ssh-audit/src/ssh_audit/policy.py

"""
   The MIT License (MIT)

   Copyright (C) 2020-2024 Joe Testa (jtesta@positronsecurity.com)

   Permission is hereby granted, free of charge, to any person obtaining a copy
   of this software and associated documentation files (the "Software"), to deal
   in the Software without restriction, including without limitation the rights
   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   copies of the Software, and to permit persons to whom the Software is
   furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   THE SOFTWARE.
"""
import copy
import json
import sys

from typing import Dict, List, Tuple
from typing import Optional, Any, Union, cast
from datetime import date

from ssh_audit import exitcodes
from ssh_audit.banner import Banner
from ssh_audit.builtin_policies import BUILTIN_POLICIES
from ssh_audit.globals import SNAP_PACKAGE, SNAP_PERMISSIONS_ERROR
from ssh_audit.ssh2_kex import SSH2_Kex


# Validates policy files and performs policy testing
class Policy:

    WARNING_DEPRECATED_DIRECTIVES = "\nWARNING: this policy is using deprecated features.  Future versions of ssh-audit may remove support for them.  Re-generating the policy file is perhaps the most straight-forward way of resolving this issue.  Manually converting the 'hostkey_size_*', 'cakey_size_*', and 'dh_modulus_size_*' directives into the new format is another option.\n"

    def __init__(self, policy_file: Optional[str] = None, policy_data: Optional[str] = None, manual_load: bool = False, json_output: bool = False) -> None:
        self._name: Optional[str] = None
        self._version: Optional[str] = None
        self._banner: Optional[str] = None
        self._compressions: Optional[List[str]] = None
        self._host_keys: Optional[List[str]] = None
        self._optional_host_keys: Optional[List[str]] = None
        self._kex: Optional[List[str]] = None
        self._ciphers: Optional[List[str]] = None
        self._macs: Optional[List[str]] = None
        self._hostkey_sizes: Optional[Dict[str, Dict[str, Union[int, str, bytes]]]] = None
        self._dh_modulus_sizes: Optional[Dict[str, int]] = None
        self._server_policy = True
        self._allow_algorithm_subset_and_reordering = False
        self._allow_larger_keys = False
        self._errors: List[Any] = []

        self._name_and_version: str = ''

        # If invoked while JSON output is expected, send warnings to stderr instead of stdout (which would corrupt the JSON output).
        if json_output:
            self._warning_target = sys.stderr
        else:
            self._warning_target = sys.stdout

        # Ensure that only one mode was specified.
        num_modes = 0
        if policy_file is not None:
            num_modes += 1
        if policy_data is not None:
            num_modes += 1
        if manual_load is True:
            num_modes += 1

        if num_modes != 1:
            raise RuntimeError('Exactly one of the following can be specified only: policy_file, policy_data, or manual_load')

        if manual_load:
            return

        if policy_file is not None:
            try:
                with open(policy_file, "r", encoding='utf-8') as f:
                    policy_data = f.read()
            except FileNotFoundError:
                print("Error: policy file not found: %s" % policy_file)
                sys.exit(exitcodes.UNKNOWN_ERROR)
            except PermissionError as e:
                # If installed as a Snap package, print a more useful message with potential work-arounds.
                if SNAP_PACKAGE:
                    print(SNAP_PERMISSIONS_ERROR)
                else:
                    print("Error: insufficient permissions: %s" % str(e))
                sys.exit(exitcodes.UNKNOWN_ERROR)

        lines = []
        if policy_data is not None:
            lines = policy_data.split("\n")

        for line in lines:
            line = line.strip()
            if (len(line) == 0) or line.startswith('#'):
                continue

            key = None
            val = None
            try:
                key, val = line.split('=')
            except ValueError as ve:
                raise ValueError("could not parse line: %s" % line) from ve

            key = key.strip()
            val = val.strip()

            if key not in ['name', 'version', 'banner', 'compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs', 'client policy', 'host_key_sizes', 'dh_modulus_sizes', 'allow_algorithm_subset_and_reordering', 'allow_larger_keys'] and not key.startswith('hostkey_size_') and not key.startswith('cakey_size_') and not key.startswith('dh_modulus_size_'):
                raise ValueError("invalid field found in policy: %s" % line)

            if key in ['name', 'banner']:

