Write a C++ code to implement this

Suppose, Alice and Bob are business partners, who live in two different countries. However, they maintain a joint bank account, where they can do online money transactions. For each transaction, they share a Public Key with each other; similarly they have a Private Key by which they produce a Secret Key. This secret key is used for money transaction (as they seeret key is same in both end). You are an Intruder, who wants to create a Duplicate Secret Key, without informing them and do all the transactions. Now, write a program (in CIJava), how you can produce this Duplicate Secret key, when you only know the Public Key of Alice and Bob