#!/usr/bin/python3

# file: statusd.py
# date: 26.07.2019
# email: berhsi@web.de

# Status server, listening for door status updates. The IPv4 address and port
# to listen on are configurable, by default localhost:10001 is used. The
# connection is secured by TLS and client side authentication.

import json
import logging
import os
import socket
import ssl
import sys
from time import time, sleep
import configparser


def certs_readable(config):
    '''
    checks at start, if the needed certificates defined (no nullstring) and
    readable.
    param 1: dictionary
    return: boolean
    '''
    for i in (config['server']['key'], config['server']['cert'],
              config['client']['cert']):
        if i == '' or os.access(i, os.R_OK) is False:
            logging.error('Cannot read {}'.format(i))
            return False
    return True


def print_config(config):
    '''
    Logs the config with level debug.
    '''
    logging.debug('Using config:')
    for section in config.sections():
        logging.debug('Section {}'.format(section))
        for i in config[section]:
            logging.debug('  {}: {}'.format(i, config[section][i]))


def print_ciphers(cipherlist):
    '''
    Prints the list of allowed ciphers.
    param1: dictionary
    return: boolean
    '''
    print('Available ciphers')
    for i in cipherlist:
        print('\n')
        for j in i.keys():
            print('{}: {}'.format(j, i[j]))
    print('\n')


def display_peercert(cert):
    '''
    Displays the values of a given certificate.
    param1: dictionary
    return: boolean
    '''
    for i in cert.keys():
        print('{}:'.format(i))
        if i in ('subject', 'issuer'):
            for j in cert[i]:
                print('\t{}'.format(j))
        else:
            print('\t{}'.format(cert[i]))


def receive_buffer_is_valid(raw_data):
    '''
    Checks validity of the received buffer contents.
    param 1: byte
    return: boolean
    '''
    if raw_data in (b'\x00', b'\x01'):
        logging.debug('Argument is valid: {}'.format(raw_data))
        return True

    logging.debug('Argument is not valid: {}'.format(raw_data))
    return False


def change_status(raw_data, api):
    '''
    Write the new status together with a timestamp into the Space API JSON.
    param 1: byte
    param 2: string
    return: boolean
    '''

    logging.debug('Change status API')
    # todo: use walrus operator := when migrating to python >= 3.8
    data = read_api(api)
    if data is False:
        return False

    status, timestamp = set_values(raw_data)
    if os.access(api, os.W_OK):
        logging.debug('API file is writable')
        with open(api, 'w') as api_file:
            logging.debug('API file open successfull')
            data["state"]["open"] = status
            data["state"]["lastchange"] = timestamp
            try:
                json.dump(data, api_file, indent=4)
            except Exception as e:
                logging.error('Failed to change API file')
                logging.error('{}'.format(e))
            logging.debug('API file changed')
    else:
        logging.error('API file is not writable. Wrong permissions?')
        return False
    logging.info('Status successfull changed to {}'.format(status))
    return True


def read_api(api):
    '''
    Reads the Space API JSON into a dict. Returns the dict on success and
    False on failure.

    param 1: string
    return: dict or boolean
    '''
    logging.debug('Open API file: {}'.format(api))

    # return early if the API JSON cannot be read
    if not os.access(api, os.R_OK):
        logging.error('Failed to read API file')
        return False

    logging.debug('API is readable')
    with open(api, 'r') as api_file:
        logging.debug('API file successfully opened')
        try:
            api_json_data = json.load(api_file)
            logging.debug('API file read successfull')
        except Exception as e:
            logging.error('Failed to read API file: {}'.format(e))
            return False
    return api_json_data


def set_values(raw_data):
    '''
    Create a timestamp, changes the value of the given byte into a string
    and returns both.
    param 1: byte
    return: tuple
    '''
    status = "true" if raw_data == b'\x01' else "false"
    timestamp = str(time()).split('.')[0]

    logging.debug('Set values for timestamp: {} and status: {}'.format(
                    timestamp, status))
    return (status, timestamp)


def main():
    '''
    The main function - open a socket, create a ssl context, load certs and
    listen for connections. At SSL context we set only one available cipher
    suite and disable compression.
    OP_NO_COMPRESSION: prevention against CRIME attack
    OP_DONT_ISERT_EMPTY_FRAGMENTS: prevention agains CBC 4 attack
    (cve-2011-3389)
    '''

    loglevel = logging.WARNING
    formatstring = '%(asctime)s: %(levelname)s: %(message)s'
    logging.basicConfig(format=formatstring, level=loglevel)

