Corrects BitString decoding

Properly handles initial octet which stores the number of unused bits
and removes the corresponding number of bits.

Adds corresponding unit tests.

src/asn1.py

@@ -544,6 +544,8 @@ def _read_value(self, cls, nr, length):  # type: (int, int, int) -> any
             value = self._decode_object_identifier(bytes_data)
         elif nr in (Numbers.PrintableString, Numbers.IA5String, Numbers.UTCTime):
             value = self._decode_printable_string(bytes_data)
+        elif nr == Numbers.BitString:
+            value = self._decode_bitstring(bytes_data)
         else:
             value = bytes_data
         return value
@@ -647,3 +649,28 @@ def _decode_object_identifier(bytes_data):  # type: (bytes) -> str
     def _decode_printable_string(bytes_data):  # type: (bytes) -> str
         """Decode a printable string."""
         return bytes_data.decode('utf-8')
+
+    @staticmethod
+    def _decode_bitstring(bytes_data):  # type: (bytes) -> str
+        """Decode a bitstring."""
+        if len(bytes_data) == 0:
+            raise Error('ASN1 syntax error')
+
+        num_unused_bits = bytes_data[0]
+        if not (0 <= num_unused_bits <= 7):
+            raise Error('ASN1 syntax error')
+
+        if num_unused_bits == 0:
+            return bytes_data[1:]
+
+        # Shift off unused bits
+        remaining = bytearray(bytes_data[1:])
+        bitmask = (1 << num_unused_bits) - 1
+        removed_bits = 0
+
+        for i in range(len(remaining)):
+            byte = int(remaining[i])
+            remaining[i] = (byte >> num_unused_bits) | (removed_bits << num_unused_bits)
+            removed_bits = byte & bitmask
+
+        return bytes(remaining)

tests/test_asn1.py

@@ -435,6 +435,24 @@ def test_printable_string(self):
         tag, val = dec.read()
         assert val == u'foo'
 
+    def test_bitstring(self):
+        buf = b'\x03\x04\x00\x12\x34\x56'
+        dec = asn1.Decoder()
+        dec.start(buf)
+        tag = dec.peek()
+        assert tag == (asn1.Numbers.BitString, asn1.Types.Primitive, asn1.Classes.Universal)
+        tag, val = dec.read()
+        assert val == b'\x12\x34\x56'
+
+    def test_bitstring_unused_bits(self):
+        buf = b'\x03\x04\x04\x12\x34\x50'
+        dec = asn1.Decoder()
+        dec.start(buf)
+        tag = dec.peek()
+        assert tag == (asn1.Numbers.BitString, asn1.Types.Primitive, asn1.Classes.Universal)
+        tag, val = dec.read()
+        assert val == b'\x01\x23\x45'
+
     def test_unicode_printable_string(self):
         buf = b'\x13\x05\x66\x6f\x6f\xc3\xa9'
         dec = asn1.Decoder()
@@ -703,6 +721,12 @@ def test_error_object_identifier_with_too_large_first_component(self):
         dec.start(buf)
         pytest.raises(asn1.Error, dec.read)
 
+    def test_error_bitstring_with_too_many_unused_bits(self):
+        buf = b'\x03\x04\x08\x12\x34\x50'
+        dec = asn1.Decoder()
+        dec.start(buf)
+        pytest.raises(asn1.Error, dec.read)
+
     def test_big_negative_integer(self):
         buf = b'\x02\x10\xff\x7f\x2b\x3a\x4d\xea\x48\x1e\x1f\x37\x7b\xa8\xbd\x7f\xb0\x16'
         dec = asn1.Decoder()