Section: C.3.2 Profile 2: management of sequence numbers which are not time-based
Content: Generation of sequence numbers: This follows the scheme for the generation of sequence numbers specified in Annex C.1.1.2. The following parameter values are suggested for reference: Length of in bits = 5. Start conditions: SQNHE = 0 for all users. Verification of sequence numbers in the USIM: Length of the array: a = 32 Protection against wrap around: Choose = 228. Choosing = 228 means that an attack to force the counter in the USIM to wrap around would require at least SEQmax/ = 215 > 32.000 successful authentications (cf. note 6 of C.2.3). Note 7 of Annex C.2.3 does not apply. Age limit for sequence numbers: There is no clock here. So, the “age” limit would be interpreted as the maximum allowed difference between SQNMS (see section 6.3) and the sequence number received. The use of such a limit is optional. The choice of a value for the parameter L affects only the USIM. It has no impact on the choice of other parameters and it entirely up to the operator, depending on his security policy. Therefore no particular value is suggested here. User anonymity: the value of SQN may allow to trace the user over longer periods. If this is a concern then SQN has to be concealed by an anonymity key as specified in section 6.3.