A method for mutual authentication between user equipment and a communications network. The network includes a mobility management entity and a home subscriber server. The method, implemented by the user equipment, includes: receiving an authentication challenge having an token based on a first index and a first authentication message calculated by the home subscriber server and based on a first sequence number; checking that a condition of a set is true, the set including: the first sequence number is the same as a second sequence number stored in the user equipment, and the first sequence number is the same as a preceding value of the second sequence number and the first index is higher than a second index stored in the client equipment; and calculating and sending, when a condition is true, an authentication result and an authentication message, based on the preceding value of the second sequence number.