pySim/ota.py

"""Code related to SIM/UICC OTA according to TS 102 225 + TS 31.115."""

# (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/>.

from pySim.construct import *
from pySim.utils import b2h
from pySim.sms import UserDataHeader
from construct import *
import zlib
import abc
import struct
from typing import Optional

# ETS TS 102 225 gives the general command structure and the dialects for CAT_TP, TCP/IP and HTTPS
# 3GPP TS 31.115 gives the dialects for SMS-PP, SMS-CB, USSD and HTTP

# CPI CPL CHI CHL SPI KIc KID TAR CNTR PCNTR RC/CC/DS data

#                                           CAT_TP  TCP/IP          SMS
# CPI                                       0x01    0x01            =IEIa=70,len=0
# CHI                                       NULL    NULL            NULL
# CPI, CPL and CHL included in RC/CC/DS     true    true
# RPI                                       0x02    0x02            =IEIa=71,len=0
# RHI                                       NULL    NULL
# RPI, RPL and RHL included in RC/CC/DS     true    true
# packet-id                             0-bf,ff     0-bf,ff
# identification packet                     false   102 225 tbl 6

# KVN 1..f; KI1=KIc, KI2=KID, KI3=DEK

# ETSI TS 102 225 Table 5 + 3GPP TS 31.115 Section 7
ResponseStatus = Enum(Int8ub, por_ok=0, rc_cc_ds_failed=1, cntr_low=2, cntr_high=3,
                      cntr_blocked=4, ciphering_error=5, undefined_security_error=6,
                      insufficient_memory=7, more_time_needed=8, tar_unknown=9,
                      insufficient_security_level=0x0A,
                      actual_response_sms_submit=0x0B,
                      actual_response_ussd=0x0C)

# ETSI TS 102 226 Section 5.1.2
CompactRemoteResp = Struct('number_of_commands'/Int8ub,
                           'last_status_word'/HexAdapter(Bytes(2)),
                           'last_response_data'/HexAdapter(GreedyBytes))

RC_CC_DS = Enum(BitsInteger(2), no_rc_cc_ds=0, rc=1, cc=2, ds=3)

# TS 102 225 Section 5.1.1 + TS 31.115 Section 4.2
SPI = BitStruct(  # first octet
    Padding(3),
    'counter'/Enum(BitsInteger(2), no_counter=0, counter_no_replay_or_seq=1,
                   counter_must_be_higher=2, counter_must_be_lower=3),
    'ciphering'/Flag,
    'rc_cc_ds'/RC_CC_DS,
    # second octet
    Padding(2),
    'por_in_submit'/Flag,
    'por_shall_be_ciphered'/Flag,
    'por_rc_cc_ds'/RC_CC_DS,
    'por'/Enum(BitsInteger(2), no_por=0,
               por_required=1, por_only_when_error=2)
)

# TS 102 225 Section 5.1.2
KIC = BitStruct('key'/BitsInteger(4),
                'algo'/Enum(BitsInteger(4), implicit=0, single_des=1, triple_des_cbc2=5, triple_des_cbc3=9,
                            aes_cbc=2)
                )

# TS 102 225 Section 5.1.3.1
KID_CC = BitStruct('key'/BitsInteger(4),
                   'algo'/Enum(BitsInteger(4), implicit=0, single_des=1, triple_des_cbc2=5, triple_des_cbc3=9,
                               aes_cmac=2)
                   )

# TS 102 225 Section 5.1.3.2
KID_RC = BitStruct('key'/BitsInteger(4),
                   'algo'/Enum(BitsInteger(4), implicit=0, crc16=1, crc32=5, proprietary=3)
                   )

