 Sigma protocols exist for proving various statements, such as those pertaining to discrete logarithms.Using these proofs, the prover can not only prove the knowledge of the discrete logarithm, but also that the discrete logarithm is of a specific form.For instance, it is possible to prove that two logarithms of formula_61 and formula_62 with respect to bases formula_63 and formula_64 are equal or fulfill some other linear relation.