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| 1 #!/usr/bin/env python |
| 2 # Copyright (c) 2012 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)) |
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