I read in a document that for a given $n = p\times q$ ($p$, $q$ primes), if you know $p$ and $q$ then you can easily solve the discrete logarithm problem, i.e. for fixed $a,b$, you can find $x$ such that $$a^x = b \bmod n$$ But I don't see how this can be done... I tried to check if the Chinese reminder theorem could help me, and this reduces to find the discrete logarithm modulo $p$ and modulo $q$, but I still don't see how we can get this new discrete logarithm without bruteforcing (which is still exponential, even if you get a square root improvement over the naive bruteforce).
Thank you!