1. A method for authenticating a user, said method comprising: obtaining a first digital representation of an iris of an eye of at least one authentic user; obtaining a second digital representation of at least one authentic overlay pattern; obtaining a third digital representation of a captured image of an iris of a user to be authenticated, where the iris of the user has been overlaid with a contact lens including a further overlay pattern thereby creating an image, in the third digital representation, comprising the iris of the user to be authenticated and the further overlay pattern; generating an expected digital representation of an iris of at least one authentic user, by combining the first digital representation with the second digital representation; attempting to authenticate the user by comparing the third digital representation with the expected digital representation; and generating, based on the comparing, an authentication signal indicative of whether authentication of the user has been successful.