#============================================================================
# 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) 2012 Oz Nahum <nahumoz@gmail.com>
#============================================================================
# Copyright (C) 2006 Ivan Kelly <ivan@ivankelly.net>
#============================================================================

"""
Encryption Module used by PwmanDatabase

Supports AES, ARC2, Blowfish, CAST, DES, DES3, IDEA, RC5.

Usage:
import pwman.util.crypto.CryptoEngine as CryptoEngine
from pwman.util.crypto import CryptoEngine

class myCallback(CryptoEngine.Callback):
    def execute(self):
        return "mykey"

params = {'encryptionAlgorithm': 'AES',
          'encryptionCallback': callbackFunction}

CryptoEngine.init(params)

crypto = CryptoEngine.get()
ciphertext = crypto.encrypt("plaintext")
plaintext = cyypto.decrypt(ciphertext)
"""

from Crypto.Cipher import Blowfish as cBlowfish
from Crypto.Cipher import AES as cAES
from Crypto.Cipher import ARC2 as cARC2
from Crypto.Cipher import ARC4 as cARC2
from Crypto.Cipher import CAST as cCAST
from Crypto.Cipher import DES as cDES
from Crypto.Cipher import DES3 as cDES3

from Crypto.Random import OSRNG


from pwman.util.callback import Callback
import pwman.util.config as config
import cPickle
import time
import sys
import ctypes

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)

# Use this to tell if crypto is successful or not
_TAG = "PWMANCRYPTO"

_INSTANCE = None

# Use this to tell if crypto is successful or not
_TAG = "PWMANCRYPTO"

class CryptoException(Exception):
    """Generic Crypto Exception."""
    def __init__(self, message):
        self.message = message
    def __str__(self):
        return "CryptoException: " + self.message

class CryptoUnsupportedException(CryptoException):
    """Unsupported feature requested."""
    def __str__(self):
        return "CryptoUnsupportedException: " +self.message

class CryptoBadKeyException(CryptoException):
    """Encryption key is incorrect."""
    def __str__(self):
        return "CryptoBadKeyException: " + self.message

class CryptoNoKeyException(CryptoException):
    """No key has been initalised."""
    def __str__(self):
        return "CryptoNoKeyException: " + self.message

class CryptoNoCallbackException(CryptoException):
    """No Callback has been set."""
    def __str__(self):
        return "CryptoNoCallbackException: " + self.message

class CryptoPasswordMismatchException(CryptoException):
    """Entered passwords do not match."""
    def __str__(self):
        return "CryptoPasswordMismatchException: " + self.message


class CryptoEngine:
    """Cryptographic Engine"""
    _timeoutcount = 0
    _instance = None
    _callback = None

    @classmethod
    def get(cls):
        """
        CryptoEngine.get() -> CryptoEngine
        Return an instance of CryptoEngine.
        If no instance is found, a CryptoException is raised.
        """
        if (CryptoEngine._instance == None):
            algo = config.get_value("Encryption", "algorithm")
            if algo == "Dummy":
                CryptoEngine._instance = DummyCryptoEngine()
            else:
                CryptoEngine._instance = CryptoEngine()
        return CryptoEngine._instance
    #get = classmethod(get)

    def __init__(self):
        """Initialise the Cryptographic Engine

        params is a dictionary. Valid keys are:
        algorithm: Which cipher to use
        callback:  Callback class.
        keycrypted: This should be set by the database layer.
        timeout:   Time after which key will be forgotten.
                             Default is -1 (disabled).
        """
        algo = config.get_value("Encryption", "algorithm")
        if len(algo) > 0:
            self._algo = algo
        else:
            raise CryptoException("Parameters missing, no algorithm given")

        callback = config.get_value("Encryption", "callback")
        if isinstance(callback, Callback):
            self._callback = callback
        else:
            self._callback = None

        keycrypted = config.get_value("Encryption", "keycrypted")
        if len(keycrypted) > 0:
            self._keycrypted = keycrypted
        else:
            self._keycrypted = None

        timeout = config.get_value("Encryption", "timeout")
        if timeout.isdigit():
            self._timeout = timeout
        else:
            self._timeout = -1
        self._cipher = None

    def encrypt(self, obj):
        """
        encrypt(obj) -> ciphertext
        Encrypt obj and return its ciphertext. obj must be a picklable class.
        Can raise a CryptoException and CryptoUnsupportedException"""
        cipher = self._getcipher()
        plaintext = self._preparedata(obj, cipher.block_size)
        ciphertext = cipher.encrypt(plaintext)

        return str(ciphertext).encode('base64')

    def decrypt(self, ciphertext):
        """
        decrypt(ciphertext) -> obj
        Decrypt ciphertext and returns the obj that was encrypted.
        If key is bad, a CryptoBadKeyException is raised
        Can also raise a CryptoException and CryptoUnsupportedException"""
        cipher = self._getcipher()
        ciphertext = str(ciphertext).decode('base64')
        plaintext = cipher.decrypt(ciphertext)
        return self._retrievedata(plaintext)

    def set_cryptedkey(self, key):
        """
        hold _keycrypted
        """
        self._keycrypted = key

    def get_cryptedkey(self):
        """
        return _keycrypted
        """
        return self._keycrypted

    def set_callback(self, callback):
        """
        set the callback function
        """
        self._callback = callback

    def get_callback(self):
        """
        return call back function
        """
        return self._callback

