Discrete logarithm

For any integer $b$ and a primitive root $s$ of prime number $p$, we can find a unique exponent $i$ such that $b=s^i\text{ mod }p$ where $1\le i\le (p-1)$.

$i$ is called the discrete logarithm of $b$ for base $s$, $\text{mod }p$.

Computing discrete logarithms appears infeasible today.