chrome/browser/policy/test/asn1der.py

   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
   7 import binascii
   8 import struct
   9
  10 # Tags as defined by ASN.1.
  11 INTEGER = 2
  12 BIT_STRING = 3
  13 NULL = 5
  14 OBJECT_IDENTIFIER = 6
  15 SEQUENCE = 0x30
  16
  17 def 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) < 128:
  27     return struct.pack(">BB", tag, len(data)) + data;
  28   assert len(data) <= 0xffff;
  29   return struct.pack(">BBH", tag, 0x82, len(data)) + data;
  30
  31 def 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   if (len(data) % 2 == 1):
  41     # Odd number of non-zero bytes - pad out our data to a full number of bytes.
  42     data = '0' + data
  43
  44   # If the high bit is set, need to prepend a null byte to denote a positive
  45   # number.
  46   if (int(data[0], 16) >= 8):
  47     data = '00' + data
  48
  49   return Data(INTEGER, binascii.unhexlify(data))
  50
  51 def Bitstring(value):
  52   """Encodes a bit string.
  53
  54   Args:
  55     value: a string holding the binary data.
  56   Returns:
  57     encoded TLV value.
  58   """
  59   return Data(BIT_STRING, '\x00' + value)
  60
  61 def Sequence(values):
  62   """Encodes a sequence of other values.
  63
  64   Args:
  65     values: the list of values, must be strings holding already encoded data.
  66   Returns:
  67     encoded TLV value.
  68   """
  69   return Data(SEQUENCE, ''.join(values))