We have already discussed a lot about the SIM Interposer – ‘Gevey’, which claims to unlock your iPhone 4 baseband 02.10.04 and 03.10.01. But why it wasn’t released earlier? How does it work? Is it legal to use Turbo SIM? A lot of questions will be answered here:
Why Gevey Suddenly Announced its Turbo SIM?
After Geohot released his first free soft-unlock, the manufacturer of these SIM hack discontinued their product as the production cost was quite high. But Gevey was waiting for the right time, they were reluctant to release it before as they risk losing a lot of their investment should the dev team come up with software unlock, before they were able to recover their cost.
How does Gevey SIM Technically Works?
SIM card holds critical information like the IMSI number, which is a unique code that corresponds to your account in the mobile carrier’s database. Let’s understands the anatomy of IMSI number by taking one example.
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. To make it more secure, now 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 allows 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 recognize the phone as a home user.
Why were they discontinued?
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.
Why now and How does it actually work to unlock iPhone 4?
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.
Step 1: You will need to use the SIM tray supplied and file your MicroSIM slightly to accommodate the EEPROM chip.
Step 2: Installing the SIM, only with something in between.
Step 3: 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.
Step 4: Dial 112 and hang up after the call is connected. The network issues a TMSI for your connection.
Step 5: 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
Step 6: 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.
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 iPhone 4 unlockers?
- It works if; Your network handles 112 calls properly according to the GSM standard and that 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 legitimate emergency number. Not an issue in China where the number is only used for informative purpose 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.
This article has originally been written by Dill Huang, a part time mobile technician. As always we will update you on every bits of iPhone 4 unlock coming-in. Just follow us on twitter and become our facebook fan.