The IMSI is a 15-digit number, appox. 50 bits. I agree, this nowadays is not very secure against brute force attacks. I tried a lot to access other data on the SIM which requires PIN, e.g. the address book. This could provide more bits to the key. I just had no success yet to query the SIM phone book programmatically Has anybody tried this successfully? It could help a lot. On the other hand, some may find that 50 bits are enough for their data on the phone. The usual thief or finder would not try to crack it brute force. Those having real sensitive data should of course not protect data using a 15 digit number...