                # If the banner value is blank, set it to "" so that the code below handles it.
                if len(val) < 2:
                    val = "\"\""

                if (val[0] != '"') or (val[-1] != '"'):
                    raise ValueError('the value for the %s field must be enclosed in quotes: %s' % (key, val))

                # Remove the surrounding quotes, and unescape quotes & newlines.
                val = val[1:-1]. replace("\\\"", "\"").replace("\\n", "\n")

                if key == 'name':
                    self._name = val
                elif key == 'banner':
                    self._banner = val

            elif key == 'version':
                self._version = val

            elif key in ['compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs']:
                try:
                    algs = val.split(',')
                except ValueError:
                    # If the value has no commas, then set the algorithm list to just the value.
                    algs = [val]

                # Strip whitespace in each algorithm name.
                algs = [alg.strip() for alg in algs]

                if key == 'compressions':
                    self._compressions = algs
                elif key == 'host keys':
                    self._host_keys = algs
                elif key == 'optional host keys':
                    self._optional_host_keys = algs
                elif key == 'key exchanges':
                    self._kex = algs
                elif key == 'ciphers':
                    self._ciphers = algs
                elif key == 'macs':
                    self._macs = algs

            elif key.startswith('hostkey_size_'):  # Old host key size format.
                print(Policy.WARNING_DEPRECATED_DIRECTIVES, file=self._warning_target)  # Warn the user that the policy file is using deprecated directives.

                hostkey_type = key[13:]
                hostkey_size = int(val)

                if self._hostkey_sizes is None:
                    self._hostkey_sizes = {}

                self._hostkey_sizes[hostkey_type] = {'hostkey_size': hostkey_size, 'ca_key_type': '', 'ca_key_size': 0}

            elif key.startswith('cakey_size_'):  # Old host key size format.
                print(Policy.WARNING_DEPRECATED_DIRECTIVES, file=self._warning_target)  # Warn the user that the policy file is using deprecated directives.

                hostkey_type = key[11:]
                ca_key_size = int(val)

                ca_key_type = 'ssh-ed25519'
                if hostkey_type in ['ssh-rsa-cert-v01@openssh.com', 'rsa-sha2-256-cert-v01@openssh.com', 'rsa-sha2-512-cert-v01@openssh.com']:
                    ca_key_type = 'ssh-rsa'

                if self._hostkey_sizes is None:
                    self._hostkey_sizes = {}
                self._hostkey_sizes[hostkey_type] = {'hostkey_size': hostkey_size, 'ca_key_type': ca_key_type, 'ca_key_size': ca_key_size}

            elif key == 'host_key_sizes':  # New host key size format.
                self._hostkey_sizes = json.loads(val)

                # Fill in the trimmed fields that were omitted from the policy.
                self._normalize_hostkey_sizes()

            elif key.startswith('dh_modulus_size_'):  # Old DH modulus format.
                print(Policy.WARNING_DEPRECATED_DIRECTIVES, file=self._warning_target)  # Warn the user that the policy file is using deprecated directives.

                dh_type = key[16:]
                dh_size = int(val)

                if self._dh_modulus_sizes is None:
                    self._dh_modulus_sizes = {}

                self._dh_modulus_sizes[dh_type] = dh_size

            elif key == 'dh_modulus_sizes':  # New DH modulus format.
                self._dh_modulus_sizes = json.loads(val)

            elif key.startswith('client policy') and val.lower() == 'true':
                self._server_policy = False
            elif key == 'allow_algorithm_subset_and_reordering' and val.lower() == 'true':
                self._allow_algorithm_subset_and_reordering = True
            elif key == 'allow_larger_keys' and val.lower() == 'true':
                self._allow_larger_keys = True

        if self._name is None:
            raise ValueError('The policy does not have a name field.')
        if self._version is None:
            raise ValueError('The policy does not have a version field.')

