An access manager manages access to a resource. At a first time, the access manager designates a variable attribute associated with a recurring public event as a shared secret between the access manager and a user. At a second time occurring after the first time, the access manager receives a shared key from the user. As received, the shared key is based on a value of the variable attribute associated with the recurring public event at a most recent recurrence of the recurring public event relative to the second time. The access manager evaluates the shared key. In response to the evaluation, the access manager grants or denies the user access to the resource.