LaForge's home page (Posts about sim)

OsmoDevCall: "Osmocom SIMtrace2 Tutorial"

Harald Welte — Tue, 18 Oct 2022 16:00:00 GMT

I've presented an SIMtrace2 Tutorial - SIM protocol tracing: how and why as part of the OsmoDevCall talk series on Osmocom and related technology.

You can find the video recording at https://media.ccc.de/v/osmodevcall-20221019-laforge-simtrace2-tutorial

OsmoDevCall: "Advanced SIM card topics: GlobalPlatform SCP02, OTA, ARA-M, ISIM"

Harald Welte — Thu, 24 Feb 2022 16:00:00 GMT

I've presented on Advanced SIM card topics: GlobalPlatform SCP02, OTA, ARA-M, ISIM as part of the OsmoDevCall talk series on Osmocom and related technology.

You can find the video recording at https://media.ccc.de/v/osmodevcall-20220225-laforge-advanced-sim-topics

OsmoDevCall: "SIM card profile creation, personalization, production"

Harald Welte — Thu, 11 Nov 2021 16:00:00 GMT

I've presented on SIM card profile creation, personalization, production as part of the OsmoDevCall talk series on Osmocom and related technology.

You can find the video recording at https://media.ccc.de/v/osmodevcall-20211022-laforge-sim

OsmoDevCall: "osmo-remsim in practice"

Harald Welte — Thu, 26 Aug 2021 16:00:00 GMT

I've presented on osmo-remsim in practice as part of the OsmoDevCall talk series on Osmocom and related technology.

You can find the video recording at https://media.ccc.de/v/osmodevcall-20210827-laforge-osmo-remsim

OsmoDevCall: "pySim-shell - next generation SIM configuration tool"

Harald Welte — Thu, 08 Apr 2021 16:00:00 GMT

I've presented a talk SS7 and SIGTRAN in 2G/3G networks as part of the OsmoDevCall talk series on Osmocom and related technology.

You can find the video recording at https://media.ccc.de/v/osmodevcall-20210409-laforge-pysim-shell

36C3 Talks on SIM card technology / Mitel DECT

Harald Welte — Sat, 04 Jan 2020 16:00:00 GMT

At 36C3 in December 2019 I had the pleasure of presenting: One full talk about SIM card technology from A to Z and another talk where I presented together with eventphone team members about Security issues in the Mitel SIP-DECT system.

The SIM card talk was surprisingly successful, both in terms of a full audience on-site, as well as in terms of the number of viewers of the recordings on media.ccc.de. SIM cards are a rather niche topic in the wider IT industry, and my talk was not covering any vulnerabilities or the like. Also, there was nothing novel in the talk: SIM cards have been around for decades, and not much has changed (except maybe eSIM and TLS) in recent years.

In any case, I'm of course happy that it was well received. So far I've received lots of positive feedback.

As I'm working [more than] full time in cellular technology for almost 15 years now, it's sometimes hard to imagine what kind of topics people might be interested in. If you have some kind of suggestion on what kind of subject within my area of expertise you'd like me to talk about, please don't hesitate to reach out.

The Mitel DECT talk also went quite well. I covered about 10 minutes of technical details regarding the reverse engineering of the firmware and the communication protocols of the device. Thanks again to Dieter Spaar for helping with that. He is and remains the best reverse engineer I have met, and it's always a privilege to collaborate on any project. It was of course also nice to see what kind of useful (and/or fun) things the eventphone team have built on top of the knowledge that was gained by protocol-level reverse engineering.

If you want to know more low-level technical detail than the 36C3 talk, I recommend my earlier talk at the OsmoDevCon 2019 about Aastra/Mitel DET base station dissection.

If only I had more time, I would love to work on improving the lack of Free / Open Source Software realted to the DECT protocol family. There's the abandoned deDECTed.org, and the equally abandoned dect.osmocom.org project. The former only deals with the loewst levels of DECT (PHY/MAC). The latter is to a large extent implemented as part of an ancient version of the Linux kernel (I would say this should all run in userspace, like we run all of GSM/UMTS/LTE in userspace today).

If anyone wants to help out, I still think working on the DECT DLC and NWK dissectors for wireshark is the best way to start. It will create a tool that's important for anyone working with the DECT protocols, and it will be more or less a requirement for development and debugging should anyone ever go further in terms of implementing those protocols on either the PP or FP side. You can find my humble beginnings of the related dissectors in the laforge/dect branch of osmocom.org/wireshark.git.

Sometimes software development is a struggle

Harald Welte — Sat, 28 Sep 2019 16:00:00 GMT

I'm currently working on the firmware for a new project, an 8-slot smart card reader. I will share more about the architecture and design ideas behind this project soon, but today I'll simply write about how hard it sometimes is to actually get software development done. Seemingly trivial things suddenly take ages. I guess everyone writing code knows this, but today I felt like I had to share this story.