        self._name_and_version = "%s (version %s)" % (self._name, self._version)


    def _append_error(self, mismatched_field: str, expected_required: Optional[List[str]], expected_optional: Optional[List[str]], actual: List[str]) -> None:
        if expected_required is None:
            expected_required = ['']
        if expected_optional is None:
            expected_optional = ['']
        self._errors.append({'mismatched_field': mismatched_field, 'expected_required': expected_required, 'expected_optional': expected_optional, 'actual': actual})


    def _normalize_hostkey_sizes(self) -> None:
        '''Normalizes the self._hostkey_sizes structure to ensure all required fields are present.'''

        if self._hostkey_sizes is not None:
            for host_key_type in self._hostkey_sizes:
                if 'ca_key_type' not in self._hostkey_sizes[host_key_type]:
                    self._hostkey_sizes[host_key_type]['ca_key_type'] = ''
                if 'ca_key_size' not in self._hostkey_sizes[host_key_type]:
                    self._hostkey_sizes[host_key_type]['ca_key_size'] = 0
                if 'raw_hostkey_bytes' not in self._hostkey_sizes[host_key_type]:
                    self._hostkey_sizes[host_key_type]['raw_hostkey_bytes'] = b''


    @staticmethod
    def create(source: Optional[str], banner: Optional['Banner'], kex: Optional['SSH2_Kex'], client_audit: bool) -> str:
        '''Creates a policy based on a server configuration.  Returns a string.'''

        today = date.today().strftime('%Y/%m/%d')
        compressions = None
        host_keys = None
        kex_algs = None
        ciphers = None
        macs = None
        dh_modulus_sizes_str = ''
        client_policy_str = ''
        host_keys_json = ''

        if client_audit:
            client_policy_str = "\n# Set to true to signify this is a policy for clients, not servers.\nclient policy = true\n"

        if kex is not None:
            if kex.server.compression is not None:
                compressions = ', '.join(kex.server.compression)
            if kex.key_algorithms is not None:
                host_keys = ', '.join(kex.key_algorithms)
            if kex.kex_algorithms is not None:
                kex_algs = ', '.join(kex.kex_algorithms)
            if kex.server.encryption is not None:
                ciphers = ', '.join(kex.server.encryption)
            if kex.server.mac is not None:
                macs = ', '.join(kex.server.mac)

            if kex.host_keys():

                # Make a deep copy of the host keys dict, then delete all the raw hostkey bytes from the copy.
                host_keys_trimmed = copy.deepcopy(kex.host_keys())
                for hostkey_alg in host_keys_trimmed:
                    del host_keys_trimmed[hostkey_alg]['raw_hostkey_bytes']

                    # Delete the CA signature if any of its fields are empty.
                    if host_keys_trimmed[hostkey_alg]['ca_key_type'] == '' or host_keys_trimmed[hostkey_alg]['ca_key_size'] == 0:
                        del host_keys_trimmed[hostkey_alg]['ca_key_type']
                        del host_keys_trimmed[hostkey_alg]['ca_key_size']

                host_keys_json = "\n# Dictionary containing all host key and size information.  Optionally contains the certificate authority's signature algorithm ('ca_key_type') and signature length ('ca_key_size'), if any.\nhost_key_sizes = %s\n" % json.dumps(host_keys_trimmed)

            if kex.dh_modulus_sizes():
                dh_modulus_sizes_str = "\n# Group exchange DH modulus sizes.\ndh_modulus_sizes = %s\n" % json.dumps(kex.dh_modulus_sizes())


        policy_data = '''#
# Custom policy based on %s (created on %s)
#
%s
# The name of this policy (displayed in the output during scans).  Must be in quotes.
name = "Custom Policy (based on %s on %s)"

# The version of this policy (displayed in the output during scans).  Not parsed, and may be any value, including strings.
version = 1