SmsCommandPacket = Struct('cmd_pkt_len'/Int16ub,
                          'cmd_hdr_len'/Int8ub,
                          'spi'/SPI,
                          'kic'/KIC,
                          'kid'/Switch(this.spi.rc_cc_ds, {'cc': KID_CC, 'rc': KID_RC }),
                          'tar'/Bytes(3),
                          'secured_data'/GreedyBytes)

class OtaKeyset:
    """The OTA related data (key material, counter) to be used in encrypt/decrypt."""
    def __init__(self, algo_crypt: str, kic_idx: int, kic: bytes,
                 algo_auth: str, kid_idx: int, kid: bytes, cntr: int = 0):
        self.algo_crypt = algo_crypt
        self.kic = bytes(kic)
        self.kic_idx = kic_idx
        self.algo_auth = algo_auth
        self.kid = bytes(kid)
        self.kid_idx = kid_idx
        self.cntr = cntr

    @property
    def auth(self):
        """Return an instance of the matching OtaAlgoAuth."""
        return OtaAlgoAuth.fromKeyset(self)

    @property
    def crypt(self):
        """Return an instance of the matching OtaAlgoCrypt."""
        return OtaAlgoCrypt.fromKeyset(self)

class OtaCheckError(Exception):
    pass

class OtaDialect(abc.ABC):
    """Base Class for OTA dialects such as SMS, BIP, ..."""

    def _compute_sig_len(self, spi:SPI):
        if spi['rc_cc_ds'] == 'no_rc_cc_ds':
            return 0
        elif spi['rc_cc_ds'] == 'rc': # CRC-32
            return 4
        elif spi['rc_cc_ds'] == 'cc': # Cryptographic Checksum (CC)
            # TODO: this is not entirely correct, as in AES case it could be 4 or 8
            return 8
        else:
            raise ValueError("Invalid rc_cc_ds: %s" % spi['rc_cc_ds'])

    @abc.abstractmethod
    def encode_cmd(self, otak: OtaKeyset, tar: bytes, apdu: bytes) -> bytes:
        pass

    @abc.abstractmethod
    def decode_resp(self, otak: OtaKeyset, apdu: bytes) -> (object, Optional["CompactRemoteResp"]):
        """Decode a response into a response packet and, if indicted (by a
        response status of `"por_ok"`) a decoded response.

        The response packet's common characteristics are not fully determined,
        and (so far) completely proprietary per dialect."""
        pass


from Crypto.Cipher import DES, DES3, AES
from Crypto.Hash import CMAC

class OtaAlgo(abc.ABC):
    iv = property(lambda self: bytes([0] * self.blocksize))
    blocksize = None
    enum_name = None

    @staticmethod
    def _get_padding(in_len: int, multiple: int, padding: int = 0):
        """Return padding bytes towards multiple of N."""
        if in_len % multiple == 0:
            return b''
        pad_cnt = multiple - (in_len % multiple)
        return b'\x00' * pad_cnt

    @staticmethod
    def _pad_to_multiple(indat: bytes, multiple: int, padding: int = 0):
        """Pad input bytes to multiple of N."""
        return indat + OtaAlgo._get_padding(len(indat), multiple, padding)

    def pad_to_blocksize(self, indat: bytes, padding: int = 0):
        """Pad the given input data to multiple of the cipher block size."""
        return self._pad_to_multiple(indat, self.blocksize, padding)

    def __init__(self, otak: OtaKeyset):
        self.otak = otak

    def __str__(self):
        return self.__class__.__name__

class OtaAlgoCrypt(OtaAlgo, abc.ABC):
    def __init__(self, otak: OtaKeyset):
        if self.enum_name != otak.algo_crypt:
            raise ValueError('Cannot use algorithm %s with key for %s' % (self.enum_name, otak.algo_crypt))
        super().__init__(otak)

    def encrypt(self, data:bytes) -> bytes:
        """Encrypt given input bytes using the key material given in constructor."""
        padded_data = self.pad_to_blocksize(data)
        return self._encrypt(data)

    def decrypt(self, data:bytes) -> bytes:
        """Decrypt given input bytes using the key material given in constructor."""
        return self._decrypt(data)

