Method for mutual authentication between a user equipment and a communication network, said network comprising a mobility management entity and a subscriber server, said method comprising: - the user equipment receiving an authentication challenge comprising an authentication token (AUTN), which depends on a first index (idx Op,c) and on a first authentication message (MAC 1) which is computed by the subscriber server and depends on a first sequence number (SQN Op,c), - the user equipment computing a second authentication message (MAC 2) on the basis of a current sequence number (SQN c) referred to as the "second sequence number", which is stored by the user equipment, and a third authentication message (MAC 3) on the basis of a preceding sequence number (SQN c -1) corresponding to the value which the current sequence number had during a preceding session associated with a successful mutual authentication, - the user equipment checking that one of the conditions belonging to a set of conditions is met, said set of conditions comprising: - the first authentication message (MAC 1) received from the server is equivalent to the second authentication message computed by the user equipment indicating that the first sequence number is identical to the second sequence number (SQN c) stored in the user equipment, and - the first authentication message (MAC 1) received from the server is equivalent to the third authentication message (MAC 3) computed by the user equipment indicating that the first sequence number is identical to the preceding sequence number (SQN c -1) and the first index is strictly greater than a second index (idx c) stored in the user equipment, - the user equipment computing and sending, when one of the conditions is met, an authentication result and a fourth authentication message (MAC 4), which depends on the preceding sequence number, said authentication result being intended to be checked in order to authenticate the user equipment. - the user equipment receiving an authentication challenge comprising an authentication token (AUTN), which depends on a first index (idx Op,c) and on a first authentication message (MAC 1) which is computed by the subscriber server and depends on a first sequence number (SQN Op,c), - the user equipment computing a second authentication message (MAC 2) on the basis of a current sequence number (SQN c) referred to as the "second sequence number", which is stored by the user equipment, and a third authentication message (MAC 3) on the basis of a preceding sequence number (SQN c -1) corresponding to the value which the current sequence number had during a preceding session associated with a successful mutual authentication, - the user equipment checking that one of the conditions belonging to a set of conditions is met, said set of conditions comprising: - the first authentication message (MAC 1) received from the server is equivalent to the second authentication message computed by the user equipment indicating that the first sequence number is identical to the second sequence number (SQN c) stored in the user equipment, and - the first authentication message (MAC 1) received from the server is equivalent to the third authentication message (MAC 3) computed by the user equipment indicating that the first sequence number is identical to the preceding sequence number (SQN c -1) and the first index is strictly greater than a second index (idx c) stored in the user equipment, - the first authentication message (MAC 1) received from the server is equivalent to the second authentication message computed by the user equipment indicating that the first sequence number is identical to the second sequence number (SQN c) stored in the user equipment, and - the first authentication message (MAC 1) received from the server is equivalent to the third authentication message (MAC 3) computed by the user equipment indicating that the first sequence number is identical to the preceding sequence number (SQN c -1) and the first index is strictly greater than a second index (idx c) stored in the user equipment, - the user equipment computing and sending, when one of the conditions is met, an authentication result and a fourth authentication message (MAC 4), which depends on the preceding sequence number, said authentication result being intended to be checked in order to authenticate the user equipment.