# When false, host keys, kex, ciphers, and MAC lists must match exactly.  When true, the target host may support a subset of the specified algorithms and/or algorithms may appear in a different order; this feature is useful for specifying a baseline and allowing some hosts the option to implement stricter controls.
allow_algorithm_subset_and_reordering = false

# When false, host keys, CA keys, and Diffie-Hellman key sizes must exactly match what's specified in this policy.  When true, target systems are allowed to have larger keys; this feature is useful for specifying a baseline and allowing some hosts the option to implement stricter controls.
allow_larger_keys = false

# The banner that must match exactly.  Commented out to ignore banners, since minor variability in the banner is sometimes normal.
# banner = "%s"

# The compression options that must match exactly (order matters).  Commented out to ignore by default.
# compressions = %s
%s%s
# The host key types that must match exactly (order matters).
host keys = %s

# Host key types that may optionally appear.
#optional host keys = ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com

# The key exchange algorithms that must match exactly (order matters).
key exchanges = %s

# The ciphers that must match exactly (order matters).
ciphers = %s

# The MACs that must match exactly (order matters).
macs = %s
''' % (source, today, client_policy_str, source, today, banner, compressions, host_keys_json, dh_modulus_sizes_str, host_keys, kex_algs, ciphers, macs)

        return policy_data


    def evaluate(self, banner: Optional['Banner'], kex: Optional['SSH2_Kex']) -> Tuple[bool, List[Dict[str, str]], str]:
        '''Evaluates a server configuration against this policy.  Returns a tuple of a boolean (True if server adheres to policy) and an array of strings that holds error messages.'''

        ret = True

        banner_str = str(banner)
        if (self._banner is not None) and (banner_str != self._banner):
            ret = False
            self._append_error('Banner', [self._banner], None, [banner_str])

        # All subsequent tests require a valid kex, so end here if we don't have one.
        if kex is None:
            error_list, error_str = self._get_errors()
            return ret, error_list, error_str

        if (self._compressions is not None) and (kex.server.compression != self._compressions):
            ret = False
            self._append_error('Compression', self._compressions, None, kex.server.compression)

        # If a list of optional host keys was given in the policy, remove any of its entries from the list retrieved from the server.  This allows us to do an exact comparison with the expected list below.
        pruned_host_keys = kex.key_algorithms
        if self._optional_host_keys is not None:
            pruned_host_keys = [x for x in kex.key_algorithms if x not in self._optional_host_keys]

        # Check host keys.
        if self._host_keys is not None:
            # If the policy allows subsets and re-ordered algorithms...
            if self._allow_algorithm_subset_and_reordering:
                for hostkey_t in kex.key_algorithms:
                    if hostkey_t not in self._host_keys:
                        ret = False
                        self._append_error('Host keys', self._host_keys, self._optional_host_keys, kex.key_algorithms)
                        break
            # The policy requires exact matching of algorithms.
            elif pruned_host_keys != self._host_keys:
                ret = False
                self._append_error('Host keys', self._host_keys, self._optional_host_keys, kex.key_algorithms)

        # Check host key sizes.
        if self._hostkey_sizes is not None:
            hostkey_types = list(self._hostkey_sizes.keys())
            hostkey_types.sort()  # Sorted to make testing output repeatable.
            for hostkey_type in hostkey_types:
                expected_hostkey_size = cast(int, self._hostkey_sizes[hostkey_type]['hostkey_size'])
                server_host_keys = kex.host_keys()
                if hostkey_type in server_host_keys:
                    actual_hostkey_size = cast(int, server_host_keys[hostkey_type]['hostkey_size'])
                    if (self._allow_larger_keys and actual_hostkey_size < expected_hostkey_size) or \
                       (not self._allow_larger_keys and actual_hostkey_size != expected_hostkey_size):
                        ret = False
                        self._append_error('Host key (%s) sizes' % hostkey_type, [str(expected_hostkey_size)], None, [str(actual_hostkey_size)])

