# encryption.py - module for handling encrypted values
# coding: utf-8
#
# Copyright (C) 2014-2016 Arthur de Jong
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library 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
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301 USA

"""Module that handles encrypted PSKC values.

This module defines an Encryption class that handles the encryption key,
algorithms and decryption.

The encryption key can be derived using the KeyDerivation class.
"""


def algorithm_key_lengths(algorithm):
    """Return the possible key lengths for the configured algorithm."""
    from pskc.exceptions import DecryptionError
    if algorithm is None:
        raise DecryptionError('No algorithm specified')
    elif algorithm.endswith('#aes128-cbc') or \
            algorithm.endswith('#aes192-cbc') or \
            algorithm.endswith('#aes256-cbc'):
        return [int(algorithm[-7:-4]) // 8]
    elif algorithm.endswith('#tripledes-cbc') or \
            algorithm.endswith('#kw-tripledes'):
        from Crypto.Cipher import DES3
        return list(DES3.key_size)
    elif algorithm.endswith('#kw-aes128') or \
            algorithm.endswith('#kw-aes192') or \
            algorithm.endswith('#kw-aes256'):
        return [int(algorithm[-3:]) // 8]
    else:
        raise DecryptionError('Unsupported algorithm: %r' % algorithm)


def decrypt(algorithm, key, ciphertext, iv=None):
    """Decrypt the ciphertext and return the plaintext value."""
    from pskc.exceptions import DecryptionError
    if key is None:
        raise DecryptionError('No key available')
    if algorithm is None:
        raise DecryptionError('No algorithm specified')
    if len(key) not in algorithm_key_lengths(algorithm):
        raise DecryptionError('Invalid key length')
    if algorithm.endswith('#aes128-cbc') or \
            algorithm.endswith('#aes192-cbc') or \
            algorithm.endswith('#aes256-cbc'):
        from Crypto.Cipher import AES
        from pskc.crypto import unpad
        if not iv:
            iv = ciphertext[:AES.block_size]
            ciphertext = ciphertext[AES.block_size:]
        cipher = AES.new(key, AES.MODE_CBC, iv)
        return unpad(cipher.decrypt(ciphertext), AES.block_size)
    elif algorithm.endswith('#tripledes-cbc'):
        from Crypto.Cipher import DES3
        from pskc.crypto import unpad
        if not iv:
            iv = ciphertext[:DES3.block_size]
            ciphertext = ciphertext[DES3.block_size:]
        cipher = DES3.new(key, DES3.MODE_CBC, iv)
        return unpad(cipher.decrypt(ciphertext), DES3.block_size)
    elif algorithm.endswith('#kw-aes128') or \
            algorithm.endswith('#kw-aes192') or \
            algorithm.endswith('#kw-aes256'):
        from pskc.crypto.aeskw import unwrap
        return unwrap(ciphertext, key)
    elif algorithm.endswith('#kw-tripledes'):  # pragma: no branch
        from pskc.crypto.tripledeskw import unwrap
        return unwrap(ciphertext, key)
    # no fallthrough because algorithm_key_lengths() fails with unknown algo


def encrypt(algorithm, key, plaintext, iv=None):
    """Encrypt the provided value with the key using the algorithm."""
    from pskc.exceptions import EncryptionError
    if key is None:
        raise EncryptionError('No key available')
    if algorithm is None:
        raise EncryptionError('No algorithm specified')
    if len(key) not in algorithm_key_lengths(algorithm):
        raise EncryptionError('Invalid key length')
    if algorithm.endswith('#aes128-cbc') or \
            algorithm.endswith('#aes192-cbc') or \
            algorithm.endswith('#aes256-cbc'):
        from Crypto import Random
        from Crypto.Cipher import AES
        from pskc.crypto import pad
        iv = iv or Random.get_random_bytes(AES.block_size)
        cipher = AES.new(key, AES.MODE_CBC, iv)
        return iv + cipher.encrypt(pad(plaintext, AES.block_size))
    elif algorithm.endswith('#tripledes-cbc'):
        from Crypto import Random
        from Crypto.Cipher import DES3
        from pskc.crypto import pad
        iv = iv or Random.get_random_bytes(DES3.block_size)
        cipher = DES3.new(key, DES3.MODE_CBC, iv)
        return iv + cipher.encrypt(pad(plaintext, DES3.block_size))
    elif algorithm.endswith('#kw-aes128') or \
            algorithm.endswith('#kw-aes192') or \
            algorithm.endswith('#kw-aes256'):
        from pskc.crypto.aeskw import wrap
        return wrap(plaintext, key)
    elif algorithm.endswith('#kw-tripledes'):  # pragma: no branch
        from pskc.crypto.tripledeskw import wrap
        return wrap(plaintext, key)
    # no fallthrough because algorithm_key_lengths() fails with unknown algo


class KeyDerivation(object):
    """Handle key derivation.

