1 #!/usr/bin/python2.5 2 # Copyright (c) 2011 The Chromium Authors. All rights reserved. 3 # Use of this source code is governed by a BSD-style license that can be 4 # found in the LICENSE file. 5 6 """Helper module for ASN.1/DER encoding.""" 7 8 import binascii 9 import struct 10 11 # Tags as defined by ASN.1. 12 INTEGER = 2 13 BIT_STRING = 3 14 NULL = 5 15 OBJECT_IDENTIFIER = 6 16 SEQUENCE = 0x30 17 18 def Data(tag, data): 19 """Generic type-length-value encoder. 20 21 Args: 22 tag: the tag. 23 data: the data for the given tag. 24 Returns: 25 encoded TLV value. 26 """ 27 if len(data) == 0: 28 return struct.pack(">BB", tag, 0); 29 assert len(data) <= 0xffff; 30 return struct.pack(">BBH", tag, 0x82, len(data)) + data; 31 32 def Integer(value): 33 """Encodes an integer. 34 35 Args: 36 value: the long value. 37 Returns: 38 encoded TLV value. 39 """ 40 data = '%x' % value 41 return Data(INTEGER, binascii.unhexlify('00' + '0' * (len(data) % 2) + data)) 42 43 def Bitstring(value): 44 """Encodes a bit string. 45 46 Args: 47 value: a string holding the binary data. 48 Returns: 49 encoded TLV value. 50 """ 51 return Data(BIT_STRING, '\x00' + value) 52 53 def Sequence(values): 54 """Encodes a sequence of other values. 55 56 Args: 57 values: the list of values, must be strings holding already encoded data. 58 Returns: 59 encoded TLV value. 60 """ 61 return Data(SEQUENCE, ''.join(values)) 62