                    # If we have expected CA signatures set, check them against what the server returned.
                    if self._hostkey_sizes is not None and len(cast(str, self._hostkey_sizes[hostkey_type]['ca_key_type'])) > 0 and cast(int, self._hostkey_sizes[hostkey_type]['ca_key_size']) > 0:
                        expected_ca_key_type = cast(str, self._hostkey_sizes[hostkey_type]['ca_key_type'])
                        expected_ca_key_size = cast(int, self._hostkey_sizes[hostkey_type]['ca_key_size'])
                        actual_ca_key_type = cast(str, server_host_keys[hostkey_type]['ca_key_type'])
                        actual_ca_key_size = cast(int, server_host_keys[hostkey_type]['ca_key_size'])

                        # Ensure that the CA signature type is what's expected (i.e.: the server doesn't have an RSA sig when we're expecting an ED25519 sig).
                        if actual_ca_key_type != expected_ca_key_type:
                            ret = False
                            self._append_error('CA signature type', [expected_ca_key_type], None, [actual_ca_key_type])
                        # Ensure that the actual and expected signature sizes match.
                        elif (self._allow_larger_keys and actual_ca_key_size < expected_ca_key_size) or \
                             (not self._allow_larger_keys and actual_ca_key_size != expected_ca_key_size):
                            ret = False
                            self._append_error('CA signature size (%s)' % actual_ca_key_type, [str(expected_ca_key_size)], None, [str(actual_ca_key_size)])

        # Check key exchanges.
        if self._kex is not None:
            # If the policy allows subsets and re-ordered algorithms...
            if self._allow_algorithm_subset_and_reordering:
                for kex_t in kex.kex_algorithms:
                    if kex_t not in self._kex:
                        ret = False
                        self._append_error('Key exchanges', self._kex, None, kex.kex_algorithms)
                        break
                # If kex-strict-?-v00@openssh.com is in the policy (i.e. the Terrapin vulnerability countermeasure), then it must appear in the server's list, regardless of the "allow_algorithm_subset_and_reordering" flag.
                if ('kex-strict-s-v00@openssh.com' in self._kex and 'kex-strict-s-v00@openssh.com' not in kex.kex_algorithms) or \
                   ('kex-strict-c-v00@openssh.com' in self._kex and 'kex-strict-c-v00@openssh.com' not in kex.kex_algorithms):
                    ret = False
                    self._append_error('Key exchanges', self._kex, None, kex.kex_algorithms)

            # The policy requires exact matching of algorithms.
            elif kex.kex_algorithms != self._kex:
                ret = False
                self._append_error('Key exchanges', self._kex, None, kex.kex_algorithms)

        # Checking Ciphers
        if self._ciphers is not None:
            # If the policy allows subsets and re-ordered algorithms...
            if self._allow_algorithm_subset_and_reordering:
                for cipher_t in kex.server.encryption:
                    if cipher_t not in self._ciphers:
                        ret = False
                        self._append_error('Ciphers', self._ciphers, None, kex.server.encryption)
                        break
            # The policy requires exact matching of algorithms.
            elif kex.server.encryption != self._ciphers:
                ret = False
                self._append_error('Ciphers', self._ciphers, None, kex.server.encryption)

        # Checking MACs
        if self._macs is not None:
            # If the policy allows subsets and re-ordered algorithms...
            if self._allow_algorithm_subset_and_reordering:
                for mac_t in kex.server.mac:
                    if mac_t not in self._macs:
                        ret = False
                        self._append_error('MACs', self._macs, None, kex.server.mac)
                        break
            # The policy requires exact matching of algorithms.
            elif kex.server.mac != self._macs:
                ret = False
                self._append_error('MACs', self._macs, None, kex.server.mac)

        if self._dh_modulus_sizes is not None:
            dh_modulus_types = list(self._dh_modulus_sizes.keys())
            dh_modulus_types.sort()  # Sorted to make testing output repeatable.
            for dh_modulus_type in dh_modulus_types:
                expected_dh_modulus_size = self._dh_modulus_sizes[dh_modulus_type]
                if dh_modulus_type in kex.dh_modulus_sizes():
                    actual_dh_modulus_size = kex.dh_modulus_sizes()[dh_modulus_type]
                    if (self._allow_larger_keys and actual_dh_modulus_size < expected_dh_modulus_size) or \
                       (not self._allow_larger_keys and actual_dh_modulus_size != expected_dh_modulus_size):
                        ret = False
                        self._append_error('Group exchange (%s) modulus sizes' % dh_modulus_type, [str(expected_dh_modulus_size)], None, [str(actual_dh_modulus_size)])

        error_list, error_str = self._get_errors()
        return ret, error_list, error_str


    def _get_errors(self) -> Tuple[List[Any], str]:
        '''Returns the list of errors, along with the string representation of those errors.'''

