Started to play with the Galaxy S (GT-I9000) phone

For many years I'm on a more or less consistent hunt for finding a reasonably open and free mobile phone. This started in 2004 with OpenEZX, has continued with Openmoko, project gnufiish and has resulted in a bit of peeking and poking in the Palm Pre. However, none of those projects ever had the success I was hoping for:

  • OpenEZX was never really finished, and only for the 1st generation phones (A780) by the time they were long end of life
  • OpenMoko Neo1973 and FreeRunner were a great project, and they are still the most open+free mobile phones that ever existed. However, they're GPRS only and the hardware is even more outdated now then it was when we created it.
  • gnufiish was an attempt of running software from the Openmoko days (such as on some E-TEN glofiish phones. However, we never could make the SPI-based modem communication work from our re-engineered Linux driver :(
  • Palm Pre is an interesting device, in that Palm provides easy root access, does not attempt to lock the device down with cryptographic signatures and provides full recovery flashing tools by means of WebOS Doctor. But once again, the proprietary communication protocol with the 3G Modem was the big blocker item for using real custom software and not the WebOS stuff they ship.

So I've constantly been on the watch for new devices that are coming out. Most of the phones you can buy in recent years are either running proprietary software like Windows Mobile, Symbian, Apples iPhone-OSX - or they run Android but then use some integrated Qualcomm Smartphone-on-a-chip product. The problem with the latter (from a Free Software point of view) is that Qualcomm is very secretive about their products, does not provide any kind of public documentation, and the ever-increasing integration between application processor and baseband processor makes it more difficult to run custom software on them.

The Samsung Galaxy S (GT-I9000) seemed like a good candidate to me, for several reasons:

  • Samsung does not use cryptographic signature techniques and gaining root as well as flashing the AP software is relatively easy
  • The phone is based on a traditional separate application processor (AP) and baseband processor (BP) design. The AP is a Samsung S5PC110, the BP is some Qualcomm MSM6xxx.
  • High-end hardware, with the S5PC110 running at 1GHz and 512MB RAM
  • Samsung provides excellent "GPL source code offers" containing the Linux kernel used in their firmware - including detailed instructions in how to build it. Also, many of the drivers are included under GPL, such as drivers for all the integrated peripherals of the SoC, some custom components like the USB multiplexor ASIC, etc. as well as the driver for the dual-ported RAM between the AP and BP for the 3G Modem communication
  • The Android RIL shipped by Samsung contains lots of debugging/decoding/dumping code that can make reverse engineering the AP/BP protocol.

So right now I'm in the exploration phase, making myself familiar with the bootloader, the flashing process, the userspace ABI of the custom (GPL licensed) kernel drivers, etc. It's a fairly pleasant experience so far, and I now have a debootstrap'ed Debian lenny on an additional ext2 partition on the SD card. This provides me with an actually useful userland I can chroot() into, such as lsof, strace, ltrace, tcpdump, etc. to do some more exploration of the phone.

The only real ugliness on the software side so far is the use of proprietary Samsung filesystems (RFS/TFS4). The only reason those filesystems existed, as far as I can tell, was to run legacy filesystems like FAT on top of raw NAND or OneNAND flash. This is mainly necessary if you want to export e.g. a FAT partition via USB Mass Storage to a Windows PC. However, the GT-I9000 doesn't have any OneNAND, but only an internal moviNAND (basically a SD-Card in a BGA package that you can solder on the board). MMC/SD cards already include the wear leveling algorithm, so there is absolutely no point (from what I can tell) in running the RFS/TFS4 stack.

In fact, in several forums people are complaining about the slow I/O performance of the Galaxy S, and they have a much better performance when using ext2/ext3 directly on that moviNAND device.

Doing RFID related research and development again

More or less a bit surprising to me, I got again involved in RFID research, on which I hadn't really done much ever since my involvement in the OpenPCD and OpenPICC projects some five-to-four years ago.

It's a lot of fun, and I didn't seem to forget much. What really bothers me a bit is that the OpenPCD / librfid / OpenPCD integration never really was completed, and that libnfc doesn't work with OpenPCD. Let's hope I'll somehow find some time to change this. It just feels wrong that OpenPCD was the first hardware project created to encourage (security) research into RFID, and now all the current tools only run on the Proxmark or on proprietary readers...

Worlds first 20 minute voice call from a Free Software GSM stack on a phone

As Dieter Spaar has pointed out in a mailing list post on the OsmocomBB developer list, he has managed to get a first alpha version of TCH (Traffic Channel) code released, supporting the FR and EFR GSM codecs.

