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# coding: utf-8
"""
Functions for parsing and dumping using the ASN.1 DER encoding. Exports the
following items:
- emit()
- parse()
- peek()
Other type classes are defined that help compose the types listed above.
"""
from __future__ import unicode_literals, division, absolute_import, print_function
import sys
from ._types import byte_cls, chr_cls, type_name
from .util import int_from_bytes, int_to_bytes
_PY2 = sys.version_info <= (3,)
_INSUFFICIENT_DATA_MESSAGE = 'Insufficient data - %s bytes requested but only %s available'
def emit(class_, method, tag, contents):
"""
Constructs a byte string of an ASN.1 DER-encoded value
This is typically not useful. Instead, use one of the standard classes from
asn1crypto.core, or construct a new class with specific fields, and call the
.dump() method.
:param class_:
An integer ASN.1 class value: 0 (universal), 1 (application),
2 (context), 3 (private)
:param method:
An integer ASN.1 method value: 0 (primitive), 1 (constructed)
:param tag:
An integer ASN.1 tag value
:param contents:
A byte string of the encoded byte contents
:return:
A byte string of the ASN.1 DER value (header and contents)
"""
if not isinstance(class_, int):
raise TypeError('class_ must be an integer, not %s' % type_name(class_))
if class_ < 0 or class_ > 3:
raise ValueError('class_ must be one of 0, 1, 2 or 3, not %s' % class_)
if not isinstance(method, int):
raise TypeError('method must be an integer, not %s' % type_name(method))
if method < 0 or method > 1:
raise ValueError('method must be 0 or 1, not %s' % method)
if not isinstance(tag, int):
raise TypeError('tag must be an integer, not %s' % type_name(tag))
if tag < 0:
raise ValueError('tag must be greater than zero, not %s' % tag)
if not isinstance(contents, byte_cls):
raise TypeError('contents must be a byte string, not %s' % type_name(contents))
return _dump_header(class_, method, tag, contents) + contents
def parse(contents, strict=False):
"""
Parses a byte string of ASN.1 BER/DER-encoded data.
This is typically not useful. Instead, use one of the standard classes from
asn1crypto.core, or construct a new class with specific fields, and call the
.load() class method.
:param contents:
A byte string of BER/DER-encoded data
:param strict:
A boolean indicating if trailing data should be forbidden - if so, a
ValueError will be raised when trailing data exists
:raises:
ValueError - when the contents do not contain an ASN.1 header or are truncated in some way
TypeError - when contents is not a byte string
:return:
A 6-element tuple:
- 0: integer class (0 to 3)
- 1: integer method
- 2: integer tag
- 3: byte string header
- 4: byte string content
- 5: byte string trailer
"""
if not isinstance(contents, byte_cls):
raise TypeError('contents must be a byte string, not %s' % type_name(contents))
contents_len = len(contents)
info, consumed = _parse(contents, contents_len)
if strict and consumed != contents_len:
raise ValueError('Extra data - %d bytes of trailing data were provided' % (contents_len - consumed))
return info
def peek(contents):
"""
Parses a byte string of ASN.1 BER/DER-encoded data to find the length
This is typically used to look into an encoded value to see how long the
next chunk of ASN.1-encoded data is. Primarily it is useful when a
value is a concatenation of multiple values.
:param contents:
A byte string of BER/DER-encoded data
:raises:
ValueError - when the contents do not contain an ASN.1 header or are truncated in some way
TypeError - when contents is not a byte string
:return:
An integer with the number of bytes occupied by the ASN.1 value
"""
if not isinstance(contents, byte_cls):
raise TypeError('contents must be a byte string, not %s' % type_name(contents))
info, consumed = _parse(contents, len(contents))
return consumed
def _parse(encoded_data, data_len, pointer=0, lengths_only=False):
"""
Parses a byte string into component parts
:param encoded_data:
A byte string that contains BER-encoded data
:param data_len:
The integer length of the encoded data
:param pointer:
The index in the byte string to parse from
:param lengths_only:
A boolean to cause the call to return a 2-element tuple of the integer
number of bytes in the header and the integer number of bytes in the
contents. Internal use only.
