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authorBen Fox-Moore <ben.foxmoore@accelleran.com>2018-09-24 15:47:02 +0200
committerPhilipp Maier <pmaier@sysmocom.de>2018-09-26 18:14:19 +0200
commit0ec147513c453e1a5a39133494427c0d84f396be (patch)
treee1cdc953d81f38a49a18edfc54fb34b1ca1d051f
parentc555e18ebb8829ff216452c6803fd7c1718c6845 (diff)
utils: fix encoding/decoding of IMSI value
When programming or reading a SIM with an IMSI shorter than 15, the IMSI value is incorrectly encoded/decoded. The code pads the the IMSI value with 0xF from the left but padding from the right would be correct. It also encodes the length as half the number of digits in the IMSI (rounded up). This isn't correct for even length IMSIs. With even length IMSIs, the odd/even parity bit bumps the last digit into an extra byte, which should be counted as well. - Fix endcoding of IMSI value - Fix decoding of IMSI value Change-Id: I9ae4ca4eb7c2965e601a7108843d052ff613beb9 Patch-by: Ben Foxmoore Closes: SYS#3552
-rw-r--r--pySim/utils.py32
1 files changed, 26 insertions, 6 deletions
diff --git a/pySim/utils.py b/pySim/utils.py
index 17dc693..ba94702 100644
--- a/pySim/utils.py
+++ b/pySim/utils.py
@@ -49,11 +49,29 @@ def rpad(s, l, c='f'):
def lpad(s, l, c='f'):
return c * (l - len(s)) + s
+def half_round_up(n):
+ return (n + 1)//2
+
+# IMSI encoded format:
+# For IMSI 0123456789ABCDE:
+#
+# | byte 1 | 2 upper | 2 lower | 3 upper | 3 lower | ... | 9 upper | 9 lower |
+# | length in bytes | 0 | odd/even | 2 | 1 | ... | E | D |
+#
+# If the IMSI is less than 15 characters, it should be padded with 'f' from the end.
+#
+# The length is the total number of bytes used to encoded the IMSI. This includes the odd/even
+# parity bit. E.g. an IMSI of length 14 is 8 bytes long, not 7, as it uses bytes 2 to 9 to
+# encode itself.
+#
+# Because of this, an odd length IMSI fits exactly into len(imsi) + 1 // 2 bytes, whereas an
+# even length IMSI only uses half of the last byte.
+
def enc_imsi(imsi):
"""Converts a string imsi into the value of the EF"""
- l = (len(imsi) + 1) // 2 # Required bytes
+ l = half_round_up(len(imsi) + 1) # Required bytes - include space for odd/even indicator
oe = len(imsi) & 1 # Odd (1) / Even (0)
- ei = '%02x' % l + swap_nibbles(lpad('%01x%s' % ((oe<<3)|1, imsi), 16))
+ ei = '%02x' % l + swap_nibbles('%01x%s' % ((oe<<3)|1, rpad(imsi, 15)))
return ei
def dec_imsi(ef):
@@ -61,13 +79,15 @@ def dec_imsi(ef):
if len(ef) < 4:
return None
l = int(ef[0:2], 16) * 2 # Length of the IMSI string
- swapped = swap_nibbles(ef[2:])
+ l = l - 1 # Encoded length byte includes oe nibble
+ swapped = swap_nibbles(ef[2:]).rstrip('f')
oe = (int(swapped[0])>>3) & 1 # Odd (1) / Even (0)
- if oe:
+ if not oe:
+ # if even, only half of last byte was used
l = l-1
- if l+1 > len(swapped):
+ if l != len(swapped) - 1:
return None
- imsi = swapped[1:l+2]
+ imsi = swapped[1:]
return imsi
def dec_iccid(ef):