    default_config = {
        'general': {
            'timeout': 3.0,
            'loglevel': 'warning'
        },
        'server': {
            'host': 'localhost',
            'port': 10001,
            'cert': './certs/server.crt',
            'key': './certs/server.key'
        },
        'client': {
            'cert': './certs/client.crt'
        },
        'api': {
            'api': './api',
            'template': './api_template'
        }
    }
    configfile = './statusd.conf'
    config = configparser.ConfigParser()
    config.read_dict(default_config)
    if not config.read(configfile):
        logging.warning('Configuration file %s not found or not readable. Using default values.',
                        configfile)

    logger = logging.getLogger()
    if not config['general']['loglevel'] in ('critical', 'error', 'warning', 'info', 'debug'):
        logging.warning('Invalid loglevel %s given. Using default level %s.',
                        config['general']['loglevel'],
                        default_config['general']['loglevel'])
        config.set('general', 'loglevel', default_config['general']['loglevel'])

    logger.setLevel(config['general']['loglevel'].upper())

    print_config(config)

    # todo: zertifikate sollten nur lesbar sein!
    if not certs_readable(config):
        logging.error('Cert check failed\nExit')
        sys.exit(1)

    context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
    context.verify_mode = ssl.CERT_REQUIRED
    context.load_cert_chain(certfile=config['server']['cert'],
                            keyfile=config['server']['key'])
    context.load_verify_locations(cafile=config['client']['cert'])
    context.set_ciphers('EECDH+AESGCM')   # only ciphers for tls 1.2 and 1.3
    context.options = ssl.OP_CIPHER_SERVER_PREFERENCE
    # ensure, compression is disabled (disabled by default anyway at the moment)
    context.options |= ssl.OP_NO_COMPRESSION
    logging.debug('SSL context created')
    # print_ciphers(context.get_ciphers())

    with socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) as mySocket:
        logging.debug('Socket created')
        mySocket.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
        keep = mySocket.getsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE)
        logging.debug('Socket keepalive: {}'format(keep))
        try:
            mySocket.bind((config['server']['host'], int(config['server']['port'])))
            mySocket.listen(5)
        except Exception as e:
            logging.error('Unable to bind and listen')
            logging.error('{}'.format(e))
            sys.exit(1)
        logging.info('Listening on {} at Port {}'.format(config['server']['host'],
                     config['server']['port']))

        while True:
            try:
                fromSocket, fromAddr = mySocket.accept()
                logging.info('Client connected: {}:{}'.format(fromAddr[0], fromAddr[1]))
                try:
                    fromSocket.settimeout(float(config['general']['timeout']))
                    logging.debug('Connection timeout set to {}'.format(
                                  config['general']['timeout']))
                except Exception:
                    logging.error('Cannot set timeout to  {}'.format(
                                  config['general']['timeout']))
                try:
                    conn = context.wrap_socket(fromSocket, server_side=True)
                    conn.settimeout(float(config['general']['timeout']))
                except socket.timeout:
                    logging.error('Socket timeout')
                except Exception as e:
                    logging.error('Connection failed: {}'.format(e))
                logging.info('Connection established')
                logging.info('Peer certificate commonName: {}'.format(
                              conn.getpeercert()['subject'][5][0][1]))
                logging.debug('Peer certificate serialNumber: {}'.format(
                              conn.getpeercert()['serialNumber']))

                raw_data = conn.recv(1)
                if receive_buffer_is_valid(raw_data) is True:
                    if change_status(raw_data, config['api']['api']) is True:
                        logging.debug('Send {} back'.format(raw_data))
                        conn.send(raw_data)
                    # change_status returns false:
                    else:
                        logging.info('Failed to change status')
                        if conn:
                            conn.send(b'\x03')
                # receive_handle returns false:
                else:
                    logging.info('Invalid argument received: {}'.format(raw_data))
                    logging.debug('Send {} back'.format(b'\x03'))
                    if conn:
                        conn.send(b'\x03')
                sleep(0.1)  # protection against dos
            except KeyboardInterrupt:
                logging.info('Keyboard interrupt received')
                sys.exit(1)
            except Exception as e:
                logging.error('{}'.format(e))
                continue
            finally:
                if mySocket:
                    logging.info('Shutdown socket')
                    mySocket.shutdown(socket.SHUT_RDWR)
    return 0


if __name__ == '__main__':
    main()