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