What this means in human readable language: He can actually make voice calls from a mobile phone that runs the Free Software OsmocomBB GSM stack on its baseband processor. This is a major milestone in the history of our project.

While Dieter has been working on the Layer1 TCH support and the setup of the voiceband path in the analog baseband chip (audio ADC/DAC), Andreas Eversberg has been quietly working on getting call control of Layer3 into a state where it can do all the signalling required for mobile-originated and mobile-terminated call.

Combining both of their work together, they have been able to make a 20 minute long voice call from a baseband processor running a Free Software GSM stack. For all we know, it is the first time anything remotely like this has been done using community-developed Free Software. Five years ago I would have thought it's impossible to pull this off with a small team of volunteers. I'm very happy to see that I was wrong, and we actually could do it. With less than half a dozen of developers, in less than nine months of unpaid, spare-time work.

Sure, the next weeks and months will be spent on bringing the code from alpha level to something more stable, fixing known issues and known bugs, etc. But I'm confident the biggest part of the work on the OsmocomBB stack is behind us. Big thanks to the developer team driving this project forward.

Wondermedia WM8505 Linux + u-boot source code

In recent months, a number of alleged GPL-violation reports regarding products (tablet computers, mini netbooks and the like) using the Wondermedia WM850x line of ARM SoCs. People have been contacting me, as I was working as VIA Open Source Liaison, and there is the general belief that VIA and Wondermedia Technology (WMT) are one company.

I had investigated this issue even before there were any reports, and I'd like to publicly state that:

  • Wondermedia is a separate company from VIA, with independent management, making their own business decisions. The 850x SoC development was started inside VIA, but is no longer part of VIA for a long time.
  • Any references to VIA in the source code or old data sheets date from that time before the SoC business became part of Wondermedia
  • I have had assurances from Wondermedia, even before there were any allegations, that similar to VIA they explicitly notify their customers about the GPL and always provide their SDK / BSP as full corresponding source code.
  • Effectively, this means that GPLv2 Section "3a" is used. WMT has provided the Linux and u-boot source code to its customers, and thus has no obligation under GPLv2 Section "3b" to provide it to anybody else (any 3rd party)
  • So, if you buy a product including a WMT SoC and u-boot/Linux, like always, GPL compliance of what has been shipped to you has to be assured by the manufacturer of the product, not the semiconductor maker

Notwithstanding all of the above, Wondermedia was willing to provide the Linux kernel and u-boot source code of their SDK to me, so I can share it with the community. As indicated, they're not legally required to do this and I'm happy they do it anyway to show their good intentions.

You can download the released source code from the ftp-server, more specifically here are the latest Linux kernel (modified 2.6.29 android derivative) and u-boot source code archives.

This software is provided without any kind of support. If you see some GPL related legal problems (i.e. you believe it is incomplete), don't hesitate to contact me. To the best of my knowledge WMT (basically a small hardware start-up with small software development team) has no resources to actively push any of this mainline.

Working on a document on smartphone hardware architecture

I've started to write upe some information on modern smartphone hardware architecture. It will be in a similar style to what I previously wrote on feature phones and gsm modem hardware, but with a specific focus on smpartphones, their multiple processors, memory sharing, AP/BP interface, audio architecture, etc.

I should have done this a long time ago. In fact, I think I should write more documents like that on various technical subjects. If you want to learn about low-level aspects of modern telephones, there is way too little published information out there.

Official wiki page on GSMTAP created

I've come up with GSMTAP about two years ago while working on airprobe. The goal was to have something similar to what radiotap does in the wifi world: A pseudo-header that adds additional information and context that is not present in the actual message.

Initially, GSMTAP was intended to be a separate link-layer type in the pcap file format, but this would preclude its use in real-time protocol analysis. So I modified it to be encapsulated in UDP packets, which are sent and received using normal UDP/IP sockets.

Over recent years, GSMTAP has not only been integrated into multiple programs of the airprobe project, but is also understood by wireshark. OpenBTS has also decided to adopt the format and can generate GSMTAP messages for debugging purposes.

After creating OsmocomBB, it was taught how to generate GSMTAP messages very quickly, too.

So by now, at least when it comes to Free Software, it is definitely the de-facto standard for capturing/transmitting and analyzing protocol messages from the GSM air interface.

However, until now, there has never been any official "homepage" of the GSMTAP header. This has changed now, the GSMTAP homepage is now part of the OsmocomBB wiki.

