A method of authenticating access to a service comprises: a) receiving at a mobile terminal, over a bi-directional near-field communication channel between the mobile terminal and a browser, at least part of the identifier of a service; b) comparing, at the mobile terminal, at least part of the identifier received at the mobile terminal with a set of identifiers stored in the mobile device; and c) authenticating access to the service on the basis of whether at least part of the identifier received at the mobile terminal matches an identifier in the set. The mobile terminal may stored a set of URLs, and may compare a received URL (or part URL) with the set of stored URLs. It may generate an alert to the user if at least part of the URL received at the mobile terminal does not match a stored URL. User names and keys are not required to be stored on the web-browser, so the web-browser does not need to maintain a password database. This improves security, since a password database would be vulnerable to malicious code.