An apparatus configured to perform or control obtaining or causing obtaining an image of at least a part of a surface of a document. The apparatus deriving or causing the derivation of a representation of the at least a part of the surface from the image. The representation comprising a set of values respectively representing a corresponding set of height levels. The apparatus generates or causes the generating of a string of characters based on the representation of the part of the surface. The string of characters comprising a first portion representative of the set of height levels of the corresponding sections of the part of the surface and a second portion generated based on a function of the representation and/or of the first portion of the string of characters. The apparatus associates or causes the associating of information representing the string of characters with identification information of the document.