Finding a Shortest Vector in a Two-Dimensional Lattice Modulo m

We find the shortest non-zero vector in the lattice of all integer multiples of the vector (a, b) modulo m, for given integers 0 < a, b < m. We reduce the problem to the computation of a Minkowski-reduced basis for a planar lattice and thereby show that the problem can be solved in O(log m(log log m)2) bit operations.