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annotate Note-about-padding @ 9:b6331ae4eea9

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Note-about-padding added
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 22 Feb 2021 02:19:43 +0000
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b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
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1 The complex logic in smswrap/ota-smswrap-sjs1.c for getting the secured message
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Mychaela Falconia <falcon@freecalypso.org>
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2 into just the right format to be accepted by the card has been copied from
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Mychaela Falconia <falcon@freecalypso.org>
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3 Osmocom Python program shadysim.py - that Python code appears to be the only
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Mychaela Falconia <falcon@freecalypso.org>
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4 existing reference for using the RFM feature of Sysmocom SIM cards. However,
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Mychaela Falconia <falcon@freecalypso.org>
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5 in the process of studying that code and extracting the logic from it, I found
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Mychaela Falconia <falcon@freecalypso.org>
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6 what appeared to be a bug in their code and logic, and sure enough, further
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Mychaela Falconia <falcon@freecalypso.org>
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7 investigation has confirmed it to be a bug indeed, a bug that is tolerated by
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Mychaela Falconia <falcon@freecalypso.org>
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8 sysmoUSIM-SJS1 cards, but not the newer sysmoISIM-SJA2 - hence the test_rfm
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Mychaela Falconia <falcon@freecalypso.org>
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9 function of that Python tool does not work on the new cards.
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Mychaela Falconia <falcon@freecalypso.org>
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10
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Mychaela Falconia <falcon@freecalypso.org>
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11 When the application message payload is encrypted with any variant of DES
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Mychaela Falconia <falcon@freecalypso.org>
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12 (including the two-key 3DES used on Sysmocom SIMs), the length of the ciphertext
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Mychaela Falconia <falcon@freecalypso.org>
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13 has to be a multiple of 8 bytes - hence if the plaintext length is not a
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Mychaela Falconia <falcon@freecalypso.org>
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14 multiple of 8 bytes, the plaintext needs to be padded. But what should happen
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Mychaela Falconia <falcon@freecalypso.org>
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15 if the plaintext length going into the cipher just happens to be a perfect
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Mychaela Falconia <falcon@freecalypso.org>
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16 multiple of 8 bytes? The correct answer (ought to be obvious) is to apply no
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Mychaela Falconia <falcon@freecalypso.org>
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17 padding at all, i.e., zero bytes of padding. However, the Python code in
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Mychaela Falconia <falcon@freecalypso.org>
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18 shadysim.py adds 8 bytes of padding, and sets the number of padding bytes in the
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Mychaela Falconia <falcon@freecalypso.org>
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19 header to 8. The resulting encrypted message should be considered malformed per
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Mychaela Falconia <falcon@freecalypso.org>
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20 standard specs, but sysmoUSIM-SJS1 cards are liberal in what they accept in this
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Mychaela Falconia <falcon@freecalypso.org>
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21 instance, thus the bug went unnoticed. The newer sysmoISIM-SJA2 cards do not
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Mychaela Falconia <falcon@freecalypso.org>
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22 accept such malformed messages with invalid padding, and it just so happens that
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Mychaela Falconia <falcon@freecalypso.org>
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23 the message generated by the test_rfm function has 40 bytes of plaintext going
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Mychaela Falconia <falcon@freecalypso.org>
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24 into the cipher, perfectly divisible by 8 - hence their test_rfm function fails
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Mychaela Falconia <falcon@freecalypso.org>
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25 on the new cards.
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Mychaela Falconia <falcon@freecalypso.org>
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26
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Mychaela Falconia <falcon@freecalypso.org>
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27 Our ota-smswrap-sjs1 program accepts an optional third argument after the two
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Mychaela Falconia <falcon@freecalypso.org>
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28 key arguments, selecting the padding mode. The default mode is correct padding;
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Mychaela Falconia <falcon@freecalypso.org>
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29 if the extra padding mode argument is set to 1, our tool replicates the bogus
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Mychaela Falconia <falcon@freecalypso.org>
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30 behaviour of the reference Python code. This bug-compatible mode has been
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Mychaela Falconia <falcon@freecalypso.org>
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31 implemented to make sure that we can generate the exact same packet starting
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Mychaela Falconia <falcon@freecalypso.org>
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32 with data/shadysim-rfm-test (verifying that we have copied the logic correctly),
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Mychaela Falconia <falcon@freecalypso.org>
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33 but it should not be used further beyond these debugging tests.