FreeCalypso > hg > themwi-ota-tools

annotate Note-about-padding @ 11:b4b4a822286c default tip

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
add INSTALL
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 21 May 2024 01:06:04 +0000
parents b6331ae4eea9
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
9
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
1 The complex logic in smswrap/ota-smswrap-sjs1.c for getting the secured message
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
2 into just the right format to be accepted by the card has been copied from
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
3 Osmocom Python program shadysim.py - that Python code appears to be the only
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
4 existing reference for using the RFM feature of Sysmocom SIM cards. However,
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
5 in the process of studying that code and extracting the logic from it, I found
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
6 what appeared to be a bug in their code and logic, and sure enough, further
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
7 investigation has confirmed it to be a bug indeed, a bug that is tolerated by
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
8 sysmoUSIM-SJS1 cards, but not the newer sysmoISIM-SJA2 - hence the test_rfm
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
9 function of that Python tool does not work on the new cards.
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
11 When the application message payload is encrypted with any variant of DES
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
12 (including the two-key 3DES used on Sysmocom SIMs), the length of the ciphertext
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
13 has to be a multiple of 8 bytes - hence if the plaintext length is not a
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
14 multiple of 8 bytes, the plaintext needs to be padded. But what should happen
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
15 if the plaintext length going into the cipher just happens to be a perfect
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
16 multiple of 8 bytes? The correct answer (ought to be obvious) is to apply no
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
17 padding at all, i.e., zero bytes of padding. However, the Python code in
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
18 shadysim.py adds 8 bytes of padding, and sets the number of padding bytes in the
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
19 header to 8. The resulting encrypted message should be considered malformed per
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
20 standard specs, but sysmoUSIM-SJS1 cards are liberal in what they accept in this
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
21 instance, thus the bug went unnoticed. The newer sysmoISIM-SJA2 cards do not
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
22 accept such malformed messages with invalid padding, and it just so happens that
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
23 the message generated by the test_rfm function has 40 bytes of plaintext going
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
24 into the cipher, perfectly divisible by 8 - hence their test_rfm function fails
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
25 on the new cards.
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
26
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
27 Our ota-smswrap-sjs1 program accepts an optional third argument after the two
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
28 key arguments, selecting the padding mode. The default mode is correct padding;
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
29 if the extra padding mode argument is set to 1, our tool replicates the bogus
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
30 behaviour of the reference Python code. This bug-compatible mode has been
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 implemented to make sure that we can generate the exact same packet starting
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
32 with data/shadysim-rfm-test (verifying that we have copied the logic correctly),
b6331ae4eea9 Note-about-padding added
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
33 but it should not be used further beyond these debugging tests.