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# Copyright (c) 2011 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))
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