 bits minimum)
base_generator = 3

def public_key(base_generator, modulus_prime, priv, name):
    public = (base_generator ** priv) % modulus_prime
    print "%s public key = %d**%d %% %d = %d" % (name, base_generator, priv, modulus_prime, public)
    return public

def shared_key(pub, priv, name):
    shared_key = (pub ** priv) % modulus_prime
    print "%s shared key = %d ** %d %% %d = %d" % (name, pub, priv, modulus_prime, shared_key)
    return shared_key

def separator(value="mid"):
    repeat = 35; char = "-"; new_line = "\n";
    position = {
        "top": char * repeat + new_line,
        "mid": char * repeat,
        "bot": new_line + char * repeat,
    }
    print position[value]

print "Modulus (prime): %d" % modulus_prime
print "Base (primitive root of Modulus): %d" % base_generator
print "Alice private key: %d" % alice_private_key
print "Bob private key: %d" % bob_private_key
separator("bot")

alice_public_key = public_key(base_generator, modulus_prime, alice_private_key, "alice")
bob_public_key = public_key(base_generator, modulus_prime, bob_private_key, "bob")
separator("mid")

alice_shared_key = shared_key(bob_public_key, alice_private_key, "alice")
bob_shared_key = shared_key(alice_public_key, bob_private_key, "bob")
separator("top")

print "Alice shared key: %d" % alice_shared_key
print "Bob shared key: %d" % bob_shared_key

"""
root@kali:~# python diffiehellman.py 999 100
Modulus (prime): 17
Base (primitive root of Modulus): 3
Alice private key: 999
Bob private key: 100

-----------------------------------
alice public key = 3**999 % 17 = 11
bob public key = 3**100 % 17 = 13
-----------------------------------
alice shared key = 13 ** 999 % 17 = 4
bob shared key = 11 ** 100 % 17 = 4
-----------------------------------

Alice shared key: 4
Bob shared key: 4
"""