Diffe-Hellman Key Exchange

Each party choose a separate secret number for themselves 

Both parties agreed on a common base number and a modulus.  The modulus must be a large prime number  the base must be a primitive root of the modulus (when the base is raised to a certain power successively, the reminder will cover all numbers within the modulus boundary.   

Each party calculate base raise to the power of their secret number then derive the modulus. Each party send the result to the other. 

Each party raise the reminder from the other party to the power of their secret number. The reminder is calculated and thus become the share secret.