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annotate doc/IMEI @ 659:761e8b0c65b0

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loadagent: first step in implementation of binary flash programming
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Mar 2020 17:55:20 +0000
parents 4644799cb515
children 232e36a227dd
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17
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1 IMEI vs. IMEISV
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2 ===============
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3
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4 There is a subtle distinction between an IMEI and an IMEISV. The first 14
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5 digits are the same between the two: the supposedly-world-unique number of a
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6 given piece of hardware. In a traditional IMEI 15-digit number the significant
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7 14 digits are followed by a Luhn check digit, whereas an IMEISV has 16 digits:
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8 the 14 significant digits of the IMEI, *no* Luhn check digit, and two digits of
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9 "software version".
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10
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11 It is up to device manufacturers and firmware designers to decide whether or
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12 not to store the Luhn check digit in the GSM device's flash or EEPROM or
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13 whatever, but it is not sent over the air: instead the IMEISV is sent. It
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14 appears that the GSM standard authors' intent was that the IMEI part is stored
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15 immutably in each manufactured device whereas the SV digits are added by the
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16 running firmware to indicate its version, but the IMEI handling scheme
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17 implemented in TI's reference firmware and retained by many of the TI-based GSM
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18 device manufacturers (at least FIC/Openmoko and Foxconn/Pirelli) dispenses away
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19 with the IMEI vs. IMEISV distinction.
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20
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21 IMEI storage and retrieval in TI's reference firmware
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22 =====================================================
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23
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24 When running on the plain Calypso as opposed to Calypso+, TI's TCS211 reference
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25 firmware supports two ways of storing and retrieving the IMEI: obfuscated and
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26 unobfuscated. In both schemes the IMEI datum is stored as a file in the
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27 device's flash file system (FFS), and even though the FFS filename calls it the
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28 IMEI, the content of this file is really treated as the IMEISV: 16 digits are
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29 stored, the firmware function responsible for reading the IMEI datum out of FFS
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30 and passing it on to the rest of the fw is called cl_get_imeisv(), the code in
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31 this function does not transform the 16 digits in any way, and the downstream
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32 recipients of these digits treat them as the IMEISV.
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33
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34 The two specific schemes offered by TCS211 fw are as follows:
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35
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36 In the unobfuscated scheme (FF_PROTECTED_IMEI not defined), the so-called IMEI
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37 but really IMEISV is stored in an FFS file named /pcm/IMEI. The file is 8 bytes
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38 long, each byte stores two IMEISV digits, and the order of the digits within
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39 each byte is reversed relative to the natural order: first the least significant
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40 nibble is used, then the most significant nibble.
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41
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42 In the obfuscated scheme (FF_PROTECTED_IMEI is defined), the so-called IMEI but
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43 really IMEISV is stored in an FFS file named /gsm/imei.enc. The file is 16
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44 bytes long: the first 8 bytes store the 16-digit IMEISV encrypted with DES,
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45 using the Calypso die ID as the key, and the last 8 bytes store that Calypso die
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46 ID DES-encrypted with itself. Underneath the obfuscation, the 16 IMEISV digits
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47 are stored in the 8 bytes in the natural order: first the most significant
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48 nibble is used, then the least significant nibble.
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49
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50 IMEI storage and retrieval schemes implemented by device manufacturers
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51 ======================================================================
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52
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53 Openmoko devices use the unobfuscated IMEI storage method unchanged from TI's
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54 reference fw: the factory-assigned IMEI is stored in an FFS file named
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55 /pcm/IMEI, and that is where the original mokoN firmwares look for it. Further
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56 blurring the distinction between the IMEI and the IMEISV, the 16 digits stored
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57 in /pcm/IMEI (which the fw treats as the IMEISV) were factory-programmed as the
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58 15-digit IMEI (with the Luhn check digit) with an appended 0, i.e., the SV
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59 digits get set to x0 where x is the Luhn check digit.
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60
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61 Foxconn, the makers of the Pirelli DP-L10, have used the obfuscated version of
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62 TI's IMEI handling mechanism instead, with an additional twist: instead of
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63 storing the 16-byte encrypted datum in /gsm/imei.enc in FFS, they have moved it
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64 into their own factory data record stored in a non-FFS sector of the flash.