        subset_and_reordering_semicolon = "; subset and/or reordering allowed" if self._allow_algorithm_subset_and_reordering else "; exact match"
        subset_and_reordering_parens = " (subset and/or reordering allowed)" if self._allow_algorithm_subset_and_reordering else ""
        error_list = []
        spacer = ''
        for e in self._errors:
            e_str = "  * %s did not match.\n" % e['mismatched_field']

            if ('expected_optional' in e) and (e['expected_optional'] != ['']):
                e_str += "    - Expected (required%s): %s\n    - Expected (optional): %s\n" % (subset_and_reordering_semicolon, Policy._normalize_error_field(e['expected_required']), Policy._normalize_error_field(e['expected_optional']))
                spacer = '              '
            else:
                e_str += "    - Expected%s: %s\n" % (subset_and_reordering_parens, Policy._normalize_error_field(e['expected_required']))
                spacer = '   '
            e_str += "    - Actual:%s%s\n" % (spacer, Policy._normalize_error_field(e['actual']))
            error_list.append(e_str)

        error_list.sort()  # To ensure repeatable results for testing.

        error_str = ''
        if len(error_list) > 0:
            error_str = "\n".join(error_list)

        return self._errors, error_str


    def get_name_and_version(self) -> str:
        '''Returns a string of this Policy's name and version.'''
        return self._name_and_version


    def is_server_policy(self) -> bool:
        '''Returns True if this is a server policy, or False if this is a client policy.'''
        return self._server_policy


    @staticmethod
    def list_builtin_policies(verbose: bool) -> Tuple[List[str], List[str]]:
        '''Returns two lists: a list of names of built-in server policies, and a list of names of built-in client policies, respectively.'''
        server_policy_descriptions = []
        client_policy_descriptions = []

        for policy_name, policy in BUILTIN_POLICIES.items():
            policy_description = ""
            if verbose:
                policy_description = "\"{:s}\": {:s}".format(policy_name, policy['changelog'])
            else:
                policy_description = "\"{:s}\"".format(policy_name)

            if policy['server_policy']:
                server_policy_descriptions.append(policy_description)
            else:
                client_policy_descriptions.append(policy_description)

        server_policy_descriptions.sort()
        client_policy_descriptions.sort()
        return server_policy_descriptions, client_policy_descriptions


    @staticmethod
    def load_builtin_policy(policy_name: str, json_output: bool = False) -> Optional['Policy']:
        '''Returns a Policy with the specified built-in policy name loaded, or None if no policy of that name exists.'''
        p = None
        if policy_name in BUILTIN_POLICIES:
            policy_struct = BUILTIN_POLICIES[policy_name]
            p = Policy(manual_load=True, json_output=json_output)
            policy_name_without_version = policy_name[0:policy_name.rfind(' (')]
            p._name = policy_name_without_version  # pylint: disable=protected-access
            p._version = cast(str, policy_struct['version'])  # pylint: disable=protected-access
            p._banner = cast(Optional[str], policy_struct['banner'])  # pylint: disable=protected-access
            p._compressions = cast(Optional[List[str]], policy_struct['compressions'])  # pylint: disable=protected-access
            p._host_keys = cast(Optional[List[str]], policy_struct['host_keys'])  # pylint: disable=protected-access
            p._optional_host_keys = cast(Optional[List[str]], policy_struct['optional_host_keys'])  # pylint: disable=protected-access
            p._kex = cast(Optional[List[str]], policy_struct['kex'])  # pylint: disable=protected-access
            p._ciphers = cast(Optional[List[str]], policy_struct['ciphers'])  # pylint: disable=protected-access
            p._macs = cast(Optional[List[str]], policy_struct['macs'])  # pylint: disable=protected-access
            p._hostkey_sizes = cast(Optional[Dict[str, Dict[str, Union[int, str, bytes]]]], policy_struct['hostkey_sizes'])  # pylint: disable=protected-access
            p._dh_modulus_sizes = cast(Optional[Dict[str, int]], policy_struct['dh_modulus_sizes'])  # pylint: disable=protected-access
            p._server_policy = cast(bool, policy_struct['server_policy'])  # pylint: disable=protected-access
            p._name_and_version = "%s (version %s)" % (p._name, p._version)  # pylint: disable=protected-access

