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SJA5: EF.USIM_AUTH_KEY: don't display padding of TUAK K

The K value in case of TUAK can be 16 or 32 bytes long.  In case
of the 16 byte variant, it is padded with FF to the end.  Use Rpad()
to remove that padding.

This is not perfect, as it would also strip any consecutive ff even
at the end of the first 16 bytes (when displaying).  But then, this is
a rather artificial situation, as proper keys have high entropy and there
shouldn't be any consecutive ff... at any point in it.

Change-Id: Ia0f9a2138f16dce72f3118001e95baa1c80f23ce
---
M pySim/sysmocom_sja2.py
1 file changed, 19 insertions(+), 1 deletion(-)

git pull ssh://gerrit.osmocom.org:29418/pysim refs/changes/34/33234/1

diff --git a/pySim/sysmocom_sja2.py b/pySim/sysmocom_sja2.py
index 8b1fc04..b5fbd2b 100644
--- a/pySim/sysmocom_sja2.py
+++ b/pySim/sysmocom_sja2.py
@@ -207,7 +207,7 @@
                                    'tuak_cfg'/TuakCfgByte,
                                    'num_of_keccak_iterations'/Int8ub,
                                    'op_opc'/HexAdapter(Bytes(32)),
-                                   'k'/HexAdapter(Bytes(32)))
+                                   'k'/HexAdapter(Rpad(Bytes(32))))
 
     def _decode_bin(self, raw_bin_data: bytearray) -> dict:
         if raw_bin_data[0] & 0x0F == 0x06:

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