Playing more with Erlang

Last year I started to occasionally play with Erlang. People who know me as die-hard C coder who tries to avoid C++, Java and Python wherever possible will probably be surprised here now.

I have no intention of changing my general position on programming languages. I don't feel comfortable using something where I don't know and/or understand the immediate impact on how this code will be executed on the actual silicon.

However, if you have a need to play with anything that uses ASN.1, but particularly the aligned/unaligned PER encoding variants, then it is pretty clear that there is nothing available as Free Software that can compare to the Erlang asn1ct/asn1rt modules.

At that time last year I was doing some rapid prototyping with the RANAP protocol, and the progress was quite quick. I never had time to return to that project, so it (and my Erlang skills) were left dormant.

In recent weeks, I have picked Erlang up again - again to work on ASN.1 encoded messages: This time TCAP and MAP. While we still need the in-progress TCAP+MAP implementation in C for OsmoSGSN, there are other tasks at hand where an Erlang-based implementation might yield a much higher productivity.

So right now I'm working on a program that parses/decodes and iterates through every MAP component in a TCAP message and replaces certain fields, re-encodes the entire message and sends it off the wire. Once that is done, I think I'll actually try to do a more complete TCAP server and implement a simplistic HLR for OsmoSGSN testing.

On the recent news items about the homebrew IMSI-catcher for 1500 USD

Some news sites seem to do very limited research and present it as big news that you can now build an IMSI-Catcher for a budget of USD 1500, using OpenBTS and a URSP.

Let me bring some clarity into this situation:

  • Fundamentally, an IMSI-Catcher is nothing special but a GSM base station (BTS) that is configured to the network country code (NCC) and mobile network code (MNC) of a commercial network operator.
  • In GSM, the phone has no way to authenticate and thus verify the legitimacy of the mobile network. This is like a "rogue access point" in a open (unencrypted/unauthenticated) WiFi network.
  • Thus, anyone who has a device that can run as a GSM base station has the ability to run an IMSI catcher.
  • There are two Free Software / Open Source projects for running your own GSM network, both have first been published in 2008: OpenBTS and OpenBSC.
  • None of those two projects are intended to be used as an IMSI-Catcher but for legitimate operation of GSM networks. However, if a user choses to configure the NCC and MNC of a commercial operator and allow "unknown/unregistered/unprovisioned IMSIs (SIMs) on his network, he will effectively have an IMSI catcher.
  • Such operation is in violation of spectrum usage regulations, even if you have a valid test/experimental license, since that license does not permit you to use somebody else's NCC/MNC.
  • Furthermore, such operation is in violation of criminal law in most jurisdictions. In Germany there is a separate offense in the criminal code, called Paragraph 317 Stoerung von Telekommunikationsanlagen, combined with Paragraph 202b Abfangen von Daten.
  • Furthermore, there are certainly civil claims to be made by the affected operator (and its subscriber) against anyone who unlawfully operates such a fake base station
  • OpenBTS and OpenBSC, as well as the problems resulting from this fake base station attack have been covered in a variety of conference presentations from 2008 through today.
  • Thus, there is nothing new about what has been presented at Defcon 18

Also, the theoretic basics ow how to operate an IMSI catcher are nothing new either. There are even a number of patents covering IMSI catchers, the first that I know of has been patented by Rohde & Schwarz in 2003. Also, see this blog post by OpenBTS founder David Burgess on this topic.

So all that you always needed is a bit of hardware and software to send radio waves containing messages formatted in the way how they are described in the (equally public) GSM specifications as published by ETSI and 3GPP. Commercial, proprietary systems have existed for a decade. From 2008 on, there is some Free / Open Source Software to operate GSM networks. The situation remains unchanged in 2010.

So please, remember this the next time somebody is trying to tell you that this is the latest invention since sliced bread.

GSM Denial of Service by flooding BTS with RACH requests

At Blackhat US 2010, there was a Talk that (among other things) apparently included the subject of a RACH DoS on GSM base stations, implemented using my Layer1 of the OsmocomBB software.

As some news sites are covering this as "news": This vulnerability has been long known in the field and was - to the best of my knowledge - first demonstrated to a public audience by Dieter Spaar at the Deepsec 2009 conference in November 2009. You can get his slides.

The difficult part for many years has not been to know about the possibility of this weakness. Anyone who has read the GSM air interface specification will inevitably see that there is a limited number of RACH slots and a limited number of dedicated channels. Once you fill more RACH slots than the cell has dedicated channels, and you keep re-filling them at a higher rate than the cell can expire those dedicated channels, you have a DoS.

