We describe an efficient quantum algorithm for computing discrete logarithms in semigroups using Shors algorithms for period finding and discrete log as subroutines. Thus proposed cryptosystems based on the presumed hardness of discrete logarithms in semigroups are insecure against quantum attacks. In contrast, we show that some generalizations of the discrete log problem are hard in semigroups despite being easy in groups. We relate a shifted version of the discrete log problem in semigroups to the dihedral hidden subgroup problem, and we show that the constructive membership problem with respect to $k ge 2$ generators in a black-box abelian semigroup of order $N$ requires $tilde Theta(N^{frac{1}{2}-frac{1}{2k}})$ quantum queries.
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