    @abc.abstractmethod
    def _encrypt(self, data:bytes) -> bytes:
        """Actual implementation, to be implemented by derived class."""
        pass

    @abc.abstractmethod
    def _decrypt(self, data:bytes) -> bytes:
        """Actual implementation, to be implemented by derived class."""
        pass

    @classmethod
    def fromKeyset(cls, otak: OtaKeyset) -> 'OtaAlgoCrypt':
        """Resolve the class for the encryption algorithm of otak and instantiate it."""
        for subc in cls.__subclasses__():
            if subc.enum_name == otak.algo_crypt:
                return subc(otak)
        raise ValueError('No implementation for crypt algorithm %s' % otak.algo_auth)

class OtaAlgoAuth(OtaAlgo, abc.ABC):
    def __init__(self, otak: OtaKeyset):
        if self.enum_name != otak.algo_auth:
            raise ValueError('Cannot use algorithm %s with key for %s' % (self.enum_name, otak.algo_crypt))
        super().__init__(otak)

    def sign(self, data:bytes) -> bytes:
        """Compute the CC/CR check bytes for the input data using key material
        given in constructor."""
        padded_data = self.pad_to_blocksize(data)
        sig = self._sign(padded_data)
        return sig

    def check_sig(self, data:bytes, cc_received:bytes):
        """Compute the CC/CR check bytes for the input data and compare against cc_received."""
        cc = self.sign(data)
        if cc_received != cc:
            raise OtaCheckError('Received CC (%s) != Computed CC (%s)' % (b2h(cc_received), b2h(cc)))

    @abc.abstractmethod
    def _sign(self, data:bytes) -> bytes:
        """Actual implementation, to be implemented by derived class."""
        pass

    @classmethod
    def fromKeyset(cls, otak: OtaKeyset) -> 'OtaAlgoAuth':
        """Resolve the class for the authentication algorithm of otak and instantiate it."""
        for subc in cls.__subclasses__():
            if subc.enum_name == otak.algo_auth:
                return subc(otak)
        raise ValueError('No implementation for auth algorithm %s' % otak.algo_auth)

class OtaAlgoCryptDES(OtaAlgoCrypt):
    """DES is insecure.  For backwards compatibility with pre-Rel8"""
    name = 'DES'
    enum_name = 'single_des'
    blocksize = 8
    def _encrypt(self, data:bytes) -> bytes:
        cipher = DES.new(self.otak.kic, DES.MODE_CBC, self.iv)
        return cipher.encrypt(data)

    def _decrypt(self, data:bytes) -> bytes:
        cipher = DES.new(self.otak.kic, DES.MODE_CBC, self.iv)
        return cipher.decrypt(data)

class OtaAlgoAuthDES(OtaAlgoAuth):
    """DES is insecure.  For backwards compatibility with pre-Rel8"""
    name = 'DES'
    enum_name = 'single_des'
    blocksize = 8
    def _sign(self, data:bytes) -> bytes:
        cipher = DES.new(self.otak.kid, DES.MODE_CBC, self.iv)
        ciph = cipher.encrypt(data)
        return ciph[len(ciph) - 8:]

class OtaAlgoCryptDES3(OtaAlgoCrypt):
    name = '3DES'
    enum_name = 'triple_des_cbc2'
    blocksize = 8
    def _encrypt(self, data:bytes) -> bytes:
        cipher = DES3.new(self.otak.kic, DES3.MODE_CBC, self.iv)
        return cipher.encrypt(data)

    def _decrypt(self, data:bytes) -> bytes:
        cipher = DES3.new(self.otak.kic, DES3.MODE_CBC, self.iv)
        return cipher.decrypt(data)