            # Ensure this struct has all the necessary fields.
            p._normalize_hostkey_sizes()  # pylint: disable=protected-access

        return p


    @staticmethod
    def _normalize_error_field(field: List[str]) -> Any:
        '''If field is an array with a string parsable as an integer, return that integer.  Otherwise, return the field joined with commas.'''
        if len(field) == 1:
            try:
                return int(field[0])
            except ValueError:
                return field[0]
        else:
            return ', '.join(field)


    def __str__(self) -> str:
        undefined = '{undefined}'

        name = undefined
        version = undefined
        banner = undefined
        compressions_str = undefined
        host_keys_str = undefined
        optional_host_keys_str = undefined
        kex_str = undefined
        ciphers_str = undefined
        macs_str = undefined
        hostkey_sizes_str = undefined
        dh_modulus_sizes_str = undefined


        if self._name is not None:
            name = '[%s]' % self._name
        if self._version is not None:
            version = '[%s]' % self._version
        if self._banner is not None:
            banner = '[%s]' % self._banner

        if self._compressions is not None:
            compressions_str = ', '.join(self._compressions)
        if self._host_keys is not None:
            host_keys_str = ', '.join(self._host_keys)
        if self._optional_host_keys is not None:
            optional_host_keys_str = ', '.join(self._optional_host_keys)
        if self._kex is not None:
            kex_str = ', '.join(self._kex)
        if self._ciphers is not None:
            ciphers_str = ', '.join(self._ciphers)
        if self._macs is not None:
            macs_str = ', '.join(self._macs)
        if self._hostkey_sizes is not None:
            hostkey_sizes_str = str(self._hostkey_sizes)
        if self._dh_modulus_sizes is not None:
            dh_modulus_sizes_str = str(self._dh_modulus_sizes)

        return "Name: %s\nVersion: %s\nAllow Algorithm Subset and/or Reordering: %r\nBanner: %s\nCompressions: %s\nHost Keys: %s\nOptional Host Keys: %s\nKey Exchanges: %s\nCiphers: %s\nMACs: %s\nHost Key Sizes: %s\nDH Modulus Sizes: %s\nServer Policy: %r" % (name, version, self._allow_algorithm_subset_and_reordering, banner, compressions_str, host_keys_str, optional_host_keys_str, kex_str, ciphers_str, macs_str, hostkey_sizes_str, dh_modulus_sizes_str, self._server_policy)


    def __getstate__(self) -> Dict[str, Any]:
        '''Called when pickling this object.  The file descriptor isn't serializable, so we'll remove it from the state and include a string representation.'''

        state = self.__dict__.copy()

        if state['_warning_target'] == sys.stdout:
            state['_warning_target_type'] = 'stdout'
        else:
            state['_warning_target_type'] = 'stderr'

        del state['_warning_target']
        return state


    def __setstate__(self, state: Dict[str, Any]) -> None:
        '''Called when unpickling this object.  Based on the string representation of the file descriptor, we'll restore the right handle.'''

        if state['_warning_target_type'] == 'stdout':
            state['_warning_target'] = sys.stdout
        else:
            state['_warning_target'] = sys.stderr

        del state['_warning_target_type']
        self.__dict__.update(state)