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, wherein said attempting to authenticate the user comprises performing an initial comparison of the third digital representation with the expected digital representation, and in response to a determination, from said initial comparison, that the third digital representation and the expected digital representation do not match: modifying the expected digital representation to alter a relative rotational alignment between the representation of the at least one authentic overlay pattern and the expected digital representation of the iris of the at least one authentic user; and performing a further comparison between the modified expected digital representation and the third digital representation; and generating, based on the comparing, an authentication signal indicative of whether authentication of the user has been successful.