client/cros/mock_policies/asn1der.py

#!/usr/bin/python2.7
# Copyright 2019 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Helper module for ASN.1/DER encoding."""

import binascii
import struct

# Tags as defined by ASN.1.
INTEGER = 2
BIT_STRING = 3
NULL = 5
OBJECT_IDENTIFIER = 6
SEQUENCE = 0x30

def Data(tag, data):
  """Generic type-length-value encoder.

  Args:
    tag: the tag.
    data: the data for the given tag.
  Returns:
    encoded TLV value.
  """
  if len(data) < 128:
    return struct.pack(">BB", tag, len(data)) + data;
  assert len(data) <= 0xffff;
  return struct.pack(">BBH", tag, 0x82, len(data)) + data;

def Integer(value):
  """Encodes an integer.

  Args:
    value: the long value.
  Returns:
    encoded TLV value.
  """
  data = '%x' % value
  if (len(data) % 2 == 1):
    # Odd number of non-zero bytes - pad out our data to a full number of bytes.
    data = '0' + data

  # If the high bit is set, need to prepend a null byte to denote a positive
  # number.
  if (int(data[0], 16) >= 8):
    data = '00' + data

  return Data(INTEGER, binascii.unhexlify(data))

def Bitstring(value):
  """Encodes a bit string.

  Args:
    value: a string holding the binary data.
  Returns:
    encoded TLV value.
  """
  return Data(BIT_STRING, '\x00' + value)

def Sequence(values):
  """Encodes a sequence of other values.

  Args:
    values: the list of values, must be strings holding already encoded data.
  Returns:
    encoded TLV value.
  """
  return Data(SEQUENCE, ''.join(values))