So rather, the difficult part was to implement it in practise, as traditionally all GSM baseband chipsets have been extremely closed, just like the very software (firmware) running on them. Today, starting from Q2/2010, it is very easy to do a proof-of-concept implementation, as we have created OsmocomBB: An Open Source baseband firmware.

Dieter Spaar's implementation predates OsmocomBB development by the better part of a year. At that time, he had to resort to binary-patching existing proprietary (binary-only) baseband firmware. So I think people should recognize his effort in doing the first practical implementation of that attack.

A real-world practical A5/1 attack using airprobe and Kraken

At Blackhat USA 2010, Karsten Nohl has been presenting on a practical real-world A5/1 cracking attack. For recent years, Karsten, myself and others have been speaking at various opportunities, indicating that a practical attack using readily-available information and tools from the Internet is very possible, and that it is only a matter of time for somebody actually does it.

While Karsten has focused on the actual cryptographic attack, I've been putting in some time in projects like airprobe (a GSM receiver/decoder).

Now finally, a team of friends at the new Security Research Labs (founded by Karsten) in Berlin has put the pieces of the puzzle together.

Airprobe has been extended to fully support decoding of TCH/F (FACCH, SACCH and traffic), as well as SDCCH/SACCH control channels, and to specify the timeslot and physical channel configuration from the command line. Using this, you can

  • decode the AGCH, wait for an IMMEDIATE ASSIGNMENT of a SDCCH
  • decode that very SDCCH and wait until encryption is turned on
  • dump an encrypted burst where you have sufficient known plaintext
  • use a different program to actually recover the A5/1 ciphering key
  • feed that key into airprobe and decrypt+decode the ASSIGNMENT COMMAND of the TCH
  • use airprobe to decrypt+decode that assigned TCH/F

The external program to recover the A5/1 ciphering key is called Kraken and is also available from the SRLabs website.

So what are the limitations? Well, so far this only works on non-hopping cells with a single ARFCN. The limitations are those of the receiver hardware (and SDR software), and not really limitations of the airprobe GSM decoder or the actual software tools.

In the past I would have assumed that non-hopping and/or single-ARFCN cells are rare, but in fact we can find them even inside a big city like Berlin, from at least two of the four German GSM operators. So that's why this attack is very practical, no matter what the GSMA might say.

I'm still alive ;)

In case you're wondering why there is such a long period with no updates: I've been travelling over the last week and barely had sufficient time to follow my e-mail and get the most high-priority work done. Hope to update the blog soon.

More musings on locked-down mobile phones

In recent days, the story about Motorola locking out its users (and developers) from their more recent Droid phones has made big news. As it seems, the exact functionality implemented by eFuses remains unclear, and the behavior of Motorola might thus not be too different from what has more or less become the industry standard.

For those of you who are not following the mobile world as close on a technical level as people like me do: In the last five years, more and more cellphone manufacturers have used cryptographic code signing to lock-down the software that you can run on the phone. Major parts of the system including the software update mechanism and the bootloader on the device contain a verification process of those cryptographic signatures to ensure that you can only software signed by the phone manufacturer.

I have seen this with the MotoMAGX phones like the ROKR2 v8, various Windows Mobile handhelds from HTC, The non-developer (non-ADP) version of the Google/Android G1 and many other phones.

This puts the user into a strange situation where he buys some hardware from the manufacturer, but yet doesn't have control over what this device does. Just imagine buying a computer, but being limited to run Windows 98 and Office 97 on it. You could not update to a later version of the operating system, and you could not install an alternative operating system such as a version of GNU/Linux. If the computer vendor decides that he will drop support for it, you will not even be able to install security updates to the operating system.

From my point of view, this is an abusive, anti-competitive behavior by the manufacturer. For no reason but his ever-growing hunger for power he makes you completely dependent on his decision. It is not in the control of the user, what operating system or even applications you can install. It is under the control of the manufacturer.

I would accept this if the phone was rented. In this case, I would only pay a small rental fee, but the phone is the property of the manufacturer and I am only using it. But the manufacturer actually sells the device. He wants to be paid the full price, but still not actually hand control over to the buyer.

Compare this with buying a CD-player that has arbitrary restrictions so it would only play CDs from one of the major music labels/distributors like EMI, but not CDs from any of the other publishers, for no technical reason whatsoever. Or buying a TV set that is locked down so you can only watch one TV channel, while you need to buy another TV for a different channel.

