The invention relates to cardboard used in authenticity products, comprising a fiber matrix having two surfaces, whereby the second surface of the fiber layer has a layer of surface sizing containing a marking agent in a particle form, its particle size being smaller than 50 μm. By incorporating the marking agent into the surface sizing, the particles of the marking agent can be attached to the board, at the same time decreasing their total consumption by 80 to 90% compared with a case where they would be added to pulp. By using particles of a size less than 50 μm, a top free from streaks and roughness can be formed, which streaks might otherwise weaken the quality of the product.