# ============================================================================
# This file is part of Pwman3.
#
# Pwman3 is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License, version 2
# as published by the Free Software Foundation;
#
# Pwman3 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 Pwman3; if not, write to the Free Software
# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
# ============================================================================
# Copyright (C) 2014 Oz Nahum <nahumoz@gmail.com>
# ============================================================================

from __future__ import print_function
from Crypto.Cipher import AES
from Crypto.Protocol.KDF import PBKDF2
import base64
import os
import sys
import binascii
import time
from pwman.util.callback import Callback
import pwman.util.config as config
import ctypes

if sys.version_info.major > 2:  # pragma: no cover
    raw_input = input

EncodeAES = lambda c, s: base64.b64encode(c.encrypt(s))
DecodeAES = lambda c, e: c.decrypt(base64.b64decode(e)).rstrip()


def zerome(string):
    """
    securely erase strings ...
    for windows: ctypes.cdll.msvcrt.memset
    """
    bufsize = len(string) + 1
    offset = sys.getsizeof(string) - bufsize
    ctypes.memset(id(string) + offset, 0, bufsize)


class CryptoException(Exception):
    pass


def get_digest(password, salt):
    """
    Get a digest based on clear text password
    """
    iterations = 5000
    return PBKDF2(password, salt, dkLen=32, count=iterations)


def authenticate(password, salt, digest):
    """
    salt and digest are stored in a file or a database
    """
    dig = get_digest(password, salt)
    return binascii.hexlify(dig) == digest


def get_cipher(password, salt):
    """
    Create a chiper object from a hashed password
    """
    iv = os.urandom(AES.block_size)
    dig = get_digest(password, salt)
    chiper = AES.new(dig, AES.MODE_ECB, iv)
    return chiper


def prepare_data(text, block_size):
    """
    prepare data before encryption so the lenght matches the expected
    lenght by the algorithm.
    """
    num_blocks = len(text)//block_size + 1
    newdatasize = block_size*num_blocks
    return text.ljust(newdatasize)


class CryptoEngine(object):  # pagma: no cover
    _timeoutcount = 0
    _instance = None
    _instance_new = None
    _callback = None

    @classmethod
    def get(cls, dbver=0.5):
        if CryptoEngine._instance:
            return CryptoEngine._instance
        if CryptoEngine._instance_new:
            return CryptoEngine._instance_new

        algo = config.get_value("Encryption", "algorithm")

        if not algo:
            raise Exception("Parameters missing, no algorithm given")

        try:
            timeout = int(config.get_value("Encryption", "timeout"))
        except ValueError:
            timeout = -1

        kwargs = {'algorithm': algo,
                  'timeout': timeout}

        if dbver >= 0.5:
            CryptoEngine._instance_new = CryptoEngine(**kwargs)
            return CryptoEngine._instance_new

    def __init__(self, salt=None, digest=None, algorithm='AES',
                 timeout=-1, reader=None):
        """
        Initialise the Cryptographic Engine
        """
        self._algo = algorithm
        self._digest = digest if digest else None
        self._salt = salt if salt else None
        self._timeout = timeout
        self._cipher = None
        self._reader = reader
        self._callback = None

    def authenticate(self, password):
        """
        salt and digest are stored in a file or a database
        """
        dig = get_digest(password, self._salt)
        if binascii.hexlify(dig) == self._digest:
            CryptoEngine._timeoutcount = time.time()
            self._cipher = get_cipher(password, self._salt)
            return True
        return False

    def _auth(self):
        """
        Read password from the user, if the password is correct,
        finish the execution an return the password and salt which
        are read from the file.
        """
        salt, digest = self._salt, self._digest
        tries = 0
        while tries < 5:
            password = self._getsecret("Please type in your master password:"
                                       ).encode('utf-8')
            if authenticate(password, salt, digest):
                return password, salt

            print("You entered a wrong password...")
            tries += 1
        raise CryptoException("You entered wrong password 5 times..")

    def encrypt(self, text):
        if not self._is_authenticated():
            p, s = self._auth()
            cipher = get_cipher(p, s)
            del(p)
            return EncodeAES(cipher, prepare_data(text, AES.block_size))

        return EncodeAES(self._cipher, prepare_data(text, AES.block_size))

    def decrypt(self, cipher_text):
        if not self._is_authenticated():
            p, s = self._auth()
            cipher = get_cipher(p, s)
            del(p)
            return DecodeAES(cipher, prepare_data(cipher_text, AES.block_size))

        return DecodeAES(self._cipher, prepare_data(cipher_text,
                                                    AES.block_size))

    def forget(self):
        """
        discard cipher
        """
        self._cipher = None

    def _is_authenticated(self):
        if not self._is_timedout() and self._cipher is not None:
            return True
        return False

    def _is_timedout(self):
        if self._timeout > 0:
            if (time.time() - CryptoEngine._timeoutcount) > self._timeout:
                self._cipher = None
            return True
        return False

    def changepassword(self, reader=raw_input):
        if self._callback is None:
            raise CryptoException("No callback class has been "
                                  "specified")
        self._keycrypted = self._create_password()
        return self._keycrypted

    @property
    def callback(self):
        """
        return call back function
        """
        return self._callback

    @callback.setter
    def callback(self, callback):
        if isinstance(callback, Callback):
            self._callback = callback
            self._getsecret = callback.getsecret
            self._getnewsecret = callback.getnewsecret
        else:
            raise Exception("callback must be an instance of Callback!")

    def _create_password(self):
        """
        Create a secret password as a hash and the salt used for this hash.
        Change reader to manipulate how input is given.
        """
        salt = base64.b64encode(os.urandom(32))
        passwd = self._getsecret("Please type in the secret key:")
        key = self._get_digest(passwd, salt)
        hpk = salt+'$6$'.encode('utf8')+binascii.hexlify(key)
        return hpk.decode('utf-8')

    def _get_digest(self, password, salt):
        """
        Get a digest based on clear text password
        """
        iterations = 5000
        return PBKDF2(password, salt, dkLen=32, count=iterations)

    def set_cryptedkey(self, key):
        # TODO: rename this method!
        salt, digest = key.split('$6$')
        self._digest = digest.encode('utf-8')
        self._salt = salt.encode('utf-8')

    def get_cryptedkey(self):
        # TODO: rename this method!
        """
        return _keycrypted
        """
        return self._salt + '$6$' + self._digest