I actually think the antitrust authorities should investigate this behavior of the mobile phone industry. Simply compare it with the PC situation and look at the fact how often Microsoft has been judged in some kind of anti-competitive behavior in the PC world. In the mobile phone industry, the situation is worse than it ever was in the PC world, yet we do not see big antitrust cases being brought forward.

And please don't buy those pseudo-arguments that this has any relation to regulatory/FCC approval or the safety of mobile networks themselves. The entire software stack interacting with the mobile network runs on a separate processor (the baseband processor) anyway. It doesn't matter what you install on the application processor. Once again, compare it to laptops: You can insert a 3G miniPCI, expressCard or USB dongle. Inside this dongle you run the communications stack on a processor that is completely different from your main processor that runs your regular OS (be it GNU/Linux, OS X, Windows, Solaris or whatever makes you happy).

Motorola locking down the DroidX and Droid2 in a nasty way

There are plenty of reports in recent days about the level of locking-down that Motorola is apparently doing on their most recent Android products, the Droid 2 and the Droid X.

This goes as far as to an (I believe unconfirmed) report claiming that not only there is the more or less typical DRM on software (i.e. cryptographic signature validation chain), but there also is an eFuse that that is blown if something happens wrong during the booting process.

To the best of my knowledge (and I'm doing mobile phone reverse engineering for about 6 years now), this is the first time I hear of something like this. If true, it sounds pretty dangerous to me. What if something goes wrong during an update (such as a power failure during software update)? What if you really have a non-correctable multi-bit error in your NAND Flash? In that case, cryptographic verification of the firmware fails and the eFuse would be blown, resulting in your device being a brick. This could eventually backfire massively to Motorola.

The best comment from the thread:
You can legally buy a gun that only shoots in the direction of the person pulling the trigger, but it doesn't mean it's a good idea.

Reading something like this almost makes me very depressed. Motorola is benefitting from the billions-of-dollar-worth development of existing Free Software projects like the Linux kernel, but they now want to take away the fundamental right to run modified versions of that very software. Somebody needs to slap them with a very large trout.

I'm not really surprised that they are doing it, though. Motorola has shown that direction even years ago when they first used SELinux as part of their later pre-Android Linux phones (EZX and MAGX). They didn't use it to enhance the security of the user, but to enhance the security _from_ the user.

Please also note this great post by Bradley M. Kuhn on the subject matter. If you don't know Bradley, he's been doing GPL enforcement for the last 12 years - for the Free Software Foundation and the Software Freedom Law Center. In his post, he actually thanks Motorola to publicly state that they actually want to lock their phones down (as opposed to Apple).

What's even more interesting though is his elaboration on the scripts to control compilation and installation clause of GPLv2. This is indeed something that most people tend to overlook when it comes to GPL[v2] compliance and we see this a lot during our work.

And in fact, for a very long time, I have been teaching and educating this fact during my GPL related talks and trainings: In software specific for embedded devices, the scripts to control installation are incomplete, if you do not provide a means to install the software onto the actual device. Where else would you be reasonably install the Linux kernel image that is made specifically to work on such a particular mobile phone model? Due to the custom nature of Linux kernels for embedded targets, it wouldn't even run anywhere else.

I've never taken any such issue to court so far - but it was a frequent dispute in out-of-court GPL enforcement we've been doing at I'm definitely curious to see what will be the first court case addressing that issue. The ever power-hungry manufacturers of mobile phones seem like they deserve it.

Apparently Motorola has released some statement that denies they use eFuses to brick the device. All it does is to render the device unable to boot until some Motorola-certified/signed/authorized software is loaded on the device again. They did not specify how that could be done, though. Still, even without the eFuse bricking, I find it outrageous that the Industry (including Motorola) expect their customers pay hundreds of dollars for a device that is then still owned by Motorola rather than that very customer. It's like selling something but still retaining ownership of it. Doesn't that make you feel strange, too?

Implementing the TCAP protocol, heading towards OsmoSGSN SS7 support

The protocol by which traditional GSM core network components interact is called MAP (Mobile Application Part). MAP itself is a user of the TCAP (Transaction Capabilities Application Part) protocol, which in turn runs on a SS7 protocol stack (i.e. SCCP over MTP or M3UA or SUA over SCTP).

For those users of OpenBSC who have a need to interoperate with other GSM networks (roaming), the circuit-switched part of OpenBSC has so far relied on the use of a proprietary MSC (by means of the A interface). This closed MSC then talks MAP/TCAP/SS7 to roaming partners.

However, on the GPRS front, we now have OsmoSGSN. However, as opposed to the BSC on the circuit switched side, the SGSN directly interacts with the core GSM network components (both of the home network and the roaming partners).