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65 The content of the 16 digits treated as the IMEISV by the G23M component of the
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66 fw is the same as Openmoko's: 15-digit IMEI with the Luhn check digit followed
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67 by a 0 digit.
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68
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69 Compal, the makers of Motorola C1xx phones, have similarly moved their IMEI out
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70 of FFS into their own proprietary flash data structures, and we have never
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71 decoded the latter, hence we don't know exactly where and how their IMEI is
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72 stored. If you wish to run FreeCalypso firmware on these phones, you have to
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73 set your own IMEISV for our fw even if you are not seeking to make it different
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74 from the factory-assigned one, as we don't know how to retrieve the latter.
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75
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76 Changing the IMEI
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77 =================
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78
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79 When someone says that they wish to change the IMEI on their phone, they need
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80 to be a little clearer as to what they really mean, as there are two possible
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81 interpretations of the just-stated wish:
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82
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83 1. Transmitting a different IMEISV toward the network by running your own
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84 firmware on the device,
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85
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86 or
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87
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88 2. Changing the IMEI seen by the device's original proprietary firmware.
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89
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90 Interpretation 1 is much easier than interpretation 2: when you are writing your
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91 own firmware for an "alien" GSM device (hardware designed and made by someone
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92 other than you), it is much easier to just set your own IMEISV and be done with
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93 it than to figure out how to retrieve the factory-assigned one. Thus those
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94 device manufacturers who try to make it more difficult to change their IMEIs
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95 are actually creating the opposite effect: people will just set their own IMEISV
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96 when running their own fw on their hw.
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97
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98 Openmoko devices are a rare exception in that if you write your own IMEISV into
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99 /pcm/IMEI in FFS, your new IMEISV will take effect not only with FreeCalypso
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100 firmware, but also with the legacy mokoN fw versions, because they all look in
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101 /pcm/IMEI. The same does NOT hold with Compal/Motorola or Foxconn/Pirelli
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102 phones, however: if you wish to change their IMEI to be seen by their original
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103 proprietary firmwares, you are on your own, as we do not currently have any
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104 tools for accomplishing such a feat.
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105
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106 IMEI handling in FreeCalypso
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107 ============================
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108
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109 The FreeCalypso family of projects has adopted the following IMEI storage and
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110 retrieval scheme both for our own FreeCalypso-made hardware and for FreeCalypso
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111 firmwares running on alien hardware: all of our firmware versions regardless of
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112 target will look first in /etc/IMEISV, then in /pcm/IMEI when needing to obtain
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113 the IMEISV for GSM operation. This is the new unified convention; previously
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114 we used varying IMEISV retrieval schemes depending on the target and in
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115 different FC firmware projects. The new unified convention is backward-
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116 compatible with our previous schemes on every target.
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117
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118 The /etc/IMEISV file is a FreeCalypso invention. The file is 8 bytes long, and
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119 stores the 16 digits of the IMEISV in the natural order: first the most
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120 significant nibble is used, then the least significant nibble. This nibble
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121 order makes the IMEISV number directly readable in a hex dump of the file, and
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122 the filename /etc/IMEISV makes it clear that the last two digits are the SV and
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123 are not required to be equal to the Luhn check digit and 0.
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124
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125 Both /etc/IMEISV and /pcm/IMEI can be written with the fc-fsio utility's
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126 set-imeisv command:
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127
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128 set-imeisv fc XXXXXXXX-YYYYYY-ZZ # write /etc/IMEISV
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129 set-imeisv pcm XXXXXXXX-YYYYYY-ZZ # write /pcm/IMEI
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130
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131 When working on Openmoko devices, we recommend writing your IMEISV into
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132 /pcm/IMEI (set-imeisv pcm command) and not creating an /etc/IMEISV file: newer
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133 FC firmware versions will look in both locations, but older FC fw versions and
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134 the legacy mokoN ones look only in /pcm/IMEI. On all other targets we recommend
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135 using the new /etc/IMEISV storage format, i.e., you should use the set-imeisv fc
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136 variant.