Let $γ$ be a generator of a cyclic group $G$ of order $n$. The least index of a self-mapping $f$ of $G$ is the index of the largest subgroup $U$ of $G$ such that $f(x)x^{-r}$ is constant on each coset of $U$ for some positive integer~$r$. We determine the index of the univariate Diffie-Hellman mapping $d(γ^a)=γ^{a^2}$, $a=0,1,\ldots,n-1$, and show that any mapping of small index coincides with~$d$ only on a small subset of $G$. Moreover, we prove similar results for the bivariate Diffie-Hellman mapping $D(γ^a,γ^b)=γ^{ab}$, $a,b=0,1,\ldots,n-1$. In the special case that $G$ is a subgroup of the multiplicative group of a finite field we present improvements.