So in order to run OsmoSGSN interacting with existing HLRs, we need to add a MAP/TCAP/SS7 interface to it. Once this has been done for the SGSN, we of course can do the same for the MSC-part that is currently integrated with OpenBSC.

As there are existing implementations of SCTP (inside the Linux kernel) and SUA (sualibrary), TCAP is the next step in the protocol stack that needs to be implemented. I've been digging into TCAP for the last week(s), and believe I finally understood every part of its operation.

You can think of TCAP as something that facilitates the transport of request-response type transactions over a datagram oriented transport layer. It intends to have lower overhead than a connection-oriented service (e.g. establishing TCP sessions) and supports features such as aggregating multiple user-messages (called components) in a single actual transport-layer message. The idea is to reduce the overhead of message headers and routing.

TCAP is (unfortunately) specified in ASN.1 and thus requires significant effort to parse and construct. Right now I'm using Lev Walkin's asn1c ASN.1 C code generator to generate the parser and constructor functions. The actual TCAP protocol logic is once again implemented in plain C, using the various concepts and utility functions established in OpenBSC (and now part of libosmocore).

The implementation is making good progress and I hope I can do some early testing in about a week from now, and successively move straight to the MAP protocol, implementing at least those parts that we need for GPRS authentication and attach / routing area updates.

COSCUP 2010 conference schedule has been posted

The Schedule of the COSCUP 2010 conference has been posted on the conference homepage. I'm happy to see such a large number of talks from a wide range of speakers - including many friends from my time in Taiwan a couple of years back for Openmoko...

As it seems from this chinese blog entry, the organizers were overwhelmed by the number of attendee registrations, with all 610 available seats being occupied within 85 minutes of opening the registration. It seems they are in need of a bigger venue next year ;)

Family visit is keeping me busy

In case you're expecting a quick response from me these days, please apologize. I'm currently having family visiting me in Berlin, and I very much enjoy being the personal tourist guide for some days...

I shall be back to normal by the end of the week.

Major update in OpenBSC GPRS/EDGE support

Through the last couple of days, I've been in extreme bug-squashing mode for the GPRS/EDGE code base in OpenBSC (mostly the OsmoSGSN program). I'm now at a point where I can reliably establish PDP contexts and access the Internet from a variety of different phones with different baseband chipsets and GPRS protocol stack implementations. All so-far-known bugs regarding fragmentation/reassembly, sequence numbering and other issues have been fixed. There definitely are plenty more, but we first need to find them.

Since it's working reliably now, it's quite fascinating what the various phones do after connecting to the GPRS network. Like Windows Mobile phones sending Netbios Name Service updates (and requests), which I think is funny considering that they are sent to a network that is typically considered to be the public Internet.

But to be fair and not anti-Windows, my Google/Android G1 also makes some https connections back to Google - and I don't know what they are for [yet].

In any case, with OpenBSC, OsmoSGSN and OpenGGSN anyone interested in doing true security (and privacy) research with mobile phones is now able to do so. Using those programs, you can run your own GPRS+EDGE network and can see first hand what your phones are doing on a cellular network, what kind of data they are sending back home. In this setup, there is no packet filtering, NAT, deep packet inspection and no intrusion detection systems between your PC and the IP stack on your phone.

The reason why you see paging by IMSI in real-world GSM networks

During my work on airprobe and OsmocomBB I've been wondering why you see paging by IMSI in real-world GSM networks.

A quick recap: The IMSI is the world-wide unique serial number of your SIM. Since it is easy to identify and track people, the TMSI was introduced as a temporary identifier that is frequently re-allocated over encrypted channels. The only reason for the TMSI to exist is to prevent tracking of a subscriber by watching where his IMSI appears on the paging channel.

According to the theory, the IMSI is only used when first registering to any GSM network. At that time, a TMSI is allocated to the SIM card in the phone, and this TMSI is used for the next transaction(s). Later, this TMSI is re-allocated and re-allocated, but the IMSI shouldn't show up again in any paging requests.

Even if you switch mobile networks (i.e. in the roaming case), you would once send the IMSI as part of a LOCATION UPDATE REQUEST or IDENTITY RESPONSE, but the network has no need to page the SIM by IMSI.

So far the theory. If you look at the Paging Channel (PCH) of cells in real-world networks, you see a significant (10-20%) amount of paging requests that contain paging by IMSI. This seems strange on first sight, given the theory described above.

