Finding the private key in Diffie Hellman Lets say there are 2 parties X and Y. They have agreed on prime number P and generator g. X uses the secret key K.
After the key exchange they calculated the common secret key g^4.

What are the 2 possible values for the secret key of Y?
That the parties have agreed on a DH secret means the following:
$$g^4=(g^K)^Y=g^{KY}\pmod P$$
Now we can just look at the exponents:
$$4=KY\pmod{P-1}$$
Which may or may not have the unique solution for $Y$:
$$Y=4\cdot K^{-1}\pmod{P-1}$$

That is if $\gcd(K,P-1)&gt;1$ and we know that $K^{-1}$ must exist, then it will have multiple possible values. The precise values depend on $P$ and $K$.