FreeCalypso > hg > fc-sim-sniff

annotate doc/Motivation @ 54:2855330ab96f

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simsniff-dec: bug in PPS decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Oct 2023 23:32:43 +0000
parents 510bef2b2000
children
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1 Q: What is the principal idea behind SIMtrace, as distinct from the specific
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2 implementation realized by "standard" Osmocom SIMtrace?
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3
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4 A: The two principal objectives of SIMtrace are:
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5
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6 1) Passive sniffing of communication between a phone-type device and a SIM,
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7 ideally as transparent and non-invasive as possible.
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8
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9 2) Card emulation: the SIMtrace apparatus presents itself to the phone (or
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10 modem or other phone-type device) as a SIM, either emulating the entire
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11 SIM CardOS functionality in software or communicating with a real SIM
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12 located somewhere remotely, across the Internet.
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13
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14 Q: What are the shortcomings of the existing Osmocom SIMtrace implementation of
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15 the above goals?
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16
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17 A: In the opinion of Mother Mychaela of FreeCalypso, the electrical aspects of
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18 Osmocom SIMtrace implementation are its biggest shortcoming. The following
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19 problems are most acute currently:
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20
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21 * Current SIMtrace v2 hardware is not 5V-tolerant: connecting this apparatus to
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22 an old phone that puts out 5V (class A) on its SIM socket can damage the
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23 hardware, as class A SIM voltages exceed the absolute maximum rating spec of
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24 the AT91SAM3S4B microcontroller on the SIMtrace v2 board, which is connected
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25 directly to the SIM bus.
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26
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27 * One option would be to revive the previous hardware generation as in SIMtrace
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28 v1, replacing the AT91SAM3S with AT91SAM7S. However, all firmware maintained
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29 by Osmocom is written for SAM3S only, thus a backport to SAM7S would involve
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30 significant work. Given that the resulting solution would still be far from
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31 my idea of perfection, I find it difficult to justify investing in that
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32 software effort - instead I would rather work on a more philosophically-proper
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33 solution.
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34
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35 * AT91SAMx-based SIMtrace, both v1 and v2, works (most of the time, but not 100%
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36 reliably) with 1.8V phone-SIM combination (a phone that prefers class C and a
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37 SIM that supports it) only by accident. The Vih spec (the minimum required
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38 voltage on a signal line for it to register reliably as a 1) is 2.0 V for
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39 AT91SAM7S or 2.31 V (0.7 * Vddio, Vddio = 3.3 V) for AT91SAM3S, but the actual
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40 voltage on SIM interface lines in class C operation will never rise above
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41 1.8 V. The electrical interface on this hw operates severely out of spec,
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42 and I find it rather miraculous that it works at all. Not surprisingly,
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43 reports are starting to trickle in with user experiences of it actually NOT
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44 working sometimes.
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45
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46 * Even if the SIM interface is restricted (by the phone, by the SIM, or by
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47 SIMtrace MITM function tampering with ATR or file characteristics bytes) to
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48 operating in class B (3.0 V nominal) only, the existing AT91SAMx SIMtrace
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49 boards are still electrically unclean. Looking at the schematics, one can see
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50 that both CLK and I/O lines are pulled up (with resistors) to the SIMtrace
40
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51 board's 3.3V rail, which is a higher voltage than what the phone will put out
0
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52 (3.0 V or 1.8 V), and in the case of SIMtrace v1 with a 5V phone, that pull-up
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53 will turn into a pull-midway-down instead.
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54
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55 * My philosophy is that the tracing apparatus should be making only a high-
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56 impedance connection to the SIM bus and nothing more, while the SIM bus itself
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57 is galvanically connected from the phone to the physical SIM without passing
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58 through any switches or other potential Heisenbug-inducing artifacts.
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59
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60 My first thought was to gently modify the existing AT91SAMx-based SIMtrace
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61 design for electrically clean multivolt operation:
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62
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63 * Replace the electrical switches for SIM VCC (FPF2109) and SIM RST/CLK/IO
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64 (CB3Q3244) with either a relay (my initial thought, but way too power-hungry)
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65 or a manually operated 5PDT slide switch;
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66
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67 * Insert a Nexperia 74LVC4T3144 dual-supply buffer between the SIM bus and the
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68 MCU, providing a sniffing path that not only supports all 3 voltage classes,
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69 but is electrically clean, making only a high-impedance connection to the SIM
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70 bus as I desire;
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71
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72 * Connect a 74LVC1G07 open drain driver (fed with TxD from the MCU) to the SIM
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73 bus I/O line, providing a signal path for card emulation mode. (In trace mode
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74 the firmware would be responsible for never turning on this OD driver, keeping
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75 the tracing apparatus High-Z.)
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76
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77 However, as I was reading AT91SAMx datasheets more carefully in preparation for
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78 embarking on a project to turn the above idea into reality, I saw a big problem:
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79 when the USART is put into ISO 7816-3 mode, it uses the chip's TxD pin (switched
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80 to open drain operation) for both Rx and Tx, and there is no option to keep
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81 separate RxD and TxD pins with an external receiving buffer and an external OD
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82 driver.
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83
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84 It would probably be possible to build an all-voltage SIM interface with
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85 AT91SAMx, perhaps by using one of those bidirectional level shifter ICs that
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86 somehow automagically handle driving direction reversals. But I personally am
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87 not too inclined to trust those automagical bidirectional translators, they
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88 just don't align with my design philosophy - I would much much rather have
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89 unidirectional buffers, one for sniffing and another for OD-driving the I/O
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90 line in card emulation mode. Seeing that AT91SAMx is incompatible with such
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91 electrical design, I decided to screw AT91SAMx and go for a radically different
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92 approach.