class OtaAlgoAuthDES3(OtaAlgoAuth):
    name = '3DES'
    enum_name = 'triple_des_cbc2'
    blocksize = 8
    def _sign(self, data:bytes) -> bytes:
        cipher = DES3.new(self.otak.kid, DES3.MODE_CBC, self.iv)
        ciph = cipher.encrypt(data)
        return ciph[len(ciph) - 8:]

class OtaAlgoCryptAES(OtaAlgoCrypt):
    name = 'AES'
    enum_name = 'aes_cbc'
    blocksize = 16 # TODO: is this needed?
    def _encrypt(self, data:bytes) -> bytes:
        cipher = AES.new(self.otak.kic, AES.MODE_CBC, self.iv)
        return cipher.encrypt(data)

    def _decrypt(self, data:bytes) -> bytes:
        cipher = AES.new(self.otak.kic, AES.MODE_CBC, self.iv)
        return cipher.decrypt(data)

class OtaAlgoAuthAES(OtaAlgoAuth):
    name = 'AES'
    enum_name = 'aes_cmac'
    blocksize = 16 # TODO: is this needed?
    def _sign(self, data:bytes) -> bytes:
        cmac = CMAC.new(self.otak.kid, ciphermod=AES, mac_len=8)
        cmac.update(data)
        ciph = cmac.digest()
        return ciph[len(ciph) - 8:]



class OtaDialectSms(OtaDialect):
    """OTA dialect for SMS based transport, as described in 3GPP TS 31.115."""
    SmsResponsePacket = Struct('rpl'/Int16ub,
                               'rhl'/Int8ub,
                               'tar'/Bytes(3),
                               'cntr'/Bytes(5),
                               'pcntr'/Int8ub,
                               'response_status'/ResponseStatus,
                               'cc_rc'/Bytes(this.rhl-10),
                               'secured_data'/GreedyBytes)

    def encode_cmd(self, otak: OtaKeyset, tar: bytes, spi: dict, apdu: bytes) -> bytes:
        # length of signature in octets
        len_sig = self._compute_sig_len(spi)
        pad_cnt = 0
        if spi['ciphering']: # ciphering is requested
            # append padding bytes to end up with blocksize
            len_cipher = 6 + len_sig + len(apdu)
            padding = otak.crypt._get_padding(len_cipher, otak.crypt.blocksize)
            pad_cnt = len(padding)
            apdu += padding

        kic = {'key': otak.kic_idx, 'algo': otak.algo_crypt}
        kid = {'key': otak.kid_idx, 'algo': otak.algo_auth}

        # CHL = number of octets from (and including) SPI to the end of RC/CC/DS
        # 13 == SPI(2) + KIc(1) + KId(1) + TAR(3) + CNTR(5) + PCNTR(1)
        chl = 13 + len_sig

        # CHL + SPI (+ KIC + KID)
        c = Struct('chl'/Int8ub, 'spi'/SPI, 'kic'/KIC, 'kid'/KID_CC, 'tar'/Bytes(3))
        part_head = c.build({'chl': chl, 'spi':spi, 'kic':kic, 'kid':kid, 'tar':tar})
        #print("part_head: %s" % b2h(part_head))

        # CNTR + PCNTR (CNTR not used)
        part_cnt = otak.cntr.to_bytes(5, 'big') + pad_cnt.to_bytes(1, 'big')
        #print("part_cnt: %s" % b2h(part_cnt))

        envelope_data = part_head + part_cnt + apdu
        #print("envelope_data: %s" % b2h(envelope_data))