I have the following plausible explanation for this:

  • The VLR keeping the IMSI-TMSI mappings doesn't have non-volatile storage. This means at a VLR restart, all the TMSI allocations will be lost, and the network has to resort to paging by IMSI.
  • The VLR has a limited amount of RAM, which can store a limited number of IMSI-TMSI mappings. Especially if the operator is interested in saving money, the amount of memory is insufficient for all subscribers in the network. This means, the VLR will expire some old entries in the mapping table to store new entries. Thus, mobile phones whose last transaction with the GSM network was relatively long ago are likely candidates for such VLR expiration. Once a phone for an expired entry needs to be paged again, paging will happen by IMSI.
  • Last, but not least: GSM networks do not page a phone by the last known cell, but by the last known location area of the phone. A location area might be relatively big. This means that at any cell you will see a lot of paging messages, even for phones that are not even anywhere near this cell. If there is no response within the location area, the MSC might decide to do paging on a larger radius, possibly the entire MSC area. Since such MSC-wide paging is likely to occur for phones that haven't shown activity for a long time (and thus might have moved or disappeared without properly unregistering from the network), those are the exact same phones for which the IMSI-TMSI mappings have expired from the VLR. Thus, the rate of paging-by-IMSI looks disproportionately high.

So the relatively high percentage of paging by IMSI vs. TMSI should not be taken as a measurement with regard to the total number of transactions or even the total number of subscribers. It is simply the mechanics of the network resulting in a distortion of those figures caused by phones that have never properly unregistered from the network.

Back from OpenBTS workshop

I've just returned back from the First OpenBTS workshop held by David Burgess and hosted by Dieter Spaar in south-east Bavaria (Germany). While I'm not involved with OpenBTS so far (except from using it occasionally), I still thought the community surrounding Free Software / Open Source in the GSM field is small enough to make me participate.

On the request of the participants, I also did a short demonstration of both OpenBSC and OsmocomBB. And just like I managed to crash OpenBTS by accidentally sending invalid messages, my OpenBSC demo crashed at some point [due to a not-yet-known bug regarding SMS delivery. I suppose the intrusive changes of the BSC/MSC split are to be blamed for that. But I don't mind, we need that split...

I definitely had a great time meeting the participants of the workshop. There definitely is a very diverse crowd with equally diverse reasons for their interest in using and/or deploying OpenBTS.

Finally, there was a chance to discuss the need for a common 'application interface' in both OpenBSC and OpenBTS. Using that interface, external applications (e.g. implementing USSD or RRLP) could be written in a way to work with both OpenBTS and OpenBSC. I hope we can get started on this soon and remove another bit of fragmentation in what is already a fairly small special interest community...

Given the excellent weather conditions, the motorbike ride to and from the venue went fine - despite being at 650 km distance from my home.

Adding frequency hopping support to OpenBSC

During the last couple of days, I've been adding the bits required to support frequency-hopping BTSs in OpenBSC. Now everything looks great in the protocol traces - but unfortunately it still doesn't work, at least not with the Siemens BS-11 that I have access to.

Will continue to try to make it work. The big advantage of having a hopping BTS under our control is that we can define the hopping sequence - something quite useful once we get to the point where we'd like to add frequency hopping to the telephone-side stack (OsmocomBB).

The good news is that I had to fix lots of bugs in the A-bis OML dissector for wireshark that I wrote some time ago. It's now much more complete and definitely a big step further towards eventually getting it included in wireshark mainline.

A fairy tale about ICCIDs, IMSIs and iPads

One of the big news of the last week is AT&T's leak of 114,000 iPad customer records including the e-mail address and ICCID

While that leak is certainly a big issue in itself, there are some people, most notably Chris Paget, who claim that this is much more serious than generally assumed. The main claim here seems to be that order to translate an ICCID into an IMSI, you need to query the HLR.

I have been reading GSM protocol specifications on every level for the past years, and never have I seen the ICCID being mentioned anywhere. The GSM specifications do not require this information to be stored in the HLR, and the MAP protocol (used on the C interface between MSC and HLR, see 3GPP TS 29.002) does not even know how to encode/specify it.

Also, there is no technical need for it. The ICCID is never used nor needed in any part of the GSM protocol. Also, the GSM network typically doesn't store any information that is not absolutely necessary for its operation. The only identifier of a SIM card that the network protocols care about is the IMSI.

So unless the US operators in question have either some kind of proprietary extensions to both their HLR and the MAP protocol, there is to the best of my knowledge no way how you can relate the ICCID to the IMSI.

