# dnssec.py

# DNS extension for automatic GPG key verification

#

# Copyright (C) 2012-2018 Red Hat, Inc.

#

# This copyrighted material is made available to anyone wishing to use,

# modify, copy, or redistribute it subject to the terms and conditions of

# the GNU General Public License v.2, or (at your option) any later version.

# This program is distributed in the hope that it will be useful, but WITHOUT

# ANY WARRANTY expressed or implied, including the implied warranties 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, write to the

# Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

# 02110-1301, USA.  Any Red Hat trademarks that are incorporated in the

# source code or documentation are not subject to the GNU General Public

# License and may only be used or replicated with the express permission of

# Red Hat, Inc.

#

from __future__ import print_function

from __future__ import absolute_import

from __future__ import unicode_literals

from enum import Enum

import base64

import hashlib

import logging

import re

from dnf.i18n import _

import dnf.rpm

import dnf.exceptions

logger = logging.getLogger("dnf")

RR_TYPE_OPENPGPKEY = 61

class DnssecError(dnf.exceptions.Error):

    """

    Exception used in the dnssec module

    """

    def __repr__(self):

        return "<DnssecError, value='{}'>"\

            .format(self.value if self.value is not None else "Not specified")

def email2location(email_address, tag="_openpgpkey"):

    # type: (str, str) -> str

    """

    Implements RFC 7929, section 3

    https://tools.ietf.org/html/rfc7929#section-3

    :param email_address:

    :param tag:

    :return:

    """

    split = email_address.split("@")

    if len(split) != 2:

        msg = "Email address must contain exactly one '@' sign."

        raise DnssecError(msg)

    local = split[0]

    domain = split[1]

    hash = hashlib.sha256()

    hash.update(local.encode('utf-8'))

    digest = base64.b16encode(hash.digest()[0:28])\

        .decode("utf-8")\

        .lower()

    return digest + "." + tag + "." + domain

class Validity(Enum):

    """

    Output of the verification algorithm.

    TODO: this type might be simplified in order to less reflect the underlying DNS layer.

    TODO: more specifically the variants from 3 to 5 should have more understandable names

    """

    VALID = 1

    REVOKED = 2

    PROVEN_NONEXISTENCE = 3

    RESULT_NOT_SECURE = 4

    BOGUS_RESULT = 5

    ERROR = 9

class NoKey:

    """

    This class represents an absence of a key in the cache. It is an expression of non-existence

    using the Python's type system.

    """

    pass

class KeyInfo:

    """

    Wrapper class for email and associated verification key, where both are represented in

    form of a string.

    """

    def __init__(self, email=None, key=None):

        self.email = email

        self.key = key

    @staticmethod

    def from_rpm_key_object(userid, raw_key):

        # type: (str, bytes) -> KeyInfo

        """

        Since dnf uses different format of the key than the one used in DNS RR, I need to convert

        the former one into the new one.

        """

        input_email = re.search('<(.*@.*)>', userid)

        if input_email is None:

            raise DnssecError

        email = input_email.group(1)

        key = raw_key.decode('ascii').split('\n')

        start = 0

        stop = 0

        for i in range(0, len(key)):

            if key[i] == '-----BEGIN PGP PUBLIC KEY BLOCK-----':

                start = i

            if key[i] == '-----END PGP PUBLIC KEY BLOCK-----':

                stop = i

        cat_key = ''.join(key[start + 2:stop - 1]).encode('ascii')

        return KeyInfo(email, cat_key)

class DNSSECKeyVerification:

    """

    The main class when it comes to verification itself. It wraps Unbound context and a cache with

    already obtained results.

    """

    # Mapping from email address to b64 encoded public key or NoKey in case of proven nonexistence

    _cache = {}

    # type: Dict[str, Union[str, NoKey]]

    @staticmethod

    def _cache_hit(key_union, input_key_string):

        # type: (Union[str, NoKey], str) -> Validity

        """

        Compare the key in case it was found in the cache.

        """

        if key_union == input_key_string:

            logger.debug("Cache hit, valid key")

            return Validity.VALID

        elif key_union is NoKey:

            logger.debug("Cache hit, proven non-existence")

            return Validity.PROVEN_NONEXISTENCE

        else:

            logger.debug("Key in cache: {}".format(key_union))

            logger.debug("Input key   : {}".format(input_key_string))

            return Validity.REVOKED

    @staticmethod

    def _cache_miss(input_key):

        # type: (KeyInfo) -> Validity

        """

        In case the key was not found in the cache, create an Unbound context and contact the DNS

        system

        """

        try:

            import unbound

        except ImportError as e:

            msg = _("Configuration option 'gpgkey_dns_verification' requires "

