Fraud-detecting method for authenticating that an object is covered with human skin by means of a biometric system for analysing whether an object to be authenticated is covered with human skin, said system comprising a sensor of papillary prints, said sensor comprising: - a propagation medium comprising a surface with a receiving location intended to receive the object to be authenticated, - a light source configured to illuminate the receiving location of the surface through the propagation medium with incident light rays that have propagated through the propagation medium, - an imager configured to receive light rays that have propagated through the propagation medium from the surface, said object having a refractive index and being placed in a receiving location on the surface of the propagation medium of the sensor, said propagation medium having a refractive index n medium, the method comprising the following steps: - illuminating the receiving location of the surface through the propagation medium with incident light rays (20, 22) that have propagated through the propagation medium from the light source, said incident light rays (20, 22) illuminating the receiving location making to said surface a plurality of angles of incidence (α 1, α 2) encompassing at least one critical angle defined by the refractive index n medium of the propagation medium and by a refractive index expected for an authentic human skin, - receiving by means of the imager of light rays that have propagated through the propagation medium from the surface, said light rays originating from incident light rays making to the surface angles of incidence (α 1) smaller than or equal to a critical angle defined by the refractive index n medium of the propagation medium and by the refractive index of the object to be authenticated, and acquiring an image with the imager, said image having a spatial light distribution dependent at least on the refractive index of the object, - based on the acquired image, determining a light-intensity profile dependent on a distance to the light source, the light-intensity profile being representative of the spatial light distribution in the acquired image dependent on a distance to the light source, said light-intensity profile depending on the refractive index of the object, - determining at least one characteristic derived from the light-intensity profile and dependent on the refractive index of the object, - determining the correspondence between the object and an authentic human skin by comparing the characteristic derived from the light-intensity profile to a reference characteristic representative of a light-intensity profile corresponding to a spatial distribution expected for an authentic human skin, said reference characteristic depending on a refractive index expected for an authentic human skin, and - authenticating or not that the object is covered with human skin depending on the determined correspondence. - a propagation medium comprising a surface with a receiving location intended to receive the object to be authenticated, - a light source configured to illuminate the receiving location of the surface through the propagation medium with incident light rays that have propagated through the propagation medium, - an imager configured to receive light rays that have propagated through the propagation medium from the surface, - illuminating the receiving location of the surface through the propagation medium with incident light rays (20, 22) that have propagated through the propagation medium from the light source, said incident light rays (20, 22) illuminating the receiving location making to said surface a plurality of angles of incidence (α 1, α 2) encompassing at least one critical angle defined by the refractive index n medium of the propagation medium and by a refractive index expected for an authentic human skin, - receiving by means of the imager of light rays that have propagated through the propagation medium from the surface, said light rays originating from incident light rays making to the surface angles of incidence (α 1) smaller than or equal to a critical angle defined by the refractive index n medium of the propagation medium and by the refractive index of the object to be authenticated, and acquiring an image with the imager, said image having a spatial light distribution dependent at least on the refractive index of the object, - based on the acquired image, determining a light-intensity profile dependent on a distance to the light source, the light-intensity profile being representative of the spatial light distribution in the acquired image dependent on a distance to the light source, said light-intensity profile depending on the refractive index of the object, - determining at least one characteristic derived from the light-intensity profile and dependent on the refractive index of the object, - determining the correspondence between the object and an authentic human skin by comparing the characteristic derived from the light-intensity profile to a reference characteristic representative of a light-intensity profile corresponding to a spatial distribution expected for an authentic human skin, said reference characteristic depending on a refractive index expected for an authentic human skin, and - authenticating or not that the object is covered with human skin depending on the determined correspondence.