Chapter 1 - Introduction

As I'm quite convinced of test-driven development these days, I don't want to simply write firmware code that can only execute in the target, but I'm actually working on a USB CCID (USb Class for Smart Card readers) stack which is hardware-independent, and which can also run entirely in userspace on a Linux device with USB gadget (device) controller. This way it's much easier to instrument, trace, introspect and test the code base, and tests with actual target board hardware are limited to those functions provided by the board.

So the current architecture for development of the CCID implementation looks like this:

Implement the USB CCID device using FunctionFS (I did this some months ago, and in fact developing this was a similarly much more time consuming task than expected, maybe I find time to expand on that)
Attach this USB gadget to a virtual USB bus + host controller using the Linux kernel dummy_hcd module
Talk to a dumb phoenix style serial SIM card reader attached to a USB UART, which is connected to an actual SIM card (or any smart card, for that matter)

By using a "stupid" UART based smart card reader, I am very close to the target environment on a Cortex-M microcntroller, where I also have to talk to a UART and hence implement all the beauty of ISO 7816-3. Hence, the test / mock / development environment is as close as possible to the target environment.

So I implemented the various bits and pieces and ended up at a point where I wanted to test. And I'm not getting any response from the UART / SIM card at all. I check all my code, add lots of debugging, play around with various RTS / DTR / ... handshake settings (which sometimes control power) - no avail.

In the end, after many hours of trial + error I actually inserted a different SIM card and finally, I got an ATR from the card. In more than 20 years of working with smart cards and SIM cards, this is the first time I've actually seen a SIM card die in front of me, with no response whatsoever from the card.

Chapter 2 - Linux is broken

Anyway, the next step was to get the T=0 protocol of ISO 7816-3 going. Since there is only one I/O line between SIM card and reader for both directions, the protocol is a half-duplex protocol. This is unlike "normal" RS232-style UART communication, where you have a separate Rx and Tx line.

On the hardware side, this is most often implemented by simply connecting both the Rx and Tx line of the UART to the SIM I/O pin. This in turn means that you're always getting an echo back for every byte you write.

One could discard such bytes, but then I'm targeting a microcontroller, which should be running eight cards in parallel, at preferably baud-rates up to ~1 megabit speeds, so having to read and discard all those bytes seems like a big waste of resources.

The obvious solution around that is to disable the receiver inside the UART before you start transmitting, and re-enable it after you're done transmitting. This is typically done rather easily, as most UART registers in hardware provide some way to selectively enable transmitter and/or receiver independently.

But since I'm working in Linux userspace in my development environment: How do I approximate this kind of behavior? At least the older readers of this blog will remember something called the CREAD flag of termios. Clearing that flag will disable the receiver. Back in the 1990ies, I did tons of work with serial ports, and I remembered there was such a flag.

So I implement my userspace UART backend and somehow it simply doesn't want to work. Again of course I assume I must be doing something wrong. I'm using strace, I'm single-stepping through code - no avail.

In the end, it turns out that I've just found a bug in the Linux kernel, one that appears to be there at least ever since the git history of linux-2.6.git started. Almost all USB serial device drivers do not implement CREAD, and there is no sotware fall-back implemented in the core serial (or usb-serial) handling that would discard any received bytes inside the kernel if CREAD is cleared. Interestingly, the non-USB serial drivers for classic UARTs attached to local bus, PCI, ... seem to support it.

The problem would be half as much of a problem if the syscall to clear CREAD would actually fail with an error. But no, it simply returns success but bytes continue to be received from the UART/tty :/

So that's the second big surprise of this weekend...

Chapter 3 - Again a broken card?

So I settle for implementing the 'receive as many characters as you wrote' work-around. Once that is done, I continue to test the code. And what happens? Somehow my state machine (implemented using osmo-fsm, of course) for reading the ATR (code found here) somehow never wants to complete. The last byte of the ATR always is missing. How can that be?

Well, guess what, the second SIM card I used is sending a broken, non-spec compliant ATR where the header indicates 9 historical bytes are present, but then in reality only 8 bytes are sent by the card.

Of course every reader has a timeout at that point, but that timeout was not yet implemented in my code, and I also wasn't expecting to hit that timeout.

So after using yet another SIM card (now a sysmoUSIM-SJS1, not sure why I didn't even start with that one), it suddenly works.

After a weekend of detours, each of which I would not have assumed at all before, I finally have code that can obtain the ATR and exchange T=0 TPDUs with cards. Of course I could have had that very easily if I wanted (we do have code in pySim for this, e.g.) but not in the architecture that is as close as it gets to the firmware environment of the microcontroller of my target board.