How did It Work?
SIM card holds many different types of information, but the part most involved with carrier lock is the IMSI number, which is a unique code that corresponds to your account in the mobile carrier's database.
A sample IMSI might look like this
310 150 987654321
The first two segments are known as Mobile Country Code (MCC) and Mobile Network Code (MNC) respectively, and in the example above the IMSI indicate the SIM is from USA (310) AT&T (150).
When the iPhone baseband is loaded into memory, it checks the MCC and MNC against its own network lock state stored in the seczone. If the combination is allowed, the cell radio is activated and vice versa.
The earliest iPhone baseband revisions only check IMSI twice following a restart, therefore it is very easy to send spoof information in order to bypass the check. Nevertheless, the baseband was soon updated to validate SIM more aggressively and the method soon became obsolete.
The Long Con
To guard against eavesdropping that plagued pre-GSM cellular networks, the initial connection to a network does not only involve IMSI, but a 4-byte TMSI to identify each handset before IMSI is sent processed.
The base station then send a 16-byte nonce to the handset, where is signed with a 128bit DES key (Ki, stored encrypted on the SIM) and sent back. The network checked the string against their database and allow that number to connect to the network.
You see, the IMSI is not essential; as long as your key is valid, you will be able to get service. This is exactly what the various SIM hacks where a fake IMSI is sent along with the correct key. The IMSI is redirected to another network and lost, and the signed nonce usually allows you to register (without cross-checking IMSI)
With this technique, "Data Roaming" needs to be enabled to allow data connection because the baseband is acting on the assumption that the phone is roaming however the network do correctly recognise the phone as a home user.
Demise
The result of SIM hacks has always been erratic since the outcome depends on the implementation of the hack as well as the policies of the network; and people were inundated with a long list of issues and makeshift solutions. e.g. Cellular data may and may not work depending on the chemistry; a common remedy is to periodically put the hacked SIM into an unlocked "nanny" phone to "keep the link healthy".
The use of SIM hacks fell sharply after iOS 2.2.1 update where the new baseband were made aware of the method and reject dodgy IMSIs, even the better designed interposers suffered frequent failures. It is about the same time software unlocks were developed and it pushed SIM hacks further into disuse. However the technology has been kept alive since a number of GSM handsets, mainly those coming from Japan, remains vulnerable.
Partial Revival
Apparently somebody figured out that while the i4 baseband has been patched to prevent fake IMSI from working, it is still possible to force activate the baseband using the emergency dialer.
After that, it is relatively the same trick all over again. Let's see what it does at each step.
You will need to use the SIM tray supplied and file your MicroSIM slightly to accommodate the EEPROM chip.
Installing the SIM, only with something in between.
The phone will search for signal, comes up with no service and finally settling on this "one bar" icon. Earlier hacks required the fake IMSI to be programmed manually however this device is obviously capable of rapidly cycling a list of IMSI until an accepted MCC/MNC combination is found.It may also spoof ICCID since the iOS is known to cross-reference
Dial 112 and hang up after the call is connected. The network issues a TMSI for your connection.
Toggle flight mode On/Off. What exactly happens is not too clear but apparently the interposer ROM block electrical connection to prevent the BB from detecting the fake IMSI
The signal bars appear, we are safe:-) That is after the network ignored the fake IMSI (which the phone has no knowledge of) but allowed the SIM onboard because it is able to validate that.
Why 112?
112 had a long history as an emergency number for practical reasons; back then when the GSM standard was being drafted in Europe, 112 was chosen to be the universal emergency number to call that can be called from any GSM phone, free of charge, with or without a SIM card on any compliant network.
However long before GSM service was established, 112 has been used to report landline faults in China; the coincidence made it unsuitable as a true emergency number. The network still connects 112 as an unbilled emergency call, only to play an automated message in both Chinese and English informing callers of the correct emergency numbers to dial
What does it mean to unlockers?
- It works if A.your network handles 112 calls properly according to the GSM standard; B.they are tolerant to TSMI spoofing and does not actively validate your SIM again for incoming calls.
- Unlike its ancestors, the i4 SIM interposer is not a drop-in-and-forget device. The exact precedure must be performed should the device restart, lose reception for an extended period of time or move to another PLMN. In all these situations the TMSI expires and has to be obtained again. Theoretically it is possible for a daemon to automate the process similar to ZeroG, but that only makes thing more convoluted.
- It is, without question, unethical or downright illegal to use the technique anywhere 112 is a legitmate emergency number. Not a huge issue in China where the number is only used for informative purposes and the networks cannot be bothered to fix the issue.
- All firmware/baseband combinations for the i4 up to iOS4.3 are vulnerable, however the exploit may be patched in any future software updates or via the carrier. If apple can influence providers to block Cydia it is not impossible for them to press them to fix the exploit. The only way to permanently unlock your baseband is NCK.
- SIM interposer should not harm your phone hardware, however your network could request IMEI and identify your device during the emergency call. Your identity cannot be faked and it is possible that they will ban your account. There is a reason why SIM cards remain legally the property of the service provider: you are not supposed to tamper with them without breaching contract.
- Notwithstanding all the problems, SIM interposer does not cause any battery drain since it is only active transiently, nor would it cause signal loss because it does not change cellular transmission other than the initial validation step.
An overpriced (US$50), unreliable and legally questionable mod; works for some but your mileage may vary; get it if you are desperate or wait for the 40-bit NCK hack by dev team.
