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Bob and Alice Have a Secret…

Prof Bill Buchanan OBE
3 min readDec 20, 2020

Bob has a secret and Alice has the same secret. Why can’t they create a shared encryption key based on their secrets? Well, they can do this with Password Authentication Key Exchange (PAKE). So, let’s look at a simple method using discrete logs, and then we will convert it to elliptic curve methods. While discrete logs have been used in the past for Diffie-Hellman key exchange methods, we are increasing moving towards elliptic curve implementations.

SPEKE (Simple Password Exponential Key Exchange) — Discrete Logs

SPEKE (Simple Password Exponential Key Exchange) supports password-authenticated key agreement. Bob and Alice share a secret password (π) and a shared prime number (p). This password then hashed and used to determine a generator (g):

g=H(π)² (mod p)

The square function of the hash makes sure that g is a generator for the prime number p. After this, we can use a standard Diffie-Hellman type exchange. For this, Alice generates a random number a and Bob generates a random number b. Alice then sends:

A=g^a (mod p)

and Bob sends:

B=g^b (mod p)

Alice computes the shared key as:

K1=B^a(mod p)

and Bob computes the shared key as:

K2=A^b (mod p)

The resulting key is:

K=B^a(modp)=(g^b(mod p))^a (mod p)=g^{ab}(mod p)

The code is [here]:

import sys
import hashlib
import random
from Crypto.Util.number import getPrime
from Crypto.Random import get_random_bytes
pi = "HellHe"
if (len(sys.argv)>1):
if (len(sys.argv)>2):
p = getPrime(primebits, randfunc=get_random_bytes)
g=pow(int(hashlib.sha1(pi.encode()).hexdigest(), 16),2,p)
a = random.randint(0, p-1)
b = random.randint(0, p-1)
Alice_to_send = pow(g,a,p)
Bob_to_send = pow(g,b,p)
AliceK= pow(Bob_to_send,a,p)
BobK= pow(Alice_to_send,b,p)
print ("Password: ",pi)
print ("g: ",g)…

Prof Bill Buchanan OBE

Professor of Cryptography. Serial innovator. Believer in fairness, justice & freedom. Based in Edinburgh. Old World Breaker. New World Creator. Building trust.