    def changepassword(self):
        """
        Creates a new key. The key itself is actually stored in
        the database in crypted form. This key is encrypted using the
        password that the user provides. This makes it easy to change the
        password for the database.
        If oldKeyCrypted is none, then a new password is generated."""
        if (self._callback == None):
            raise CryptoNoCallbackException("No call back class has been \
specified")
        if (self._keycrypted == None):
            # Generate a new key, 32 bits in length, if that's
            # too long for the Cipher, _getCipherReal will sort it out
            random = OSRNG.new()
            key = str(random.read(32)).encode('base64')
        else:
            password = self._callback.getsecret("Please enter your current \
password")
            cipher = self._getcipher_real(password, self._algo)
            plainkey = cipher.decrypt(str(self._keycrypted).decode('base64'))
            key = self._retrievedata(plainkey)
        
        newpassword1 = self._callback.getsecret("Please enter your new \
password")
        newpassword2 = self._callback.getsecret("Please enter your new \
password again")
        if (newpassword1 != newpassword2):
            raise CryptoPasswordMismatchException("Passwords do not match")
        newcipher = self._getcipher_real(newpassword1, self._algo)
        self._keycrypted = str(newcipher.encrypt(self._preparedata(key,
                           newcipher.block_size))).encode('base64')
        # newpassword1, newpassword2 are not needed any more so we erase 
        # them
        zerome(newpassword1)
        zerome(newpassword2)
        # we also want to create the cipher if there isn't one already
        # so this CryptoEngine can be used from now on
        if (self._cipher == None):
            self._cipher = self._getcipher_real(str(key).decode('base64'),
                                                self._algo)
            CryptoEngine._timeoutcount = time.time()

        return self._keycrypted

    def alive(self):
        """
        check if we have cipher
        """
        if (self._cipher != None):
            return True
        else:
            return False

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

    def _getcipher(self):
        """
        get cypher from user, to decrypt DB
        """
        if (self._cipher != None
            and (self._timeout == -1
            or (time.time() - CryptoEngine._timeoutcount) < self._timeout)):
            return self._cipher
        if (self._callback == None):
            raise CryptoNoCallbackException("No Callback exception")
        if (self._keycrypted == None):
            raise CryptoNoKeyException("Encryption key has not been generated")

        max_tries = 5
        tries = 0

        key = None

        while tries < max_tries:
            try:
                password = self._callback.getsecret("Please enter your \
password")
                tmpcipher = self._getcipher_real(password, self._algo)
                plainkey = tmpcipher.decrypt(str(self._keycrypted).decode(
                    'base64'))
                key = self._retrievedata(plainkey)
                break
            except CryptoBadKeyException:
                print "Wrong password."
                tries += 1

        if not key:
            raise Exception("Wrong password entered %s times; giving up" \
                    % max_tries)

        self._cipher = self._getcipher_real(str(key).decode('base64'),
                self._algo)

        CryptoEngine._timeoutcount = time.time()
        return self._cipher


    def _getcipher_real(self, key, algo):
        """
        do the real job of decrypting using functions
        form PyCrypto
        """
        if (algo == "AES"):
            key = self._padkey(key, [16, 24, 32])
            cipher = cAES.new(key, cAES.MODE_ECB)
        elif (algo == 'ARC2'):
            cipher = cARC2.new(key, cARC2.MODE_ECB)
        elif (algo == 'ARC4'):
            raise CryptoUnsupportedException("ARC4 is currently unsupported")
        elif (algo == 'Blowfish'):
            cipher = cBlowfish.new(key, cBlowfish.MODE_ECB)
        elif (algo == 'CAST'):
            cipher = cCAST.new(key, cCAST.MODE_ECB)
        elif (algo == 'DES'):
            self._padkey(key, [8])
            cipher = cDES.new(key, cDES.MODE_ECB)
        elif (algo == 'DES3'):
            key = self._padkey(key, [16, 24])
            cipher =  cDES3.new(key, cDES3.MODE_ECB)
        elif (algo == 'XOR'):
            raise CryptoUnsupportedException("XOR is currently unsupported")
        else:
            raise CryptoException("Invalid algorithm specified")
        return cipher

    def _padkey(self, key, acceptable_lengths):
        """
        pad key with extra string
        """
        maxlen = max(acceptable_lengths)
        keylen = len(key)
        if (keylen > maxlen):
            return key[0:maxlen]
        acceptable_lengths.sort()
        acceptable_lengths.reverse()
        newkeylen = None
        for i in acceptable_lengths:
            if (i < keylen):
                break
            newkeylen = i
        return key.ljust(newkeylen)

    def _preparedata(self, obj, blocksize):
        """
        prepare data before encrypting
        """
        plaintext = cPickle.dumps(obj)
        plaintext = _TAG + plaintext
        numblocks = (len(plaintext)/blocksize) + 1
        newdatasize = blocksize*numblocks
        return plaintext.ljust(newdatasize)

    def _retrievedata(self, plaintext):
        """
        retrieve encrypted data
        """
        if (plaintext.startswith(_TAG)):
            plaintext = plaintext[len(_TAG):]
        else:
            raise CryptoBadKeyException("Error decrypting, bad key")
        return cPickle.loads(plaintext)


class DummyCryptoEngine(CryptoEngine):
    """Dummy CryptoEngine used when database doesn't ask for encryption.
    Only for testing and debugging the DB drivers really."""
    def __init__(self):
        pass

    def encrypt(self, obj):
        """Return the object pickled."""
        return cPickle.dumps(obj)

    def decrypt(self, ciphertext):
        """Unpickle the object."""
        return cPickle.loads(str(ciphertext))

    def changepassword(self):
        return ''