path: root/pySim/construct.py

from construct.lib.containers import Container, ListContainer
from construct.core import EnumIntegerString
import typing
from construct import *
from construct.core import evaluate, BitwisableString
from construct.lib import integertypes
from pySim.utils import b2h, h2b, swap_nibbles
import gsm0338
import codecs
import ipaddress

"""Utility code related to the integration of the 'construct' declarative parser."""

# (C) 2021-2022 by Harald Welte <laforge@osmocom.org>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.


class HexAdapter(Adapter):
    """convert a bytes() type to a string of hex nibbles."""

    def _decode(self, obj, context, path):
        return b2h(obj)

    def _encode(self, obj, context, path):
        return h2b(obj)

class Utf8Adapter(Adapter):
    """convert a bytes() type that contains utf8 encoded text to human readable text."""

    def _decode(self, obj, context, path):
        # In case the string contains only 0xff bytes we interpret it as an empty string
        if obj == b'\xff' * len(obj):
                return ""
        return codecs.decode(obj, "utf-8")

    def _encode(self, obj, context, path):
        return codecs.encode(obj, "utf-8")


class BcdAdapter(Adapter):
    """convert a bytes() type to a string of BCD nibbles."""

    def _decode(self, obj, context, path):
        return swap_nibbles(b2h(obj))

    def _encode(self, obj, context, path):
        return h2b(swap_nibbles(obj))

class InvertAdapter(Adapter):
    """inverse logic (false->true, true->false)."""
    @staticmethod
    def _invert_bool_in_obj(obj):
        for k,v in obj.items():
            # skip all private entries
            if k.startswith('_'):
                continue
            if v == False:
                obj[k] = True
            elif v == True:
                obj[k] = False
        return obj

    def _decode(self, obj, context, path):
        return self._invert_bool_in_obj(obj)

    def _encode(self, obj, context, path):
        return self._invert_bool_in_obj(obj)

class Rpad(Adapter):
    """
    Encoder appends padding bytes (b'\\xff') or characters up to target size.
    Decoder removes trailing padding bytes/characters.

    Parameters:
        subcon: Subconstruct as defined by construct library
        pattern: set padding pattern (default: b'\\xff')
        num_per_byte: number of 'elements' per byte. E.g. for hex nibbles: 2
    """

    def __init__(self, subcon, pattern=b'\xff', num_per_byte=1):
        super().__init__(subcon)
        self.pattern = pattern
        self.num_per_byte = num_per_byte

    def _decode(self, obj, context, path):
        return obj.rstrip(self.pattern)

    def _encode(self, obj, context, path):
        target_size = self.sizeof() * self.num_per_byte
        if len(obj) > target_size:
            raise SizeofError("Input ({}) exceeds target size ({})".format(
                len(obj), target_size))
        return obj + self.pattern * (target_size - len(obj))

class MultiplyAdapter(Adapter):
    """
    Decoder multiplies by multiplicator
    Encoder divides by multiplicator

    Parameters:
        subcon: Subconstruct as defined by construct library
        multiplier: Multiplier to apply to raw encoded value
    """

    def __init__(self, subcon, multiplicator):
        super().__init__(subcon)
        self.multiplicator = multiplicator

    def _decode(self, obj, context, path):
        return obj * 8

    def _encode(self, obj, context, path):
        return obj // 8


class GsmStringAdapter(Adapter):
    """Convert GSM 03.38 encoded bytes to a string."""

    def __init__(self, subcon, codec='gsm03.38', err='strict'):
        super().__init__(subcon)
        self.codec = codec
        self.err = err

    def _decode(self, obj, context, path):
        return obj.decode(self.codec)

    def _encode(self, obj, context, path):
        return obj.encode(self.codec, self.err)

class Ipv4Adapter(Adapter):
    """
    Encoder converts from 4 bytes to string representation (A.B.C.D).
    Decoder converts from string representation (A.B.C.D) to four bytes.
    """
    def _decode(self, obj, context, path):
        ia = ipaddress.IPv4Address(obj)
        return ia.compressed

    def _encode(self, obj, context, path):
        ia = ipaddress.IPv4Address(obj)
        return ia.packed

class Ipv6Adapter(Adapter):
    """
    Encoder converts from 16 bytes to string representation.
    Decoder converts from string representation to 16 bytes.
    """
    def _decode(self, obj, context, path):
        ia = ipaddress.IPv6Address(obj)
        return ia.compressed

    def _encode(self, obj, context, path):
        ia = ipaddress.IPv6Address(obj)
        return ia.packed


def filter_dict(d, exclude_prefix='_'):
    """filter the input dict to ensure no keys starting with 'exclude_prefix' remain."""
    if not isinstance(d, dict):
        return d
    res = {}
    for (key, value) in d.items():
        if key.startswith(exclude_prefix):
            continue
        if type(value) is dict:
            res[key] = filter_dict(value)
        else:
            res[key] = value
    return res


