pySim.tlv: Add COMPACT_TLV_IE TLV variant

the COMPACT-TLV variant is a TLV variant that ISO7816 uses for encoding
tag and length into a single octet. This is used (for example) in ATR
historical bytes.

Let's add support for this to our pySim TLV encoder/decoder.

Change-Id: I9e98d150b97317ae0c6be2366bdaaeaeddf8031c

pySim/tlv.py

@@ -246,6 +246,35 @@ class TLV_IE(IE):
         return dec, remainder
 
 
+class COMPACT_TLV_IE(TLV_IE):
+    """TLV_IE formatted as COMPACT-TLV described in ISO 7816"""
+
+    @classmethod
+    def _parse_tag_raw(cls, do: bytes) -> Tuple[int, bytes]:
+        return do[0] >> 4, do
+
+    @classmethod
+    def _decode_tag(cls, do: bytes) -> Tuple[dict, bytes]:
+        rawtag, remainder = cls._parse_tag_raw(do)
+        return {'tag': rawtag}, remainder
+
+    @classmethod
+    def _parse_len(cls, do: bytes) -> Tuple[int, bytes]:
+        return do[0] & 0xf, do[1:]
+
+    def _encode_tag(self) -> bytes:
+        """Not needed as we override the to_tlv() method to encode tag+length into one byte."""
+        raise NotImplementedError
+
+    def _encode_len(self):
+        """Not needed as we override the to_tlv() method to encode tag+length into one byte."""
+        raise NotImplementedError
+
+    def to_tlv(self, context: dict = {}):
+        val = self.to_bytes(context=context)
+        return bytes([(self.tag << 4) | (len(val) & 0xF)]) + val
+
+
 class BER_TLV_IE(TLV_IE):
     """TLV_IE formatted as ASN.1 BER described in ITU-T X.690 8.1.2."""
 

tests/test_tlv.py

@@ -17,8 +17,9 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import unittest
-from construct import Int8ub
+from construct import Int8ub, GreedyBytes
 from pySim.tlv import *
+from pySim.utils import h2b
 
 class TestUtils(unittest.TestCase):
     def test_camel_to_snake(self):
@@ -117,5 +118,28 @@ class TestIE(unittest.TestCase):
         self.assertEqual(ie.to_bytes(), b'\x42')
         self.assertEqual(ie.to_ie(), b'\x42')
 
+class TestCompact(unittest.TestCase):
+    class IE_3(COMPACT_TLV_IE, tag=0x3):
+        _construct = GreedyBytes
+    class IE_7(COMPACT_TLV_IE, tag=0x7):
+        _construct = GreedyBytes
+    class IE_5(COMPACT_TLV_IE, tag=0x5):
+        _construct = GreedyBytes
+    # pylint: disable=undefined-variable
+    class IE_Coll(TLV_IE_Collection, nested=[IE_3, IE_7, IE_5]):
+        _construct = GreedyBytes
+    def test_ATR(self):
+        atr = h2b("31E073FE211F5745437531301365")
+        c = self.IE_Coll()
+        c.from_tlv(atr)
+        self.assertEqual(c.children[0].tag, 3)
+        self.assertEqual(c.children[0].to_bytes(), b'\xe0')
+        self.assertEqual(c.children[1].tag, 7)
+        self.assertEqual(c.children[1].to_bytes(), b'\xfe\x21\x1f')
+        self.assertEqual(c.children[2].tag, 5)
+        self.assertEqual(c.children[2].to_bytes(), b'\x45\x43\x75\x31\x30\x13\x65')
+        self.assertEqual(c.to_tlv(), atr)
+
+
 if __name__ == "__main__":
 	unittest.main()