You need to either add fetch=FetchType.EAGER inside your ManyToMany annotations to automatically pull back child entities:
@ManyToMany(fetch = FetchType.EAGER)
A better option would be to implement a spring transactionManager by adding the following to your spring configuration file:
<bean id="transactionManager"
class="org.springframework.orm.hibernate4.HibernateTransactionManager">
<property name="sessionFactory" ref="sessionFactory" />
</bean>
<tx:annotation-driven />
You can then add an @Transactional annotation to your authenticate method like so:
@Transactional
public Authentication authenticate(Authentication authentication)
This will then start a db transaction for the duration of the authenticate method allowing any lazy collection to be retrieved from the db as and when you try to use them.