def normalize_construct(c):
    """Convert a construct specific type to a related base type, mostly useful
    so we can serialize it."""
    # we need to include the filter_dict as we otherwise get elements like this
    # in the dict: '_io': <_io.BytesIO object at 0x7fdb64e05860> which we cannot json-serialize
    c = filter_dict(c)
    if isinstance(c, Container) or isinstance(c, dict):
        r = {k: normalize_construct(v) for (k, v) in c.items()}
    elif isinstance(c, ListContainer):
        r = [normalize_construct(x) for x in c]
    elif isinstance(c, list):
        r = [normalize_construct(x) for x in c]
    elif isinstance(c, EnumIntegerString):
        r = str(c)
    else:
        r = c
    return r


def parse_construct(c, raw_bin_data: bytes, length: typing.Optional[int] = None, exclude_prefix: str = '_', context: dict = {}):
    """Helper function to wrap around normalize_construct() and filter_dict()."""
    if not length:
        length = len(raw_bin_data)
    parsed = c.parse(raw_bin_data, total_len=length, **context)
    return normalize_construct(parsed)

def build_construct(c, decoded_data, context: dict = {}):
    """Helper function to handle total_len."""
    return c.build(decoded_data, total_len=None, **context)

# here we collect some shared / common definitions of data types
LV = Prefixed(Int8ub, HexAdapter(GreedyBytes))

# Default value for Reserved for Future Use (RFU) bits/bytes
# See TS 31.101 Sec. "3.4 Coding Conventions"
__RFU_VALUE = 0

# Field that packs Reserved for Future Use (RFU) bit
FlagRFU = Default(Flag, __RFU_VALUE)

# Field that packs Reserved for Future Use (RFU) byte
ByteRFU = Default(Byte, __RFU_VALUE)

# Field that packs all remaining Reserved for Future Use (RFU) bytes
GreedyBytesRFU = Default(GreedyBytes, b'')


def BitsRFU(n=1):
    '''
    Field that packs Reserved for Future Use (RFU) bit(s)
    as defined in TS 31.101 Sec. "3.4 Coding Conventions"

    Use this for (currently) unused/reserved bits whose contents
    should be initialized automatically but should not be cleared
    in the future or when restoring read data (unlike padding).

    Parameters:
        n (Integer): Number of bits (default: 1)
    '''
    return Default(BitsInteger(n), __RFU_VALUE)


def BytesRFU(n=1):
    '''
    Field that packs Reserved for Future Use (RFU) byte(s)
    as defined in TS 31.101 Sec. "3.4 Coding Conventions"

    Use this for (currently) unused/reserved bytes whose contents
    should be initialized automatically but should not be cleared
    in the future or when restoring read data (unlike padding).

    Parameters:
        n (Integer): Number of bytes (default: 1)
    '''
    return Default(Bytes(n), __RFU_VALUE)


def GsmString(n):
    '''
    GSM 03.38 encoded byte string of fixed length n.
    Encoder appends padding bytes (b'\\xff') to maintain
    length. Decoder removes those trailing bytes.

    Exceptions are raised for invalid characters
    and length excess.

    Parameters:
        n (Integer): Fixed length of the encoded byte string
    '''
    return GsmStringAdapter(Rpad(Bytes(n), pattern=b'\xff'), codec='gsm03.38')

class GreedyInteger(Construct):
    """A variable-length integer implementation, think of combining GrredyBytes with BytesInteger."""
    def __init__(self, signed=False, swapped=False, minlen=0):
        super().__init__()
        self.signed = signed
        self.swapped = swapped
        self.minlen = minlen

    def _parse(self, stream, context, path):
        data = stream_read_entire(stream, path)
        if evaluate(self.swapped, context):
            data = swapbytes(data)
        try:
            return int.from_bytes(data, byteorder='big', signed=self.signed)
        except ValueError as e:
            raise IntegerError(str(e), path=path)

    def __bytes_required(self, i, minlen=0):
        if self.signed:
            raise NotImplementedError("FIXME: Implement support for encoding signed integer")

        # compute how many bytes we need
        nbytes = 1
        while True:
            i = i >> 8
            if i == 0:
                break
            else:
                nbytes = nbytes + 1

        # round up to the minimum number
        # of bytes we anticipate
        if nbytes < minlen:
            nbytes = minlen

        return nbytes

    def _build(self, obj, stream, context, path):
        if not isinstance(obj, integertypes):
            raise IntegerError(f"value {obj} is not an integer", path=path)
        length = self.__bytes_required(obj, self.minlen)
        try:
            data = obj.to_bytes(length, byteorder='big', signed=self.signed)
        except ValueError as e:
            raise IntegerError(str(e), path=path)
        if evaluate(self.swapped, context):
            data = swapbytes(data)
        stream_write(stream, data, length, path)
        return obj

# merged definitions of 24.008 + 23.040
TypeOfNumber = Enum(BitsInteger(3), unknown=0, international=1, national=2, network_specific=3,
                    short_code=4, alphanumeric=5, abbreviated=6, reserved_for_extension=7)
NumberingPlan = Enum(BitsInteger(4), unknown=0, isdn_e164=1, data_x121=3, telex_f69=4,
                     sc_specific_5=5, sc_specific_6=6, national=8, private=9,
                     ermes=10, reserved_cts=11, reserved_for_extension=15)
TonNpi = BitStruct('ext'/Flag, 'type_of_number'/TypeOfNumber, 'numbering_plan_id'/NumberingPlan)