    The algorithm property contains the key derivation algorithm to use. For
    PBDKF2 the following parameters are set:

      pbkdf2_salt: salt value
      pbkdf2_iterations: number of iterations to use
      pbkdf2_key_length: required key length in bytes
      pbkdf2_prf: name of pseudorandom function used
    """

    def __init__(self):
        self.algorithm = None
        # PBKDF2 properties
        self.pbkdf2_salt = None
        self.pbkdf2_iterations = None
        self.pbkdf2_key_length = None
        self.pbkdf2_prf = None

    def derive_pbkdf2(self, password):
        from Crypto.Protocol.KDF import PBKDF2
        from pskc.mac import get_hmac
        from pskc.exceptions import KeyDerivationError
        prf = None
        if self.pbkdf2_prf:
            prf = get_hmac(self.pbkdf2_prf)
            if prf is None:
                raise KeyDerivationError(
                    'Pseudorandom function unsupported: %r' %
                    self.pbkdf2_prf)
        if not all((password, self.pbkdf2_salt, self.pbkdf2_key_length,
                   self.pbkdf2_iterations)):
            raise KeyDerivationError('Incomplete PBKDF2 configuration')
        return PBKDF2(
            password, self.pbkdf2_salt, dkLen=self.pbkdf2_key_length,
            count=self.pbkdf2_iterations, prf=prf)

    def derive(self, password):
        """Derive a key from the password."""
        from pskc.exceptions import KeyDerivationError
        if self.algorithm is None:
            raise KeyDerivationError('No algorithm specified')
        if self.algorithm.endswith('#pbkdf2'):
            return self.derive_pbkdf2(password)
        else:
            raise KeyDerivationError(
                'Unsupported algorithm: %r' % self.algorithm)

    def setup_pbkdf2(self, password, salt=None, salt_length=16,
                     key_length=None, iterations=None, prf=None):
        from Crypto import Random
        from pskc.algorithms import normalise_algorithm
        self.algorithm = normalise_algorithm('pbkdf2')
        if salt is None:
            salt = Random.get_random_bytes(salt_length)
        self.pbkdf2_salt = salt
        if iterations:
            self.pbkdf2_iterations = iterations
        elif self.pbkdf2_iterations is None:
            self.pbkdf2_iterations = 12 * 1000
        if key_length:  # pragma: no branch (always specified)
            self.pbkdf2_key_length = key_length
        if prf:
            self.pbkdf2_prf = normalise_algorithm(prf)
        return self.derive_pbkdf2(password)


class Encryption(object):
    """Class for handling encryption keys that are used in the PSKC file.

    Encryption generally uses a symmetric key that is used to encrypt some
    of the information stored in PSKC files (typically the seed). This
    class provides the following values:

      id: identifier of the key
      algorithm: the encryption algorithm used
      key_names: list of names for the key
      key_name: (first) name of the key (usually there is only one)
      key: the key value itself (binary form)
      iv: optional initialization vector for CBC based encryption
      fields: a list of Key fields that will be encrypted on writing

    The key can either be assigned to the key property or derived using the
    derive_key() method.
    """

    def __init__(self, pskc):
        self.pskc = pskc
        self.id = None
        self._algorithm = None
        self.key_names = []
        self.key = None
        self.iv = None
        self.derivation = KeyDerivation()
        self.fields = []

    @property
    def key_name(self):
        """Provide the name of the (first) key."""
        if self.key_names:
            return self.key_names[0]

    @key_name.setter
    def key_name(self, value):
        self.key_names = [value]

    @property
    def algorithm(self):
        """Provide the encryption algorithm used."""
        if self._algorithm:
            return self._algorithm

    @algorithm.setter
    def algorithm(self, value):
        from pskc.algorithms import normalise_algorithm
        self._algorithm = normalise_algorithm(value)

    def derive_key(self, password):
        """Derive a key from the password."""
        self.key = self.derivation.derive(password)

    def _setup_encryption(self, kwargs):
        for k in ('id', 'algorithm', 'key_name', 'key_names', 'fields'):
            v = kwargs.pop(k, None)
            if v is not None:
                setattr(self, k, v)
        # default encryption to AES128-CBC
        if not self.algorithm:
            self.algorithm = 'aes128-cbc'
        # default to encrypting the secret only
        if not self.fields:
            self.fields = ['secret', ]
        # if we're using a CBC mode of encryption, add a MAC
        if self.algorithm.endswith('-cbc'):
            self.pskc.mac.setup()

    def setup_preshared_key(self, **kwargs):
        """Configure pre-shared key encryption when writing the file.

        The following arguments may be supplied:
          key: the encryption key to use
          id: encryption key identifier
          algorithm: encryption algorithm
          key_length: encryption key length in bytes
          key_name: a name for the key
          key_names: a number of names for the key
          fields: a list of fields to encrypt

        None of the arguments are required, reasonable defaults will be
        chosen for missing arguments.
        """
        self._setup_encryption(kwargs)
        self.key = kwargs.pop('key', self.key)
        if not self.key:
            from Crypto import Random
            self.key = Random.get_random_bytes(kwargs.pop(
                'key_length', self.algorithm_key_lengths[-1]))

    def setup_pbkdf2(self, password, **kwargs):
        """Configure password-based PSKC encryption when writing the file.

        The following arguments may be supplied:
          password: the password to use (required)
          id: encryption key identifier
          algorithm: encryption algorithm
          key_length: encryption key length in bytes
          key_name: a name for the key
          key_names: a number of names for the key
          fields: a list of fields to encrypt
          salt: PBKDF2 salt
          salt_length: used when generating random salt
          iterations: number of PBKDF2 iterations
          prf: PBKDF2 pseudorandom function

        Only password is required, for the other arguments reasonable
        defaults will be chosen.
        """
        self._setup_encryption(kwargs)
        # pass a key length to PBKDF2
        kwargs.setdefault('key_length', self.algorithm_key_lengths[-1])
        self.key = self.derivation.setup_pbkdf2(password, **kwargs)

    @property
    def algorithm_key_lengths(self):
        """Provide the possible key lengths for the configured algorithm."""
        return algorithm_key_lengths(self.algorithm)

    def decrypt_value(self, cipher_value, algorithm=None):
        """Decrypt the cipher_value and return the plaintext value."""
        return decrypt(
            algorithm or self.algorithm, self.key, cipher_value, self.iv)

    def encrypt_value(self, plaintext):
        """Encrypt the provided value and return the cipher_value."""
        return encrypt(self.algorithm, self.key, plaintext, self.iv)