pySim.tlv: Correctly parse COMPREHENSION-TLV without comprehension bit

The uppermost bit of COMPREHENSION-TLV tags indicates whether the
recipient is required to "comprehend" that IE or not. So every IE
actually has two tag values: one with and one without that bit set.

As all our existing TLV definitions of COMPR_TLV_IE have that bit set,
let's assume this is the default, but use the same definition also for
the situation where that bit is not set.

Change-Id: I58d04ec13be0c12d9fb8cb3d5a0480d0defb6c95
---
M pySim/tlv.py
1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/pySim/tlv.py b/pySim/tlv.py
index 8bf43c6..5835d33 100644
--- a/pySim/tlv.py
+++ b/pySim/tlv.py
@@ -224,12 +224,16 @@
         val = self.to_bytes(context=context)
         return self._encode_tag() + self._encode_len(val) + val
 
+    def is_tag_compatible(self, rawtag) -> bool:
+        """Is the given rawtag compatible with this class?"""
+        return rawtag == self._compute_tag()
+
     def from_tlv(self, do: bytes, context: dict = {}):
         if len(do) == 0:
             return {}, b''
         (rawtag, remainder) = self.__class__._parse_tag_raw(do)
         if rawtag:
-            if rawtag != self._compute_tag():
+            if not self.is_tag_compatible(rawtag):
                 raise ValueError("%s: Encountered tag %s doesn't match our supported tag %s" %
                                  (self, rawtag, self.tag))
             (length, remainder) = self.__class__._parse_len(remainder)
@@ -283,6 +287,15 @@
     def _parse_len(cls, do: bytes) -> Tuple[int, bytes]:
         return bertlv_parse_len(do)
 
+    def is_tag_compatible(self, rawtag: int) -> bool:
+        """Override is_tag_compatible as we need to mask out the
+        comprehension bit when doing compares."""
+        ctag = self._compute_tag()
+        if ctag > 0xff:
+            return ctag & 0x7fff == rawtag & 0x7fff
+        else:
+            return ctag & 0x7f == rawtag & 0x7f
+
     def _encode_tag(self) -> bytes:
         return comprehensiontlv_encode_tag(self._compute_tag())
 
@@ -367,6 +380,8 @@
             tag, _r = first._parse_tag_raw(remainder)
             if tag is None:
                 break
+            if issubclass(first, COMPR_TLV_IE):
+                tag = tag | 0x80 # HACK: always assume comprehension
             if tag in self.members_by_tag:
                 cls = self.members_by_tag[tag]
                 # create an instance and parse accordingly

