Let K denote a number field, and G a finite abelian group. The ring of algebraic integers in K is denoted in this paper by O K , and A denotes any O K -order in K [G]. The paper describes an algorithm that explicitly computes the Picard group Pic(A), and solves the corresponding (refined) discrete logarithm problem. A tamely ramified extension L/K of prime degree l of an imaginary quadratic number field K is used as an example; the class of O L in Pic(O K [G]) can be numerically determined.