        # 2-byte CPL. CPL is part of RC/CC/CPI to end of secured data, including any padding for ciphering
        # CPL from and including CPI to end of secured data, including any padding for ciphering
        cpl = len(envelope_data) + len_sig
        envelope_data = cpl.to_bytes(2, 'big') + envelope_data
        #print("envelope_data with cpl: %s" % b2h(envelope_data))

        if spi['rc_cc_ds'] == 'cc':
            cc = otak.auth.sign(envelope_data)
            envelope_data = part_cnt + cc + apdu
        elif spi['rc_cc_ds'] == 'rc':
            # CRC32
            crc32 = zlib.crc32(envelope_data) & 0xffffffff
            envelope_data = part_cnt + crc32.to_bytes(4, 'big') + apdu
        elif spi['rc_cc_ds'] == 'no_rc_cc_ds':
            envelope_data = part_cnt + apdu
        else:
            raise ValueError("Invalid rc_cc_ds: %s" % spi['rc_cc_ds'])

        #print("envelope_data with sig: %s" % b2h(envelope_data))

        # encrypt as needed
        if spi['ciphering']: # ciphering is requested
            ciph = otak.crypt.encrypt(envelope_data)
            envelope_data = part_head + ciph
            # prefix with another CPL
            cpl = len(envelope_data)
            envelope_data = cpl.to_bytes(2, 'big') + envelope_data
        else:
            envelope_data = part_head + envelope_data

        #print("envelope_data: %s" % b2h(envelope_data))

        return envelope_data

    def decode_resp(self, otak: OtaKeyset, spi: dict, data: bytes) -> ("OtaDialectSms.SmsResponsePacket", Optional["CompactRemoteResp"]):
        if isinstance(data, str):
            data = h2b(data)
        # plain-text POR:   027100000e0ab000110000000000000001612f
        # UDHL RPI IEDLa  RPL  RHL TAR    CNTR       PCNTR STS
        # 02   71  00     000e 0a  b00011 0000000000 00    00  01 612f
        # POR with CC:      027100001612b000110000000000000055f47118381175fb01612f
        # POR with CC+CIPH: 027100001c12b000119660ebdb81be189b5e4389e9e7ab2bc0954f963ad869ed7c
        if data[0] != 0x02:
            raise ValueError('Unexpected UDL=0x%02x' % data[0])
        udhd, remainder = UserDataHeader.fromBytes(data)
        if not udhd.has_ie(0x71):
            raise ValueError('RPI 0x71 not found in UDH')
        rph_rhl_tar = remainder[:6] # RPH+RHL+TAR; not ciphered
        res = self.SmsResponsePacket.parse(remainder)

        if spi['por_shall_be_ciphered']:
            # decrypt
            ciphered_part = remainder[6:]
            deciph = otak.crypt.decrypt(ciphered_part)
            temp_data = rph_rhl_tar + deciph
            res = self.SmsResponsePacket.parse(temp_data)
            # remove specified number of padding bytes, if any
            if res['pcntr'] != 0:
                # this conditional is needed as python [:-0] renders an empty return!
                res['secured_data'] = res['secured_data'][:-res['pcntr']]
            remainder = temp_data

        # is there a CC/RC present?
        len_sig = res['rhl'] - 10
        if spi['por_rc_cc_ds'] == 'no_rc_cc_ds':
            if len_sig:
                raise OtaCheckError('No RC/CC/DS requested, but len_sig=%u' % len_sig)
        elif spi['por_rc_cc_ds'] == 'cc':
            # verify signature
            # UDH is part of CC/RC!
            udh = data[:3]
            # RPL, RHL, TAR, CNTR, PCNTR and STSare part of CC/RC
            rpl_rhl_tar_cntr_pcntr_sts = remainder[:13]
            # remove the CC/RC bytes
            temp_data = udh + rpl_rhl_tar_cntr_pcntr_sts + remainder[13+len_sig:]
            otak.auth.check_sig(temp_data, res['cc_rc'])
        # TODO: CRC
        else:
            raise OtaCheckError('Unknown por_rc_cc_ds: %s' % spi['por_rc_cc_ds'])

        # TODO: ExpandedRemoteResponse according to TS 102 226 5.2.2
        if res.response_status == 'por_ok':
            dec = CompactRemoteResp.parse(res['secured_data'])
        else:
            dec = None
        return (res, dec)