And thus, as a result, the IMSI-catcher attack described will not work since you don't know the IMSI of the SIM card (associated with the customer record) that you want to catch.

If anyone can show me hard technical facts about ICCIDs being used in the HLRs of the operators in question, I am happy to post here I was wrong. Otherwise, I would hope everyone else could also come down to the hard technical facts, i.e. which particular MAP message is used for this alleged ICCID-to-IMSI query.

UPDATE: As some people have discovered, the three US operators themselves have decided that they use the same number to generate both the ICCID and the IMSI. So if you have one, you can compute the other. No need for HLR access, no need for the MAP protocol. So the information leak is in fact unrelated to the GSM protocol but simply a matter of how unfortunate those particular three operators assign their unique identifiers.

More thoughts on FSF action against Apple over GNU Go

Last week, I blogged about the FSF action against Apple. This week, I intend to add a bit to that.

As it has been pointed out to me, Apple has immediately removed the GPL-infringing software from its app store. This of course means they have refrained from further infringing the GPL. It is not publicly known if they have made a declaration to cease and desist or not.

So yes, by removing the software that was distributed in violation of the GPL terms, Apple has done legally the right thing: Reduce the danger/risk of committing further (knowing) infringement.

The FSF (and probably the Free Software community in general) of course want something else: For Apple to alter their app store terms in a way that would enable software authors to have Apple distribute their GPL licensed software in it. While this might be possible very easily with small modifications to their legal terms and to the implementation of the app store, it is probably not quite easy to make a legal claim and try to force this upon Apple.

Anyone always has the choice to either distribute GPL licensed software compliant with its license terms - or not distribute it at all. If Apple prefers the latter, this is very unfortunate (and you might call it anti-social or even anti-competitive) but something that they can very well do.

The only questions that I see remaining from a legal point of view: What about the previous GPL infringements? What can (and/or has) Apple to do in return to the previous distribution of infringing software? This is where the legal pressure of the copyright holders leaves room for negotiation. Instead of monetary damages (which don't really resolve what the GPL aims to do), there could possibly be a solution where Apple has to provide the GPL license text and complete corresponding source code to the Go program through their app store. And while they're at it, they might just solve the distributing source code for copyleft style licensed software problem in a generic way. Or they might just decide that they're stupid and stubborn and not interested in solving any problems in the first place.

My take on the FSF action against Apple over GNU Go

About two weeks ago, the FSF announced that it has taken action against the Apple App Store over their distribution of GNU Go. This has apparently set off some people like lefty and triggered a length and wide debate.

I personally very much support the action the FSF has taken. Anyone involved in distribution of copyrighted material is required to do due diligence on checking that he actually has a license to do so. This is not really related to the GPL.

Yes, this means that I can take GPL enforcement action to a retail store that is selling/distributing infringing products, and I can make them provide a declaration to cease and desist from further infringements. Of course, that declaration would only be valid for this single retail store. This is why in our work, we always try to go after whatever entity is responsible for the majority or all of those infringements, rather than after a single store owner.

The reason for this is simple: In many cases, it is impossible for you as the rights holder to find out who sold the product to the retail store, and track the entire supply chain back to whoever caused the GPL violation in the first place. Also, some of those entities might reside in a different jurisdiction, so you go after the first element in the supply chain that is in your own jurisdiction, to minimize the legal risk for you as plaintiff and maximize the output in terms of your local market.

But the case with Apple is different. They are not a small retailer down the road, but the entity responsible for providing the infringing software to (almost?) all of its users. They are running that App store as a commercial company and earn money from running it (even if individual apps might be free of charge). Free Software and copyleft licenses like the GPL are a very real phenomenon in the software industry today, so they should better have thought about a proper solution, not just for GNU Go but for the tens of thousands of existing GPL licensed software projects which people might want to port or re-use in iPhone applications.

They are already doing all kinds of verification/checking/review of software for other reasons (things many people might call censoring), and as part of that process they could just as well determine the license of the software, and provide a source code download link from their store. What is the big deal? If they (or other similar app store / market / ... providers) had thought how to address the problem, there are easy and pragmatic solutions to solve them in the architecture of such a app store / marketplace system.

Also, the fact that the FSF is taking legal steps is not wrong. Even if some people might dispute whether they actually have a valid case or not (I believe they do): This is what legal cases are for: To create a clear legal situation for all participants in the dispute, and to set precedent for future similar cases. Even only from that point of view it is good that they're doing this case. At the end of it, the legal situation will be more clear, both for Apple as well as for people who want to distribute GPL licensed software through their store.