                    "libunbound ({})".format(e))

            raise dnf.exceptions.Error(msg)

        ctx = unbound.ub_ctx()

        if ctx.set_option("verbosity:", "0") != 0:

            logger.debug("Unbound context: Failed to set verbosity")

        if ctx.set_option("qname-minimisation:", "yes") != 0:

            logger.debug("Unbound context: Failed to set qname minimisation")

        if ctx.resolvconf() != 0:

            logger.debug("Unbound context: Failed to read resolv.conf")

        if ctx.add_ta_file("/var/lib/unbound/root.key") != 0:

            logger.debug("Unbound context: Failed to add trust anchor file")

        status, result = ctx.resolve(email2location(input_key.email),

                                     RR_TYPE_OPENPGPKEY, unbound.RR_CLASS_IN)

        if status != 0:

            logger.debug("Communication with DNS servers failed")

            return Validity.ERROR

        if result.bogus:

            logger.debug("DNSSEC signatures are wrong")

            return Validity.BOGUS_RESULT

        if not result.secure:

            logger.debug("Result is not secured with DNSSEC")

            return Validity.RESULT_NOT_SECURE

        if result.nxdomain:

            logger.debug("Non-existence of this record was proven by DNSSEC")

            return Validity.PROVEN_NONEXISTENCE

        if not result.havedata:

            # TODO: This is weird result, but there is no way to perform validation, so just return

            # an error

            logger.debug("Unknown error in DNS communication")

            return Validity.ERROR

        else:

            data = result.data.as_raw_data()[0]

            dns_data_b64 = base64.b64encode(data)

            if dns_data_b64 == input_key.key:

                return Validity.VALID

            else:

                # In case it is different, print the keys for further examination in debug mode

                logger.debug("Key from DNS: {}".format(dns_data_b64))

                logger.debug("Input key   : {}".format(input_key.key))

                return Validity.REVOKED

    @staticmethod

    def verify(input_key):

        # type: (KeyInfo) -> Validity

        """

        Public API. Use this method to verify a KeyInfo object.

        """

        logger.debug("Running verification for key with id: {}".format(input_key.email))

        key_union = DNSSECKeyVerification._cache.get(input_key.email)

        if key_union is not None:

            return DNSSECKeyVerification._cache_hit(key_union, input_key.key)

        else:

            result = DNSSECKeyVerification._cache_miss(input_key)

            if result == Validity.VALID:

                DNSSECKeyVerification._cache[input_key.email] = input_key.key

            elif result == Validity.PROVEN_NONEXISTENCE:

                DNSSECKeyVerification._cache[input_key.email] = NoKey()

            return result

def nice_user_msg(ki, v):

    # type: (KeyInfo, Validity) -> str

    """

    Inform the user about key validity in a human readable way.

    """

    prefix = _("DNSSEC extension: Key for user ") + ki.email + " "

    if v == Validity.VALID:

        return prefix + _("is valid.")

    else:

        return prefix + _("has unknown status.")

def any_msg(m):

    # type: (str) -> str

    """

    Label any given message with DNSSEC extension tag

    """

    return _("DNSSEC extension: ") + m

class RpmImportedKeys:

    """

    Wrapper around keys, that are imported in the RPM database.

    The keys are stored in packages with name gpg-pubkey, where the version and

    release is different for each of them. The key content itself is stored as

    an ASCII armored string in the package description, so it needs to be parsed

    before it can be used.

    """

    @staticmethod

    def _query_db_for_gpg_keys():

        # type: () -> List[KeyInfo]

        # TODO: base.conf.installroot ?? -----------------------\

        transaction_set = dnf.rpm.transaction.TransactionWrapper()

        packages = transaction_set.dbMatch("name", "gpg-pubkey")

        return_list = []

        for pkg in packages:

            packager = dnf.rpm.getheader(pkg, 'packager')

            email = re.search('<(.*@.*)>', packager).group(1)

            description = dnf.rpm.getheader(pkg, 'description')

            key_lines = description.split('\n')[3:-3]

            key_str = ''.join(key_lines)

            return_list += [KeyInfo(email, key_str.encode('ascii'))]

        return return_list

    @staticmethod

    def check_imported_keys_validity():

        keys = RpmImportedKeys._query_db_for_gpg_keys()

        logger.info(any_msg(_("Testing already imported keys for their validity.")))

        for key in keys:

            try:

                result = DNSSECKeyVerification.verify(key)

            except DnssecError as e:

                # Errors in this exception should not be fatal, print it and just continue

                logger.warning("DNSSEC extension error (email={}): {}"

                             .format(key.email, e.value))

                continue

            # TODO: remove revoked keys automatically and possibly ask user to confirm

            if result == Validity.VALID:

                logger.debug(any_msg("GPG Key {} is valid".format(key.email)))

                pass

            elif result == Validity.PROVEN_NONEXISTENCE:

                logger.debug(any_msg("GPG Key {} does not support DNS"

                                    " verification".format(key.email)))

            elif result == Validity.BOGUS_RESULT:

                logger.info(any_msg("GPG Key {} could not be verified, because DNSSEC signatures"

                                    " are bogus. Possible causes: wrong configuration of the DNS"

                                    " server, MITM attack".format(key.email)))

            elif result == Validity.REVOKED:

                logger.info(any_msg("GPG Key {} has been revoked and should"

                                    " be removed immediately".format(key.email)))

            else:

                logger.debug(any_msg("GPG Key {} could not be tested".format(key.email)))