:return:
A 2-element tuple:
- 0: A tuple of (class_, method, tag, header, content, trailer)
- 1: An integer indicating how many bytes were consumed
"""
if data_len < pointer + 2:
raise ValueError(_INSUFFICIENT_DATA_MESSAGE % (2, data_len - pointer))
start = pointer
first_octet = ord(encoded_data[pointer]) if _PY2 else encoded_data[pointer]
pointer += 1
tag = first_octet & 31
# Base 128 length using 8th bit as continuation indicator
if tag == 31:
tag = 0
while True:
num = ord(encoded_data[pointer]) if _PY2 else encoded_data[pointer]
pointer += 1
tag *= 128
tag += num & 127
if num >> 7 == 0:
break
length_octet = ord(encoded_data[pointer]) if _PY2 else encoded_data[pointer]
pointer += 1
if length_octet >> 7 == 0:
if lengths_only:
return (pointer, pointer + (length_octet & 127))
contents_end = pointer + (length_octet & 127)
else:
length_octets = length_octet & 127
if length_octets:
pointer += length_octets
contents_end = pointer + int_from_bytes(encoded_data[pointer - length_octets:pointer], signed=False)
if lengths_only:
return (pointer, contents_end)
else:
# To properly parse indefinite length values, we need to scan forward
# parsing headers until we find a value with a length of zero. If we
# just scanned looking for \x00\x00, nested indefinite length values
# would not work.
contents_end = pointer
# Unfortunately we need to understand the contents of the data to
# properly scan forward, which bleeds some representation info into
# the parser. This condition handles the unused bits byte in
# constructed bit strings.
if tag == 3:
contents_end += 1
while contents_end < data_len:
sub_header_end, contents_end = _parse(encoded_data, data_len, contents_end, lengths_only=True)
if contents_end == sub_header_end and encoded_data[contents_end - 2:contents_end] == b'\x00\x00':
break
if lengths_only:
return (pointer, contents_end)
if contents_end > data_len:
raise ValueError(_INSUFFICIENT_DATA_MESSAGE % (contents_end, data_len))
return (
(
first_octet >> 6,
(first_octet >> 5) & 1,
tag,
encoded_data[start:pointer],
encoded_data[pointer:contents_end - 2],
b'\x00\x00'
),
contents_end
)
if contents_end > data_len:
raise ValueError(_INSUFFICIENT_DATA_MESSAGE % (contents_end, data_len))
return (
(
first_octet >> 6,
(first_octet >> 5) & 1,
tag,
encoded_data[start:pointer],
encoded_data[pointer:contents_end],
b''
),
contents_end
)
def _dump_header(class_, method, tag, contents):
"""
Constructs the header bytes for an ASN.1 object
:param class_:
An integer ASN.1 class value: 0 (universal), 1 (application),
2 (context), 3 (private)
:param method:
An integer ASN.1 method value: 0 (primitive), 1 (constructed)
:param tag:
An integer ASN.1 tag value
:param contents:
A byte string of the encoded byte contents
:return:
A byte string of the ASN.1 DER header
"""
header = b''
id_num = 0
id_num |= class_ << 6
id_num |= method << 5
if tag >= 31:
header += chr_cls(id_num | 31)
while tag > 0:
continuation_bit = 0x80 if tag > 0x7F else 0
header += chr_cls(continuation_bit | (tag & 0x7F))
tag = tag >> 7
else:
header += chr_cls(id_num | tag)
length = len(contents)
if length <= 127:
header += chr_cls(length)
else:
length_bytes = int_to_bytes(length)
header += chr_cls(0x80 | len(length_bytes))
header += length_bytes
return header
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