Section: C.3.1 Profile 1: management of sequence numbers which are partly time-based
Content: Generation of sequence numbers: This follows the general scheme for the generation of sequence numbers specified in Annex C.1.1.1. The following parameter values are suggested for reference: Time unit of the clock: 1 second Length of in bits = 5. Length of SEQ2 in bits = n : 24 This means that GLC will wrap around after p = 2n = 224 seconds = 194 days. This ensures that most users will have become active at least once during this period. This implies a length of SEQ1 in bits = 19. Start conditions: Choose SQNHE = 0 for all users and GLC = 1. Arrival rate temporarily higher than clock rate: Choose D = 216. D may be chosen quite large as long as the conditions in C.1.1.1 (4)(ii) and (iii) are satisfied. Choosing D = 216 = 65536 means that the condition in C.1.1.1 (4)(i) is satisfied unless more than 65536 requests for batches arrive within over 18 hours which is practically impossible. Verification of sequence numbers in the USIM: This follows the handling of sequence numbers in the USIM specified in Annex C.2. Length of the array: a = 32. This satisfies the requirement in section 6.3.2 that the mechanism for the verification of sequence numbers shall ensure that a sequence number can still be accepted if it is among the last x sequence numbers generated. 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). We have > p, as required in note 7 of C.2.3. Age limit for sequence numbers: 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. To give an example: if the policy stipulates that authentication vectors older than x seconds shall be rejected then L has to be set to x as the time unit of the clock is 1 second. User anonymity: the value of SQN does not allow to trace the user over longer periods. Therefore, there may be no need to conceal SQN by an anonymity key as specified in section 6.3.