asn1der.py revision 2a99a7e74a7f215066514fe81d2bfa6639d9eddd 
1# Copyright (c) 2011 The Chromium Authors. All rights reserved.
2# Use of this source code is governed by a BSD-style license that can be
3# found in the LICENSE file.
4
5"""Helper module for ASN.1/DER encoding."""
6
7import binascii
8import struct
9
10# Tags as defined by ASN.1.
11INTEGER = 2
12BIT_STRING = 3
13NULL = 5
14OBJECT_IDENTIFIER = 6
15SEQUENCE = 0x30
16
17def Data(tag, data):
18  """Generic type-length-value encoder.
19
20  Args:
21    tag: the tag.
22    data: the data for the given tag.
23  Returns:
24    encoded TLV value.
25  """
26  if len(data) == 0:
27    return struct.pack(">BB", tag, 0);
28  assert len(data) <= 0xffff;
29  return struct.pack(">BBH", tag, 0x82, len(data)) + data;
30
31def Integer(value):
32  """Encodes an integer.
33
34  Args:
35    value: the long value.
36  Returns:
37    encoded TLV value.
38  """
39  data = '%x' % value
40  return Data(INTEGER, binascii.unhexlify('00' + '0' * (len(data) % 2) + data))
41
42def Bitstring(value):
43  """Encodes a bit string.
44
45  Args:
46    value: a string holding the binary data.
47  Returns:
48    encoded TLV value.
49  """
50  return Data(BIT_STRING, '\x00' + value)
51
52def Sequence(values):
53  """Encodes a sequence of other values.
54
55  Args:
56    values: the list of values, must be strings holding already encoded data.
57  Returns:
58    encoded TLV value.
59  """
60  return Data(SEQUENCE, ''.join(values))
61