# Is it hard to compute $g^{ab}$ when given $(g, g^a, g^b, \frac{a}{b})$?

We know that the CDH problem, computing $$g^{ab}$$ from given $$(g, g^a, g^b)\in\mathbb Z_p^3$$, is hard. Is it still hard with an auxilary information $$\frac{a}{b}\bmod q$$ (where both $$p$$ and $$q$$ are large primes with $$q|p-1$$, and $$g$$ is a generator with order $$q$$) ?

It is indeed a hard problem - in fact, it is at least as hard as the square Diffie-Hellman problem (SDH), which states that given $$(g,g^a)$$, it is infeasible to compute $$g^{a^2}$$. It is a standard and well-studied assumption, and it can be reduced to CDH (correcting a previous version of this answer where I said it does not - I was confusing with the decisional version for which no such reduction is known).
Intuition: intuitively, SDH is exactly a CDH instance together with the constraint $$a/b=1$$. Hence, it is a particular case of the problem you consider, where $$a/b$$ is known.
Reduction: given an algorithm $$A$$ solving your problem, here is how you solve an SDH instance: on input $$g,g^a$$, pick a random exponent $$\lambda$$, and compute $$g^{\lambda^{-1}a^2} \gets A(g,g^a,g^{\lambda^{-1}a},\lambda)$$. I let you check that the input to $$A$$ is indeed distributed as a random instance of your problem. Then, compute $$(g^{\lambda^{-1}a^2})^{\lambda} = g^{a^2}$$.
For the reduction from SDH to CDH, it's a standard one; the trick is to use the identity $$(x+y)\cdot (x-y) = x^2-y^2$$. Setting $$x = (a+b)/2$$ and $$y = (a-b)/2$$ gives $$ab = ((a+b)^2-(a-b)^2)/4$$, from which the reduction to CDH is straightforward, with two calls to an SDH oracle.